NIGERIAN COLLEGE OF AVIATION TECHNOLOGY, ZARIA

Flying School

RADIO NAVIGATION AIDS

For

SP-Courses [CPL]

CPL Radio Navigation

© Flying School 2016

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 TABLE OF CONTENTS

1.0 . BASIC RADIO THEORY AND PRINCIPLES

2.0 . VHF DIRECTION FINDR (VDF)
3.0 . ADF/ NDB
4.0 . VORAND DVOR
5.0 . INSTRUCTMENT LANDING SYSTEM (ILS)
6.0 . MICROWAVE LANDING SYSTEM (MLS)
7.0 . BASIC RADAR PRINCIPLES
8.0 . DISTANCE MEASURING EQUIPMENT (DME)
9.0 . GROUND RADAR
10.0 SECONDARY SURVAILANCE RADAR (SSR)
11.0 AIRBORNE WEATHER RADAR (AWR)
12.0 GROUND PROXXIMIXITY WRANING SYSTEM (GPWS)
13.0 RADIO ALTIMETER
14.0 AREA NAVIGATOR (RNAV)
15.0 TRAFFIC COLLISION AVIODANCE SYSTEM (TCAS)
16.0 GLOBAL NAVIGATION SATELLITE SYSYTEM (GNSS).
17.0 DOPPLER NAVIGATION SYSTEM

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BASIS RADIO PRINCIPLES

1.1 INTRODUCTION
All matter or material consists of atoms and an atom comprises of a central nucleus around
which a number of electrons orbit. The electrons are negatively charged while the nucleus
also contains (normally) an equal number of positively charged protons and neutral neutrons.
If the total number of electrons in an atom is equal to the number of protons, the atom is
electrically balanced and is called a neutral particle. If the particle gains an electron, it
becomes negatively charged. If it loses an electron it becomes positively charged.

Bringing together 2 atoms of different charges causes an electron flow from the negative
particle to the positive one. This transfer, Caused by the gradient of charges between the 2
atoms, is an electron flow.

A flow of electrons along a conductor is an electric current. A direct current flows

continuously in the same direction. If the strength of the current varies rhythmically, but does
not charge direction, the current is said to be pulsating. If the direction of current flow
reverses periodically it is known as an alternating current.

When electrons flow along a conductor a magnetic field is built up around that conductor.
When the electric current ceases to flow, the magnetic field collapses. This change in the
magnetic field induces an electric current in the opposite direction to the original current. This
current in turn creates a new magnetic field, and the cycle is repeated. An alternating current
is produced, building up to a maximum in one direction, decreasing to zero, and then
increasing to a maximum in the opposite direction, and so on.

1.2 ELECTROMAGNETISM
The build- up and collapse of each electric and magnetic field occurs very rapidly. Apart from
a small loss dissipated as heat in overcoming the resistance of the circuit, the energy is fully
transferred between the two fields at small numbers of oscillations per second. However, if
the cycle is speeded up such that the time required for each field to build up, or collapse,
becomes more than one half of a cycle, then some of the energy is radiated out into space.
This energy has both electrical and magnetic properties and is known as electromagnetic
radiation.

1.3 WAVE TERMINOLOGY

Electromagnetic waves are described using certain terms:
1. Cycle: one complete waveform is known as a cycle

2. Phase: the point reached by the wave front at any instant during the formation of
a 360o cycle is referred to as the phase of the wave and is recorded in degrees.

3. Wavelength: the distance traveled by the energy in the direction of the propagation
during one cycle is the wavelength and is denoted by the Greek letter lambda (λ ) and is
expressed in meters (m) . Note: 10mm = 1cm

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 100cm = 1m
1,000m = 1km
4. Amplitude: the maximum displacement of a peak from zero is the amplitude and is
a measure of the power of the wave.

5. Frequency: the number of cycle which passes through a fixed point in one second
is called the frequency. One cycle per second is called a Hertz (Hz). It should be noted
that frequencies in the em spectrum are measured in many thousands of Hertz, prefixes are
used.
1,000mm Hz= 1kHz (kilo–Hertz)
1,000,000mm Hz= 1kHz (mega - Hertz)
1,000,000,000 Hz = GHz (Giga – Hertz)

6. Period: This is the time taken to complete one cycle and is measured in seconds

7. Attenuation: This the loss in signal strength of a transmission as range

increases from the transmitter attenuation is mainly due to absorption, scattering and
geometrical dispersion. Dispersion is the reduction of the radio signal as it spreads out
over an ever-increasing area after leaving the transmitter.

8. Noise: Any signal other than the desired signal which is detectable at receiver
tuned to the desired frequently is noise.

9. Reflection: This is the bouncing back of a signal when it strikes a surface

10. Refraction: This is the bending away of radio waves from the normal
as they cross boundaries between two media of different densities

11. Diffraction: This is the ability of a wave to follow the curvature of an object
in its path.

Fig 1.1 Sine wave

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1.4 RADIO WAVE FORMULA
Radio waves are assumed to travel at a constant speed (c) of 3.0 x 108m/s
(or 161 800 NM/s) which is equal to the speed of light. The relationship between this
speed, frequency and wavelength is:

C=f λ ………………………………………..(1.1)
Where:
C = velocity of propagation (300 x 106 m/s)
ƒ = frequency (Hz)
λ = wavelength (m)

Example 1. If the wavelength of a radio signal is 200m,

What is its frequency?

ƒ = C = 300 x106 Hz = 1.5 MHz

λ 200

Example 2: If the frequency of a radio signal is GHz,

what is the wave length?

λ = c = 300 x106 m = 1 m = 0.1 m

ƒ 3x109 10

Table 1.1: The radio frequency spectrum and aviation application

Radio frequency Frequency Range Equivalent Applications
band wavelength
VLF 3-30 kHz Very long 100- No current
10km (kilometric) applications
LF 30- 300kHz Long 10 -1 km Loran C, NDB
(kilometric)
MF 300kHz – 3MHz Medium 1 km – NDB
100m
HF 30- 30MHz Short 100 – 10m SSB
(decametric) Communications
VHF 30 – 300 MHz 10 – 1m ( metric) Air / Ground
Communications
VOR ILS LLZ
Marker Beacons
UHF 300 MHz – 3GHz 1m – 10cm ILS GP, DME SSR,
(decimetric) SATNAV,SATCOM
SHF 3 – 30 GHz 10 – 1cm Radio Altimeter,
(centimetric) weather Radar
Ground Radar
EHF 30 – 300 GHz 10 – 1mm Nocurrent
(millimetric) applications

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1.5. The Radio Spectrum

The electromagnetic spectrum of frequencies includes cosmic, gamma and X- rays (frequencies up to
and in excess of 1012 GHz), through ultraviolet, visible light, heat and infrared. However the radio
spectrum stretches from 3 kHz to 300 GHz. The radio spectrum is sub divided as in table 1.1

Radio aids are distributed amongst the spectrum as shown in table 1.1 and 1.2.
Equipment Frequencies
Loran C 100 kHz
ADF 190 – 1700Khz
HF Communication 2 – 25 MHz
ILS Marker 75 MHz

VOR 108 - 112MHz

VHF Communication 118 - 137 MHz
ILS Glide Slope 328.6 – 335.4 MHz
DME 960 - 1215 MHz
SSR / TCAS 1030 - 1090 MHz
Radio Altimeter 4200 - 400 MHz
MLS 5031 - 5091 MHz
Weather Radar (C Band) 5.5 GHz
Weather Radar (X Band) 9.4GHz
Table 1.2 Aviation related frequencies

Radio work is confined to these bands because of severe attenuation of the radio energy at the higher
frequencies (wavelengths less than 2 cm), and the problems of atmospheric static, large inefficient
aerials, and the consequent high power inputs required at low frequencies.

1.6 POLARISATION
The electrons in motion in a transmission aerial set up an electric field along the aerial, and
the associated magnetic field. Both these fields are at right angles to the direction of motion of
the energy. By convention, the alignment of the electric field is referred to as the polarization
of the radio wave.
In vertically polarized signal, the electric component is vertical with respect to the Earth. It is
produced by a vertical aerial, see fig 1.2.

Fig. 1.2 Polarization

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 Similarly, „horizontal polarization‟ is associated with an aerial horizontal to the earth‟s
surface. For communications in the bands up to VHF, vertical polarization is preferred
because horizontally polarized signals suffer severely from ground attenuation. At VHF and
above, either type of polarization may be used, but horizontal polarization is preferable where
the effect of ground reflections needs to be taken into account.

A receiving aerial has currents induced in it by the electromagnetic field. When radio waves
are propagated in free space, their polarization remains constants. Therefore, they appear
either vertically or horizontally polarized to the receiving aerial, which must be oriented to the
same polarization in order to receive the maximum signal strength.

In theory, if the receiving and transmitting antennas are not of the same polarity, reception
will vary according to the cosine of the angle between their respective planes. For example, if
the angle between aerials is 45º (cos 45º = 0.71), the signal will be received at greater than
half strength. When both are vertical, the angle is 0º, and cos 0º = 1, therefore full strength is
received.

Fig, 1.3 illustrates this relationship (TX means „transmitter‟, RX means „receiver‟).

Fig. 1.3 Angles between transmitter and receiver

1.7 PHASE DIFFERENCE

It is sometimes useful to compare the phase of two waves which arrive via different paths
from the same source, or from different sources. If the two waves are exactly in step, they are
said to be „in phase‟ otherwise, they are „out of phase.‟ The different between the phases of
the two waves provides a measure of the relationship between them and is quoted as an angle
between 0º and 360º, see fig 1.4.

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 Fig. 1.4 Phase Difference

1.7.1. COMBINED SIGNALS

In examples 1 and 2 the combination of the two waves would be detected by a receiver tuned to their
frequency. This resultant can be obtained by adding the amplitudes of each curve algebraically. If we
combine two waves having different frequencies and amplitudes, the resultant waveform is more

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complex, but its amplitude is still equal to the algebraic sum of the original amplitudes. The rate, at
which the two waves fall in and out of step with each other, is known as the „beat frequency‟ and it is
equal to the difference in frequency between the two waves.

BEAT FREQUENCY OSCILLATOR (BFO)

The beat frequency is important to the audible reception of unmodulated radio transmissions. Audio
waves have a frequency so low (20Hz to 20 kHz) that converting them to radio signals for normal
communications purposes would be impractical.

On the other hand, radio signals in the normal communications frequency bands (LF upwards) are
well above the audible range. In these bands (e.g. to hear wireless telegraphy or the tone signals of an
unmodulated NDB) the radio signal must be combined (beaten or heterodyned) with another radio
signal. This radio signal is generated separately in the receiver by a „beat frequency oscillator‟ (BFO)
to produce waveforms which fall within the audio – frequency range. The resultant signal is
amplified and may then be heard by the operator.

1.8. MODULATION
Modulation is the process by which one of the characteristics of a radio carrier wave is varied in
sympathy with the changes in amplitude and frequency of another signal (usually an audio wave). If
the transmission is modulated, (see fig. 1.5,) no BFO signal is required in the reair and the BFO (or
„IDENT/TONE) switch should be switched off. The receiver demodulates the incoming signal to
separate the original audio wave from the radio carrier waves and pass it to the user. The most
common types of modulation used in radio aids are amplitude modulation (AM) and frequency
modulation (FM). In radar aids, pulse modulation is sometimes used.

Fig. 1.5 Basic Transmitter

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1.8.1. AMPLITUDE MODULATION
During amplitude modulation the amplitude of the radio carrier wave is altered in accordance
with the changes in frequency and amplitude of the sound wave required. However, the
frequency of the carrier wave is constant, (see fig1.6).

Fig. 1.6 Amplitude Modulation

1.8.2. SIDEBANDS
When a radio carrier wave is amplitude modulated, spectral components falling into
frequency bands on either side of the carrier frequency are formed. These are called the
„upper‟ and „ lower‟ sidebands‟ according to whether the frequency range is above or below
the carrier frequency. For example, if a carrier wave of 1000 kHz is amplitude modulated by a
10kHz audio signal, then two side frequencies will be produced, which are the sum and the
difference of the carrier and audio frequency. The three frequencies in the transmission will
be 990, 1000 and 1010 kHz. If speech or music forms the audio signal then sidebands of
frequencies will be transmitted to cover the sums and differences of the carrier wave and the
range of frequencies in the audio signal. The bandwidth of the transmission is therefore twice
the maximum frequency of the audio wave.

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1.8.3. FREQUENCY MODULATION
During frequency modulation, (see fig.1.7) the amplitude of the carrier wave remains
constant, but its frequency is made to vary either side of a mean value in accordance with the
changes in frequency and amplitude of the sound wave required.

Fig. 1.7 Frequency Modulation

1.8.4. PULSE MODULATION

Pulse modulation consists of switching a carrier wave on and off as required. The upper part
of Fig. 1.8 shows a continuous carrier wave. Below it, you can see the carrier wave being
switched on and off for short periods to produce pulses of RF. This is the principle of radar; a
short pulse is transmitted and then an echo listened for.

Fig 1.8 Pulse Modulation

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1.9. TYPES OF EMISSION
In the AIP, the different radio facilities have their emission methods denoted by the prefix A
for Amplitude modulation, F for frequency modulation ad P0N for pulse modulation.

The emission designator consists of three characters:

 1st character – a letter denoting the type of modulation of the main carrier wave,
e.g. AM, FM, etc.
 2nd character – a number indicating the carrier, or the complexity of the signal
 3rd character – a letter showing the type of information to be transmitted, e.g.
radio telephony, facsimile, or a combination of types

A list of aviation related designators is shown in table 1.3.

EMISSION DESCRIPTION AIDS REMARKS
DESIGNATOR
NOX NON Unmodulated carrier wave Decca Protected frequencies
so no ident used
A3E Amplitude – modulated Broadcast station R /
double side – band T comms, Ground DF
telephony
NONA1A Unmodolated CW (tone), Long – range, marine BFO must be on to
interrupted at intervals to & others NDBs hear both tone and
send beacons identification ident
in morse
NONA2A Unmodolated CW Shorter – range, BFO on to hear tones.
(tone),on which amplitude NDBs. Locators &fan BFO off to hear ident
– modulation is markers beacons
superimposed at intervals
to send beacon ident in
morse
A8W Composite AM emission ILS localizer & glide
comprising separate path
modulation tones and
beacon ident in morse
A9W Composite AM emission VOR
plus beacon ident in morse
F Frequency modulation Radio altimeter
PON Pulse modulation DME,SSR,Loran C
NOX / G1D Differential phase shift MLS
keying (DPSK) data
transmission
Table 1.3. Types of emission and their applications

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1.10 ANTENNAS
A radio transmitter produces electromagnetic energy, also called radio waves, and to be able
to transmit the intelligence in these waves one needs a devices called an antenna. This is a
particular conductor, that when connected to a radio is able to radiate the electromagnetic
energy produced by the radio into receiver, so as to catch the radio waves and carry them into
the receiver circuit.

ANTENNA CHARACTERISTICS
There are three aspects which affect the characteristics of an antenna. The characteristics which
makes an antenna good for transmitting, also makes it good for receiving. These characteristics are:
 The length of the antenna
 The polarization
 The direction of the antenna

The efficiency of an antenna is related to its length, and for an antenna to be most efficient, the length
should be one half of the wavelength. Such a relation allows the antenna current to be at its
maximum, thereby producing a maximum electromagnetic field. The term polarization is used to
describe the direction or plane of oscillation of the electrical field. In general, one can claim that a
vertical transmitting antenna produces a vertically polarized radio wave, with the electrical field
oscillations occurring in the vertical plane, and the magnetic field oscillations in the horizontal plane.
To secure efficient reception, the receiving antenna should therefore be vertical if the transmitting
antenna is vertical. If there is a horizontal transmitting antenna, the receiving antenna should also be
horizontal. In general, communication in the LF, MF, and HF bands use horizontally polarized
antenna. Higher frequency system use vertically polarized antennas.

POLAR DIAGRAMS
An antenna is the part of the radio system which is designed to radiate or receive electromagnetic
energy. Antennas do not necessarily perform equally well in all directions.

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 Fig 1.9 Common Antenna Types

A polar diagram that indicates how well an antenna receives or transmits in different directions is
called a „radiation pattern‟. The distance from the location of the antenna to a point on the pattern
indicates the relative signal strength of the radiation in the direction determined by these two points.
When used for reception the pattern is called a „reception pattern‟.

The field strength of a transmitting antenna or the sensitivity of a receiving antenna, can be plotted
around the installation to produce a polar diagram. An omni directional antenna, such as one might
find on an aircraft for air to ground communication, or on a hand- held transceiver, will exhibit the
polar diagram shown in fig. 1.10.

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 Fig.1.10 Polar Diagram for an omnidirectional antenna

The „standard‟ antenna is half a wavelength in size and is called a dipole, (see fig 1.11).

Fig. 1.11 Dipole antenna

Its polar diagram is in the shape of a figure of eight, (see fig. 1.12). Although this particular design
is not usually found on aircraft, it does feature on ground installations, such as in the VOR (VHF
Omni – range) beacon. A loop antenna, which is used for directional finding, also exhibits a figure f
light polar diagram.

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 Fig 1.12 Figure of Eight Polar Diagram

A directional antenna, such as would be used on an ILS (instrument landing system) installation, or a
TV aerial, is generally referred to as a „Yagi‟ antenna (named after its Japanese inventor), and might
look like the ones shown in fig. 1.14.

Figures 1.13 Yagi Antenna

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 Fig. 1.4 Polar diagram for a Yagi antenna

Fig 1.15 Examples of Polar Digrams of Radiation Patterns

1.11. Propagation of Radio Waves

For convenience, it is usual to classify radio waves as:
 Ground waves
 Sky waves
 Direct waves

In practice, the radio signal can be a mixture of the above named waves, depending upon the
frequency used and the relative positions of the transmitter and receiver aerials.

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1.11.1. GROUND WAVES
Energy which leaves the transmitter and travels close to the ground tends to follow the
curvature of the Earth using a property known as „diffraction‟. The amount of diffraction is
dependent on the wavelength and decrease steadily as the wavelength is decreased. Ground
waves make it possible to receive signals well beyond the visible horizon.
The range at which ground waves can be received depends upon:
 Frequency used
 Nature of the terrain over which they pass
 Power of the transmitter
 Height of the aerials
 Interference

1. FREQUENCY
Attenuation, owing mainly to radio energy being dissipated into the earth‟s surface, reduces the range
of ground waves. Attenuation is greater at higher frequencies. Some approximate ground ranges for
vertically polarized transmissions are:
VLF – over 2000 NM
LF – about 1500 NM
MF – about 1000 NM
HF – about 100 NM
VHF and above – negligible
Horizontally polarized transmissions are attenuated much more severely over theses frequencies and
are not used for long range communication.

2. TERRAIN
The amount of radio energy lost in the earth and, hence the length of the ground wave, varies with the
electrical conductivity of its surface. Sea water is the best conducting surface, so distances achieved
over the sea are greater than those achieved over land.

Dry sand is the worst conductor and ranges over the desert are reduced to about 20% of those over
the sea, for the same frequency and transmission power. Between these two extremes, ranges depend
mainly on the moisture content of the ground, with rocks acting similarly to dry soil. The shape of the
surface is also important with ranges longer over flat ground than over hilly ground, but long
wavelengths are less affected by this factor than short wavelengths.

3. POWER
The ground wave is dependent upon power only up to the maximum range appropriate to the
frequency in use and the surface over which the wave is passing. Any further increase in power has
little effect, except in helping to combat interference.

Power in important if the maximum ground wave is to be achieved at low frequencies. Range is
roughly proportional to the square root of TX power, but power is often limited to prevent
interference between transmitters.

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4. HEIGHT OF AERIAL
Attenuation is greater close to the surface of the earth so an increase in height will normally increase
the range over which ground waves can be received. This applies particularly at the higher
frequencies.

5. INTERFRENCE
Interference can reduce the useful range of ground waves considerably and may be due to any form
of disturbance within the transmitter/ receiver equipment or while the ground wave is travelling
between the transmitter and receiver.

1.11.2. SKY WAVES

THE LONOSPHERE
Sky waves are returned to earth by refraction in the ionosphere. The ionosphere is that part of
the earth‟s outer atmosphere where solar (and cosmic) radiation removes electrons from the
gas molecules and atoms, resulting in positive ions and negative free electrons. The regions of
the ionosphere are:

 The ‘D’ layer

The D layer forms by day only at a height of between about 50km and 100km. This is the
principal source of attenuation of LF and MF waves and of the refraction of VLF waves
during daylight

 The ‘E’ layer

The second region of ionization occurs at a height of about 100km to 150km and is known as
the E layer. The height of this layer is more constant than the fluctuating F layer but at night it
becomes weaker and sometimes disappears completely.

 The ‘F’ layer

The F layer exists at a height of about 150 km to 1000km above the Earth‟s surface. During
daylight it usually splits into two parts, the F1 being about 150km to 250 km and the F2 about
300km to 1000 km above the earth‟s surface. At night it becomes one layer about 300km to
600km high, (See fig. 1.16)

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 Fig. 1.16 The Ionosphere

The main source of energy for ionization is the sun, and we find that the densities, thicknesses
and heights of the layers all vary with received solar radiation. The effects on radio waves
therefore also vary. Differences are found between day and night, summer and winter,
different parts of the sun‟s eleven – year sunspot cycle, and during unusual sunspot activity.
Effects are especially changeable around sunrise ad sunset.

Effect of the Ionosphere on Radio Waves

Radio energy reaching the ionosphere may be completely attenuated, may penetrate through it after
some refraction, or may be refracted to such an extent that it returns to Earth.

i. At VLF virtually all the energy is refracted back to earth, bouncing off the D layer by day and
the E layer by night. The waves then reflects repeatedly between the Earth‟s surface and the
ionosphere. This method of propagation is known as ionospheric ducting and was used in the
past for VLF navigation systems such as omega.

ii. At LF and MF, the D Layer causes severe attenuation of radio energy, and sky waves are
therefore very weak or non –existent by day. As a result, ground waves are the usual mode of
propagation in these bands. By night, the D layer disperses and sky waves reappear (fig.
1.17).

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 Fig. 1.17 Effect of the Ionosphere

iii. At HF, sky waves are used for long – range communications by day and night but not all of
the energy is refracted back to earth, depending upon signal frequency, the strength of the
ionosphere, and the angle at which the energy meets the ionosphere. The angle at the
transmitter from the Earth‟s vertical to the first sky wave refracting from the ionosphere is
known as the „critical angle‟. This angle increases as transmission frequency increases. The
distance travelled by the first sky wave over the Earth‟s surface from the transmitter to the
receiver is known as „skip distance‟. The area of no communications from the end of the
ground wave to the point of arrival at the Earth‟s surface of the first sky wave is called dead
space or skip zone. For a given receiver/ transmitter distance and given ionospheric
conditions, there is a maximum usable frequency (MUF) where skip distance exactly matches
the distance between the transmitter and receiver. The optimum working frequency (OWF) is
lower than the MUF to allow for diurnal variation of the ionosphere, with night OWFs being
lower than day OWFs.

iv. At VHF and above, transmitted energy either passes through the ionosphere or is absorbed
by it. A sky wave communication system is not possible at these frequencies.

1.11.3 DIRECT WAVES

The ‘line of sight’ principle
Direct waves follow a path similar to the line of sight so their surface range is limited by the
curvature of the Earth. Direct waves are present at all propagation frequencies. But the term is
more commonly used with VHF and above because at these frequencies the ground wave is
very short, and because there is normally no sky wave refraction from the ionosphere. Direct
wave range depends upon:
o Height of the transmitter aerial
o Height of the receiver aerial
o Power of the transmitter
o Terrain and other obstructions between the transmitter and the receiver
o Atmosphere refraction

Maximum Theoretical Range(MTR)

Refraction is caused by layers of different density in the atmosphere which bend the wave
front towards the earth. Under normal propagation conditions the range is appropriately 10%

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 greater than the visual range. The maximum theoretical (MTR) range for VHF and UHF
direct wave communications can be calculated by:

MTR = 1.25(√hTX +√ hR X)………………………………….(1.2)

Where, hTX and hR X are heights of transmitter and receiver aerials, respectively. The heights
(h) are given in ft (AMSL) and the range is in nautical miles.
Example: MTR for an aircraft receiver at 10,000 ft with a ground transmitter at 100 ft is:
MTR = 1.25(√100 + √10 000) NM
= 1.25(10 + 100) NM
= 137.5 NM
Range is 137.5 NM.

2. Power of the Transmitter

Range is a function of power. To double the range you must increase the power fourfold
(inverse square law).

3. TERRAIN
Screening of direct waves by high ground does occur, with a resulting decrease in reception
range. In addition, radio waves at higher frequencies are particularly sensitive to reflections
and scattering from buildings or other solid obstructions. Range can be reduced in certain
directions around the transmitter as a result of interference between direct waves and reflected
direct waves received simultaneously.

4. ATMOSPHERIC REFRACTION
The atmosphere is not homogeneous – pressure, temperature and humidity can all change
with height causing the dielectric properties of the atmosphere to change. It would be
reasonable to expect that RF (Radio Frequency) would travel through the air in straight lines
as a direct wave, but it bends, or refracts, with the changes in conditions outlined above. The
diagram in fig. 1.18 considers the atmosphere to be a series of laiyers and as the RF goes
through each layer, it is bent at the normal between layers. The result of this is that the radio
horizon extends beyond the visible horizon. As you have already seen, for conditions of the
standard atmosphere, the range of an RF wave can be given by equation 1.2.:

Fig 1.18 Atmospheric refraction

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 That‟s all very well for normal conditions, but abnormal meteorological conditions can alter
the degree of refraction of an RF wave:

 SUPER REFRACTION
Known as „anomalous propagation‟ or „duct propagation‟, this effect is associated with a
marked temperature inversion and rapid decrease in humidity with height. The inversion layer
is normally close to the Earth‟s surface rising only to a few thousand ft. These conditions are
most often found at the surface over land when a high – pressure system dominates. A warm
air mass over a cold sea can create the same effect, which is to trap the RF wave in a duct of
cold air, (see fig. 1.19). The duct behaves a bit like a waveguide; the refractive index is so
high that the signal appears to leapfrog between the surface and the top of the layer. Super
refraction is more likely where a landmass has wide temperature variations between day and
night, typically in tropical and sub tropical regions. The effect is most common in the VHF,
UHF and SHF bands and can cause interference between control towers which are hundreds
of miles apart but on the same frequency

Fig. 1.19 Duct propagation

 SUB – REFRACTION
Occasionally, the refraction index in the atmosphere is less than normal and, therefore, will
result in a reduced overall range (attenuation) due to less bending of the radio waves.
Typically, this will occur in fog or where the relative humidity is 100%. This reduces the
likelihood of mutual interference apart from the possibility of multi – path signals at short
ranges.

COASTAL REFRACTION
When a radio signal passes a coastline, it often changes direction. This phenomenon is called
„coastal refraction‟.

1.11.4 ATMOSPHERE ATTENUATION

Atmosphere attenuation is the atmospheric resistance to the passage of RF and is due to two
main factors:
 Vibration of gas molecules in the atmosphere
 Absorption and scattering of the signal by precipitation

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 The power loss is greater with increasing frequency and / or increasing precipitation.
Atmospheric attenuation is neglible until the upper end of the UHF band when it increases
rapidly to limit the highest usable frequency to about 10GHz. The exception to this is where
extremely short range radars may be used i.e. surface movement indicator equipment at large
airports and in ports / docks.

Rain and water in the atmosphere absorb and scatters radio waves. Remember that water is a
good conductor and the shape of a water droplet makes it a good reflector. Together, these
factors cause scattering which means that what is left of the RF after absorption, may no
longer be traveling in the direction expected. Water can be in the atmosphere in several
forms: water can be in the atmosphere in several forms: rain has the most attenuative effect,
while fog, ice and dry snow attenuate RF but to a mush lesser degree.

The atmospheric attenuation caused by the presence of water vapour is more intense than that
caused by resonance. Fig. 1.20 shows the relative attenuation levels and how they vary with
frequency: generally speaking, the higher the frequency, the greater the attenuation caused by
rain / precipitation.

Fig. 1.20 Atmospheric attenuation

The graph also shows where some of the highest frequency radio applications lie in the RF
spectrum. The useable upper end of the spectrum is effectively limited by atmospheric
attenuation to about 10 GHz, with the exception of an uncharacteristic drop in attenuation at
13 14 GHz. Surface movement indicators, terrain following radars and collision avoidance
equipment capaitalise on the blip.

REFLECTIONS
In fig. 1.21, a direct wave from a transmitter to an aircraft is shown. If conditions are right,
there can also be a signal reflected from the Earth‟s surface. Since the direct and reflected
waves follow different paths, they may arrive at the receiver at a phase difference. This causes
„fading‟, and occurs most often at night.
If an aircraft is flying towards a ground station, it can suffer temporary loss or fading of VHF
communications with the station. The range at which this occurs will depend on the height of
the ground antenna above the surface, aircraft altitude, and frequency of transmission.

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 Fig 1.21 Ground Reflected wave

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2.0 VHF DIRECTION FINDER (VDF)

2.1 INTODUCTION
VDF provides a means of determining the aircraft bearing from a ground stations. VDF
stations are capable of measuring the direction of arrival of radio transmission from aircraft.
In earlier days, such a service operated in MF, HF, and VHF bands. Today, the service (for
civil aviation) operates on frequencies from 118 to 137 MHz in the VHF band.

Military airfields provide direction finding service in the UHF band, so – called UDF.

2.2 PRINCIPLE OF OPERATION

If the communication transmitter on an aircraft is tuned to the VDF frequency and the
transmitter is activated, the aerials at the VDF will detect the incoming transmission and each
aerial element will feed a signal to the VDF receiver. Since the aerial elements will all be
slightly different distances from the source of the signal, each will detect a slightly different
phase of that signal at the same instant. The value of these detected phase differences will be
directly related to the direction of the incoming signal. The phase differences are used to
drive the bearing indicator.

On some VDF units a simple digital read out gives the bearing. A ground DF station can give
true or magnetic bearing. It is common to use the so called „Q –codes‟ to represent bearings.
Listed below are the codes that are relevant to direction findings.

Fig. 2.1 VDF Aerial

QTE = True bearing from the station

QDR = Magnetic bearing from the station
QUJ = True bearing to the station
QDM = Magnetic bearing to the station

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 In practice, only QDM and QDR are normally used. The accuracy of the bearing is measured
in degrees. Bearings are categorized, in accordance with the ICAO defined classifications, as
given in the following list:

Class A accurate within + 2o

Class B accurate within + 5o
Class C accurate within + 10o
Class D accuracy less than Class C

Due to topography, some VDF stations are approved for use within certain sectors only. In
that case, specific information for that aerodrome will be given in the AIP. Stations that are
listed in the AIP provide „homing‟ service. Generally the class of bearing is no worse than
class B. (Many states will not permit class C &D bearings to be provided). Ground DF
stations should not be used as en – route navigation aids. However, in case of emergency or
where other essential navigation aids have failed their services is available. When flying VFR
under marginal weather conditions, a DF station is a useful navigation tool to fall back on.

2.3 RANGE AND ERRORS

Being a VHF transmission, the range is generally line of sight. The range will primarily
depend on the height of the transmitter and the receiver.

Some factors that will affect the expected range are:

 Intervening terrain, which can screen the transmitter / receiver path

 Atmospheric refraction. An increased refractive index (resulting from the inversions of

temperature and / or humidity) can cause super refraction and increased ranges. Sub
refraction will reduce the expected range

 Transmitter power. The bearing signal measured may be in error. The major sources of
error are:
 Ground reflections, which can cause VHF and UHF signals to reach the DF station
aerial from multiple paths. This will cause additional phase difference to be detected
which will deflect the bearing indication.
 Synchronous transmissions in which signals from other aircraft communications
equipment are detected at the DF station at the same time as the desired signal. This
causes a deflection of the measured bearing. It should be noted that this is particularly
a problem in congested airspace and / or when atmospheric conditions favour super
refraction and cause transmissions from beyond the „radio horizon‟ to be detected.

 The signal quality may be reduced if the aircraft does not fly straight and level. This is
because the radio signals are vertically polarized and reception is optimal when the
aircraft has only a small amount of pitch and bank. To ensure a good reception of the
signal, avoid requesting bearing or heading to steer during steep turns.

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3.0. NDB AND ADF

3.1: PRINCIPLE OF OPERATION

Non Directional Beacons (NDB) are ground – based transmitters which transmit radio energy
equally in all directions. The airbone system is called the Automatic direction Finder (ADF).
Its indicator always points towards the tuned NDB.

The NDB transmitter is very simple: A RF oscillator provides the carrier wave. The carrier
wave is the NDB signal used by the airborne equipment (ADF) to determine the direction of
the transmitting station. A low frequency oscillator provides the identification signal of the
transmitting station or „ident‟. The low frequency signal modulates the carrier wave in the
modulator.

The types of modulation normally used by NDB are:

 N0NA1A for long range NDBs
 N0NA2A for short and medium range NDBs

The modulation class of the NDB is usually referred to as A1A or A2A only but NDB stations
actually use two different signals together: N0N (the unmodulated signal or carrier wave) is
used by the airborne equipment to determined the direction of the signal. A1A or A2A (the
modulated signal) is used to transmit the NDB‟s identification. The nominal range of an NDB
is proportional to the square root of the transmitter power.

Types, typical associated power outputs for use are as follows:

 Locator Beacon- 15 to 40 watts. Used for intermediate approach guidance towards

establishing the final approach path of an ILS. These beacons are short range and are
normally N0N A2A.

 Airways / route Beacons – up to 200 watts. Used for track guidance and general
navigation. These beacons are normally N0N A2A.

 Long – Range Beacons also known as oceanic NDB – up to 4 kilowatts. Generally

located on islands or oceanic coastlines, these are intended to provide guidance and
navigation recourse to transoceanic flights. These beacons are normally N0N A1A

The amplified signal finally reaches the transmission aerial where it is radiated omni –
directionally. The transmission mast may be either a single mast or a large T – aerial strung
between the masts, see fig. 3.1

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 Fig. 3.1 „T‟ shaped NDB aerial
These aerial arrangements produce a vertically polarized signal. The polar diagram for the
aerial is omni directional in the horizontal plane but exhibits directional properties in the
vertical plane, as shown in fig 3.2

Fig. 3.2 Cone of silence

Above the station, marked by the points at which the radiated power has fallen to 0.5 of its
maximum value, is a conical space in which signal strength may be too low to be used. This
volume of space is called the „cone of silence‟.

The frequencies assigned by ICAO for NDB transmissions are from 190 kHz to 1750 kHz. It
should be noted that transmitters operate within the NDB band of frequencies and can be
detected by the aircraft‟s receiver. These include broadcast stations (i.e. those carrying
entertainment, news, etc.) and Marine Beacons. Stations must not be used if their details are
not published in the AIP or appropriate flight Guides. Where details of Marine Beacons are
published, users should note that a number of such beacons will be grouped together to serve
an area. These beacons will share a single transmission frequency, each .transmitting for a
period of 60 seconds in a cycle of 6min. Make sure that the bearing you are reading is for the
„correct‟ beacon.

The use of signals from such published stations guarantees that, within the published range by
day, the signal from the desired station will be at least three times stronger than any other
signal on the same or near frequency.

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3.2 ADF PRINCIPLE OF OPERATION
The Automatic direction Finder (ADF) consists of a receiver, a sense aerial, a loop aerial and
an indicator. The receiver control panel, and the indicator are located in the instrument panel,
loop and the sense aerial are normally combined in a single aerial unit, normally mounted
under the fuselage. The pilot uses the receiver control panel to dial the frequency
corresponding to the NDB for intended use.

The ADF indicator consists of a needle, which indicates the direction from which the signals
of the selected NDB ground station are being received. In its most basic form, the needle
moves against a scale calibrated in degrees from 0o – 359o. This is known as a radio compass.
The datum for the direction measurement is taken from the nose of the aircraft and therefore,
the radio compass indications are relative bearings.

3.2.1 BEARING DETERMINATION

The loop, a directional aerial, is rotated electronically and, combining information from the
loop and sense aerials. The bearing to the station is internally derived in the ADF.
One of the basic principles of electricity says that if a variable number of electromagnetic
field lines pass through a coil, a voltage will be induced in the coil. When a looped
conductor, such as the loop aerial, is hit by electromagnetic waves, voltage will be induced in
the loop. These voltages depend on the angular position of the loop relative to the incoming
electromagnetic (EM) waves. The voltage induced in the loop is at its maximum when the
loop‟s plane is parallel to the received signal. Thus the receiver will detect the greatest voltage
when the plane of the loop is parallel to the direction of propagation of the radio waves. If the
loop aerial is rotated until it is perpendicular to the direction of movement of the radio waves,
none of the EM waves will pass through the loop and the resultant signal will be a null. For
one 360o rotation of the loop aerial, the receiver will detect two maximums and two nulls.
Small angular deflections of the loop aerial near its null position produce larger changes in
voltage than similar angular changes near the loop‟s maximum position. For this reason a null
position is used for direction finding purposes.

The horizontal polar diagram of a loop aerial will have the shape of a figure „8‟, as shown in
fig. 3.3. In this figure the plane of the loop is indicated, as well as the loop‟s axis which is
perpendicular to the loop‟s plane.

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 Fig. 3.3 Cardioid polar diagram

Since the null occurs in two positions during the 360o rotation, there is a 180o ambiguity in the
bearing indication. This ambiguity is resolved by the use of a sense aerial. The function of the
sense aerial, so far as automatic direction finding is concerned, is to eliminate the ambiguity
of a loop by distinguishing between the signals received from one side of the loop and the
signals received from the other side of the loop. As we have seen, the sense aerial receives
equally well from all directions and thus it is traced out in order to show the signal strength
produced by the loop at different angles through 360o, the result is a figure eight. In adding
the steady signal from the sense aerial to the alternating signal from the loop signal, the
resultant polar diagram is a heart shaped figure, called a cardioid.

To resolve the 180o ambiguity problem, the polar diagram of the loop antenna is electronically
switched back and forth some 100 to 150 times a seconds after having been fed to the ADF.
This results in the combined cardioid polar diagram being switched between two diagrams,
one being a mirror image of the other. In this process the total signal strength reaching the
ADF receiver will vary with a frequency of 100 to 150Hz. The variation in strength will be
according to the strength received from each of the cardioids from the direction of the NDB.
There will be two directions, along the loop‟s axis, from which there will be no change in
strength when switching between the cardioids. The ADF will automatically rotate the loop
aerial into such a position using an electric motor. In this position of the loop aerial the axis of
the loop will point to the NDB. An automatic comparison of the phases of the signals from the
loop and sense antennas enables the ADF to solve the 180o ambiguity, and the ADF indicator
will hence point unambiguously towards the NDB being received.

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3.2.2 CONTROL PANELS AND INDICATORS

Fig 3.4 The ADF Control Panel

There are different types of ADF control panels, but their operational use is almost the same
and an example is shown in fig. 3.4. Function switches can be labeled in different ways.
Standard functions are OFF, ADF, ANT and BFO. On the receiver illustrated above, the left
hand switch is pushed in for ADF and left out for ANT. The center switch is for changing
frequency and the two right hand buttons operate an additional ETA /stopwatch function. The
far most knobs on the right are for frequency selection and the one to the left of those is the
Off / volume control.

„ADF‟ is the normal position when the pilot wants bearing information to be displayed
automatically by the needle.

„ANT‟ is the abbreviation of antenna and, in this position, only the signal from the sense
aerial is used. This results in no satisfactory directional information to the ADF needle. The
reason for selecting the ANT position is that it gives the best audio reception. This allows for
easier identification of the NDB station and also better understanding of any voice messages.

The BFO stands for beat frequency Oscillator. Sometimes this position is labeled TONE. It is
necessary to select the BFO „ON‟ position when idenfying NDBs that use A1A
transmissions. The BFO circuit imposes a tone onto the carrier wave signal to make it
audible to the pilot, so that the NDB signal can be identified.

Once the station has been properly tuned and identified, the mode selectors should be
switched from ANT to ADF. This is very important, since bearing information will not be
displayed unless the switch is in the ADF position. Never leave the mode selector in ANT if
you are navigating using the ADF. In the ANT mode the needle will remain stationary and not
correspond to direction to the NDB. Since there is no failure flag on an ADF receiver or
indicator, the only way to be sure that the NDB is to continuously monitor the station‟s
identification.

Each NDB is identifiable by a two or three lettered morse code identification signal, which is
transmitted together with its normal signal. This is known as its IDENT. When tuning an
NDB it is absolutely essential that the facility is correctly identified before being used for
navigation. In modern ADFs the NDB carrier frequency in kHz is selected digitally with high
electronic accuracy.

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3.2.3 BEARING INDICATORS
Bearing to the station are displayed on an indicator consisting of a bearing
scale (calibrated in degrees) and a pointer. There are three types of bearing scale with varying
degrees of sophistication. They are:
 Fixed card indicator or RBI
 The manually rotatable card
 The radio magnetic indicator (RMI)

The bearing displayed on a fixed card indicator is a relative bearing; therefore it is called a
Relative Bearing Indicator (RBI). Since the card is fixed, zero is always at the top and 180
always at the bottom. Relative bearings are measured clockwise. It is sometimes convenient,
however, to describe the bearing of the NDB in relation to the nose or tail of the aircraft, (see
fig. 3.5)

Fig. 3.5 Bearing Indicator

The relative bearing indicates the position of the station relative to the longitudinal axis of the
aircraft. If the needle points to 90o, for instance, it means that the station is 90o to the right of
the nose, off the right wing tip. If the needle points to 330o, it means that the station is 30o to
the left of the nose.

Since the card is fixed, the indicated relative bearing has to be combined with the magnetic
heading of the aircraft in order to obtain the magnetic bearing to the station, QDM.

Each time the aircraft changes its headings, it will carry the fixed card with it therefore, with
each change in heading, the RBI needle will indicate a different relative bearing. But
remember that the magnetic bearing to the station is always the sum of the magnetic heading
and the relative bearing.

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3.2.4 ROTATABLE CARD
A rotatable card type of indicator is exactly like a fixed card indicator, except that the card
can be rotated to reflect the aircraft‟s heading. When the card is aligned with the Directional
Gyro, the needle will indicate QDM and the tail of the needle will indicate QDR. This
instrument is seldom seen now except in some older aeroplanes

3.2.5 THE RADIO MAGNETIC INDICATOR (RMI)

This combines the relative bearing indicator and remote indicating Gyro compass into one
instrument, with the compass card being aligned automatically with magnetic North. The RMI
normally has two indicators formed as arrows, one is called the number one needle, the other
called the number two needle, see fig. 3.6. The indicators may be selectable to indicate ADF
or VOR information. In fig. 3.6 the number two arrow is indicating ADF information, the
relative bearing to the NDB is 72 o. The compass bearing is 135o, compass heading 240o. a
rough reading of the relative bearing may be made using the marking for every 45o on the
outside of the compass scale.
These marking are fixed:

 The QDM is continuously indicated under the pointer

 The QDR continuously indicated under the tail
 This is now the most common type of presentation

Fig. 3.6 Radio Magnetic Indicator (RMI)

3.2.1 NDB NAVIGATION

Procedure for obtaining an ADF bearing:
 Determine the frequency, identification and modulation of the required beacon and
ensure that your aircraft is within the published (promulgated) range.
 Switch on the ADF and adjust volume
 Tune the frequency and identify the station using „ ANT‟ and BFO‟ as necessary
 Select ADF on the control panel and read the bearing on the indicator

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3.3.1 LINE OF POSITION (LOP) USING THE RBI
With the help of the information we get from our instruments, we are now able to determine
the line of position along which our aircraft is positioned. To draw this LOP on the chart we
need the QDR or the QTE. In fig. 3.5 the relative bearing, as read under the pointer, is 270o.
This means that the NDB is 90o to the left of the aircraft nose. With the fixed card indicator,
the way to find the accurate QDM is to add the relative bearing of 270 o to the magnetic
heading of 360o. This gives QDM 270o. In order to obtain the QDR (the magnetic bearing
from the NDB to the aircraft), we need to add or substrate 180 to the QDM. In the above case
this gives QDR 180 o.

3.3.2 HOMING NDB

Since the ADF needle always points towards the station, the easiest way to reach the beacon is
to constantly fly with the needle pointing to the top of the indicator. This procedure is known
as HOMING.

The easiest way to home to a station is to turn the aircraft in the direction of the needle until
the needle points to the top of the indicator. This points the nose of the aircraft directly
towards the station. Once aimed at the station, any crosswind component will display the
aircraft to either side of the straight track to the station and the ADF needle will swing away
from the top of the indicator. The pilot will then have to make a correction of the heading
towards the needle in order to continue heading to the station.
This process will have to be repeated again and again since the crosswind pushes the aircraft
away from the straight track. The resulting path to the station will thus be a curved one, see
fig. 33.7

Fig. 3.7 Homing; aeroplane‟s heading and the track followed by the aeroplane

3.3.3 INTERCEPTING A TRACK

The correct way to navigate with the help of ADF and NDB can be divided in three steps: first
visualize your position, second intercept the desired track and third maintain the track to or
from the station. In (fig. 3.8) a fixed card type indicator is shown. The magnetic heading at
this time is 075o, and the desired inbound track to the NDB is 035o. The first step is to
visualize your position. You should find yourself south – west of the NDB, heading 075o,

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 (This is a rough sketch; the directions are not correctly presented). The second step is to turn
to a heading that gives you a suitable intercept. Observe the instrument readings during the
turn.

Fig. 3.8 How to intercept QDM

Now look at the corresponding plan view. The heading of 075 gives you an intercept angle of
40o. Since the desired QDM is 035o, an RBI indication of 320 will indicate that you have
intercepted the desired track.

When the needle is reaching the desired relative bearings in this case 320o, start your turn
towards the station maintaining a RB of 320o. Your aircraft will now be on the desired
inbound track.

To intercept a track outbound, follow the same procedures. First of all look at the radio
compass and visualize your position. Consider fig. 3.9

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 Fig. 3.9 How to intercept QDR

The relative bearing of 100o combined with the magnetic heading of 125, indicates that you
are North and East of the NDB. The desired track is 085 outbound. Our intercept angle is 40o.
When the relative bearing is 140o we will reach our outbound track. Then the tail of the
needle will be 40 out to the left, pointing at 320. Observe the instrument indications. When
the needle has reached 130o, start turning to intercept the outbound track. Look at the
instrument indications. Heading 085 with relative bearing 180, now you are on track.

As we have seen, in order to intercept a specific course, first you have to known your position
relative to the desired track, and then you establish a suitable interception angle.

3.4 NDB and ADF – LIMITATIONS AND ACCURACY

3.4.1 LIMITATIONS
Nearly all the limitations common to NDB navigation are a direct function of its operating in
the LOW and MEDIUM frequency bands. The signal from an NDB transmitter in the LF /
MF band actually propagates primarily along GROUND WAVE and SKY WAVE paths.

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3.4.2 ERRORS OF THE ADF
The ADF bearing is subject to a number of error sources including any or all of the following.

1. QUADRANTAL ERROR
The metal components of the aeroplane‟s structure behave as an aerial. They absorb signals at
all frequencies but more readily so at frequencies in the MF band. Once absorbed, these are
then re – radiated as weak signals but, being close to the ADF aerial, are strong enough to be
detected.

The effect of this signal is a displacement of the measured null position towards the major
electrical axis of the aeroplane creating an error that is maximum on relative bearings 045o,
135o, 225o, 315o (the quadrants). This error can usually be ignored, as it is compensated for
when the aerial is installed in the aircraft.

2. DIP (or BANK) ERROR

During turns, the horizontal member of the loop aerial will detect a signal. This will cause the
null to be displaced and a „short – term‟ erroneous bearing to be displayed. Only applies to the
fixed loop antenna.

3. COASTAL REFRAACTION
When flying over the sea below 6000ft, and using a land based beacon, the changes in
propagation properties of the signal as it passes from land to sea will cause the „wave front‟ to
be displaced, (see fig. 3.10). This will result in a bearing error.

Fig. 3.10 Coastal Refraction

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 Such bearing errors may be minimized by any or all of the following:
 Do not use beacons unless they are situated on island or near to the coast
 If using an inland NDB only use bearings at or near to 90o to the coast
 Remember that coastal refraction is less as height is increased
 A position line plotted without correction for coastal refraction will indicate a
position closer to the shore than the real position.

4. MULTIPATH SIGNALS
When flying in mountainous regions, signals may be refracted (bent) around and / or reflected
from mountains. The ADF may be affected by such multipath signals and the bearings will be
unreliable.

5. SKY WAVE (NIGHT) EFFECT

If the loop aerial is receiving sky wave signals from the same NDB at the same time as
ground wave signals are being received, the null will be suppressed or displaced in a random
manner. Some of the displacements may give stable (but wrong) bearing indications for a
period of time and are therefore very hazardous. At dawn and dusk, as the state of ionization
changes these errors are particularly unpredictable.

6. RANGE
Each NDB has an associated published range. If use of that NDB is restricted to that range,
the desired signal is protected from the harmful interference of ground waves from other
known transmitters on the same or near frequencies. It should be remembered that, from
sunset to sunrise sky wave propagation of signals in the LF and MF bands is possible.

7. NOISE
This is defined as any signal detected at the receiver other than the desired signal. This will
cause the signal to noise ratio to be reduced and will result in error as the null is displaced,
usually randomly.
Another localized source of man made noise is overhead power cables. Many of these cables
carry not only electrical power but also modulated signals used by the power companies for
communication. These modulated signals radiate from the power cables and create mini
NDBs. Such emissions are monitored but, in some states, monitoring may not be carried out.
The rule is – if unsure use with extreme caution.

8. ATMOSPHERIC NOISE
There is an average of 44,000 thunderstorms over the earth‟s surface in every period of 24
hours and more than half of these occur over or near land surfaces within 30o latitude of the
equator.

Each thunderstorm generates electromagnetic signals and these radiate in all directions from
that storms, your ADF will detect the signal and bearing indication may well be deflected
towards that storm. Such noise levels are normally quite low but they will increase:
 In temperate latitudes in the summer
 As you move towards the tropics
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  At night as a result of sky wave propagation

Noise effects can be indicated by:

 Seeing the bearing indication randomly wandering
 Using the audio output and noting audible signals such as voice/music/static
If „noise effect‟ is suspected, only use the published NDBs when well within the notified
range. You could be at half the published range before a reliable signal is received.

Accuracy
When used within the published range by day in good condition, well calibrated ADF should
give a bearing accuracy within + 5 o.

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4.0. VOR AND DOPPLER VOR (DVOR)
4.1 PRINCIPLE OF OPERATION
VOR is an abbreviation for VHF Omnidirectional Radio Range, which implies that it operates
in the VHF band. Adopted by ICAO as early as 1960, VOR has been the main short-range
navigational aid for several years. Short range infers that ranges up to 200NM can be
expected. It is still the most commonly used short-range aid. The signal transmitted by the
VOR contains directional information. As opposed to the NDB, which transmit a non-
directional signal.

The principle of operation is bearing measurement by phase comparison of two signals. This
means that the transmitter on the ground transmits signals which make it possible for the
receiver to determine its position in relation to the ground station by comparing the phases of
these two signals. In theory, the VOR produces a number of tracks all originating at the
transmitter. These tracks are called radials and are numbered from 1 to 360. The 3600 radial is
the track leaving the VOR station towards the magnetic North, and if you continue with the
cardinal points, radial 090 points to the East, the radial 180 to the South and the radial 270 to
the West, all in relation to the magnetic North,
(see fig 4.1).

Fig. 4.1 The VOR and the cardinal radicals

Before we look in detail at how the system works the following example illustrate the
principle and should make it easier to understand.

Think of a light house at sea and imagine the white light rotating at a speed of one revolution
per minute (60 sec). Every time this white narrow beam passes through magnetic North, a
green omnidirectional light flashes. Omnidirectional means that it can be seen from any
position around the lighthouse. If we are situated somewhere in the vicinity of the light
sources and are able to see them, we can measure the time interval from the green light flash
until we see the white light. The elapsed time is directly proportional to our position line in
relation to the light house.
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 The speed of 1 RPM corresponds to 60 per second, so if 30seconds elapse between the time
we see the green flash and the white rotating light, we are on the 1800 radial, or directly south
of the station (30sec x 60 sec =1800). The calculation can be done from any position and the
elapsed time is directly proportional to our angular position (radial). We could name these
light signals, calling the green one the reference (REF) signal and the white beam the variable
(VAR) signal.

4.2 GROUND INSTALLATION

The VOR system operates on frequencies between 108MHz and 117.95MHz. Channel
separation is 50 KHz and the signal have a horizontal polarization.

Frequencies between 108MHz and 111.95MHz are primarily used for the localizer part of the
ILS but can be shared with short range VORs, these VORs used frequencies having an even
decimal as the first digit after the last MHz digit, while localizers use odd decimals. When
assigning a VOR to this parts of the frequency band, it is an essential requirement that it does
not interfere with an adjacent ILS channel. The frequencies from 112.00 to 117.95MHz solely
used by VOR, both on odd and even frequencies.

4.2.1 CONVECTIONAL VOR (CVOR)

The ground equipment is set up on a fixed surveyed site and consists of a transmitter driving a
combined aerial system. The aerial system is on top of a counterpoise (fig. 4.2).

Fig. 4.2 CVOR ground station

The counterpoise acts as a reflector in order to reduce the combined effects of direct waves
and ground reflected waves. One part producing the reference (REF) signal, the other
producing the variable (VAR) signal. The reference signal is an omnidirectional continuous
wave transmission on the carrier frequency of that particular VOR station. It carries a 9960Hz
sub-carrier that is frequency modulated at 30Hz. Since this is an omnidirectional transmission,
the polar diagram of the REF signal is a circle, see fig 4.3.

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 In the receiver, it is the 30Hz component of this signal that is used as a reference for
measuring the phase difference. The variable signal VAR is transmitted from an aerial that is
effectively looped. This „loop‟ produces a figure of eight polar diagram, which is
electronically rotated at 30 revolutions per sec. when the two signal (VAR & REF) are mixed
together, the resulting polar diagram will be a cardioid, but unlike the cardioid of the ADF,
this does not have a known position. We call it a limacon. It rotates at 30 revolutions per sec,
indicating with an arrow on figure 4.3.

Fig. 4.3 Polar Diagram

The rotation of the limacon creates an effective amplitude modulation of 30Hz. The VOR
receiver splits these two signals into the two original components. The two signals are
processed through different channels and the phase of the 30Hz modulations of the fixed REF
signal and the VAR signal are compared in a phase comparator. The phase difference between
these two signals is directly proportional to an angular position with reference to the VOR
station. As explained, magnetic North is the normal reference for the radials, so when 00
phase difference is detected, the receiver is on the 3600 radial from the station. Fig. 4.3 shows
the phase difference and variable signal at the cardinal points.

The description above is valid for the conventional VOR, (CVOR). The CVORs suffer from
reflections from objects in the vicinity of the VOR site and it was found that errors due to this
could have been reduced if the horizontal antenna dimensions were increased. This was not
practical to do and a new system had to be developed: the DOPPLER VOR, (DVOR). The
CVORs are now gradually being replaced by DVORs, that will be described in the next
section.

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4.2.2 DOPPLER VOR (DVOR)

Fig. 4.4 A Doppler VOR ground station

The Doppler VOR is the second generation VOR, proving improved signal quality and
accuracy. The REF signal of the DVOR is amplitude modulated, while the VAR signal is
frequency modulated. This means that the modulations are opposite as compared to the
CVORs.

The frequency modulated signal is less subject to interference than the amplitude modulated
signal and therefore the received signals provide a more accurate bearing determination.
The Doppler effect is created by letting the VAR signal be electronically rotated, on the
circular placed aerials, at a speed of 30 revolutions per second. With a diameter of the circle
of 13.4 meters, the radial velocity of the VAR signal will be 1264 m/s. This will create a
Doppler shift, causing the frequency to increase as the signal is rotate towards the observer
and reduce as it rotates away with 30 full cycles of frequency variation per second. This
results in an effective FM of 30Hz. A receiver situated at some distance in the radiation field
continuously monitors the transmitter. When certain prescribed deviations are exceeded,
either the IDENT is taken off, or the complete transmitter is taken off the air.

The VOR receiver does not know if it is receiving a signal from a CVOR or a DVOR and the
pilot treats both types in the same way. The change of FM and AM for the REF and VAR
signals, as compared to the CVOR is compensated for by having the DVOR antenna patten
rotate the opposite way, compared to the CVOR.

4.3 AIRBORNE EQUIPMENT

The airborne system of the VOR installation consists of three main elements:

1. Aerial, Receiver and Indicator

The aerial is a small, horizontal dipole, designed to receive the horizontally polarized
signals transmitted from the ground station. Its size is designed with the frequency
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 band of 108MHz – 118 MHz in mind. The aerial must be mounted in such a place that
it offers 360o reception of the radio signals. It must also be shielded from
transmissions from the VHF communication radio aerial. Aerial are frequently
mounted of the fin. The frequency selector knob on the control panel is used to select
the required station. The signal from the aerial is filtered through the high frequency
part of the receiver and only the signals from the desired VOR station are passed
through to the detectors and filters. The receiver compares the reference signal and
variable signals in order to detect the phase difference between the two. The phase
comparator compares the phase of the two signals and the difference is fed to the
indicator. A special circuitry, within the receiver, detects the identification signal and
amplifies it for a speaker or headphones. Some VORs can also transmit „ voice‟,
either radio communication, identification, meteorological – information or other
voice transmissions. The receiver panel has a frequency selector knob, a dial
indicating the selected frequency and a selector switch with a position for IDENT and
voice.

Fig. 4.5 Aircraft antenna

The IDENT position is selected when we want to hear the identification signal of the VOR. It
is very important to check the ident before using the navaid, otherwise you cannot rely on the
displayed navigational information. The ident is transmitted according to ICAO
recommendations and consists of a three letter Morse code transmitted at a rate corresponding
to seven words a minute and the signal shall be repeated at least three times every 30 seconds.
The modulation tone is 1020 Hz.

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 The voice position is selected to improve the reproduction of speech, and it is selected when
the transmission contains voice messages (for instance ATIS), or if the station serve as a
regular voice transmitter. The indicator can be in many different forms, from the simplest to
the most complex, as a part of an electronic flight information system. We will cover the basic
indicator and its parts and functions, because it displays all the basic information provided by
the VOR receiver, and it also forms the basis of the more complex systems.

2. COURSE DEVIATION INDICATOR (CDI)

The indicator which is shown at fig 4.6 consists of three main elements:
 OBS: omni bearing selector (knob, lower left on figure, with associated compass
scale)
 TO/FROM flag
 CDI course Deviation Indicator (vertical white needle on figure)

A warning flag is also a part of the indicator, most commonly an „ OFF‟ Flag.

The OBS is used to select the desired radial in relation to the VOR station. In the model
illustrated the OBS knob is turned to bring the selected bearing on the compass scale to the
top of the indicator. In this example we have selected a bearing of 1800. On another popular
type of indicator the selected bearing is shown digital in a window on the indicator face.

Fig. 4.6 Course deviation indicator

The TO / FROM indicator tells you if the selected bearing will take you to or away from the
VOR station. The CDI indicates your position relative to the selected bearing and it will move
to the left or right according to relative position to the bearing selected. The needle moves
across a scale of dots, each representing a certain number of degrees of deviation. There are

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 indicators with a 5 dot scale (to each side) or, less commonly, some with a four dot scale.
Deflection to the last dot (left or right) represents 10o displacement from the bearing selected
(full scale deflection).

The vertical needle may deflect further out than to the last dot, and will then represent more
than 100 displacement. On a five – dot scale each dot represents 20 of deviation while, on the 2
– dot scale, each dot represents 50 .

Fig 4.7

The warning flag appears when no signal is received or when the one received is too weak.
Most common is a flag with the text „OFF‟ or „NAV‟. The warning flag circuit also monitors
the receiver itself and will appear if the receiver or indicator malfunctioning. Another way of
putting it: The OFF (NAV) flag will show until an acceptable VOR signal is received and
processed by the receiver. The OFF (NAV) flag will then disappear. If the warning flag is
appearing, the indications are not to be trusted, even if a valid identification signal is being
received.

The indications on the CDI are totally independent of aircraft heading. It displays the aircraft
position in relation to the bearing selected. When the OBS is turned to center the CDI needle,
with the FROM flag showing, the number indicated on the top of the compass scale is the
radial on which the aircraft is situated.

The radial can be plotted on an aeronautical chart, thus giving you a line of position. If the
VOR had a co-located DME, the line of position and the distance would give you a fix
position on the chart.

Imagine an aircraft east of the VOR station, on radial 0900 . If we turn the OBS so that the card
rotates 360o, the needle will be centered twice, once when the corresponding radial is across
the index, and also when the reciprocal of the radial 2700 is across the index. When the OBS

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 is set to 2700 the „TO – flag‟ is visible, indicating that a magnetic track of 2700 will take you
to the VOR station. Setting the OBS to 0900 will center the needle and the „FROM – flag‟
appear, indicating that the aircraft is on the 0900 radial.

In order to have the indications of the CDI indicating the correct direction to turn in order to
regain the selected track, there has to be a general agreement between aircraft heading and
track selected. If flying towards the VOR with a FROM indication, the CDI needle will
indicate in the opposite sense to the actual situation,(see fig. 4.8)

Fig. 4.8 How to interpret the CDI

The TO / FROM indicator divides the area around the VOR in two main halves; a TO sector
and a FROM sector. A third sector, a „change over‟ sector, separates the two. The from
indicator will appear when, considering the current radial on which the aircraft lies, the
selected bearing is extending „FROM‟ the beacon. The indication TO, when the selected
bearing reciprocal is extending „Towards‟ the beacon. The indication will change from „TO‟
to “FROM” when a radial, perpendicular to the selected bearing, is crossed. At this time the
OFF or NAV flag will most likely momentarily appear as we cross within the cone of
confusion, (see fig. 4.9).

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 Fig. 4.9 To/From sectors

4.3.3 HORIZONTAL SITUATION INDICATOR (HIS)

A more modern derivative of the CDI. This instrument is widely used and you should be
familiar with its presentation and interpretation.

As the name suggests, the HSI, (see fig. 4.10), provides the pilot with a pictorial presentation
of the aeroplane‟s navigational situation in relation to a selected track as defined by a VOR
radial (or ILS localizer beam). It also displays glide slope information, a heading reference
and, on many units, a DME range indication.

The instrument consists of a number of discrete elements as follows:

i) HORITAL SITUATION INDICATOR (HSI)

Provides a pictorial presentation of aircraft deviation relative to VOR radials or
localizer beams. It also displays glide slope deviation and gives heading reference with
respect to magnetic north.

ii) NAV WARNING FLAG

Flag is in view when the NAV receiver signal is adequate. When a NAV flag is
present in the navigation indicator the autopilot operation is not affected. The pilot
must monitor the navigation indicators for NAV flags to insure that the Autopilot and /
or flight Director are tracking valid navigation information.

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 Fig. 4.10 HIS in fail mode and HIS showing VOR indications

iii) LUBBER LINE

Indicates aircraft compass heading on compass card.

iv) COMPASS WARNING FLAG (HDG)

When flag is in view, the heading display is invalid. If a HDG flag appears and a
lateral mode (HDG, NAV, APR or APR BC) is selected, the Autopilot will be
disengaged. The Autopilot may be reengaged in the basic wings level mode along with
any vertical mode (BC = Back course).

v) COURSE SELECT POINTER

Indicates selected VOR bearing or localizer track on compass card. The selected VOR
radial or localizer track remains unchanged in the compass card if the compass card
rotates.

vi) TO / FROM INDICATOR FLAG

Indicates direction of VOR station relative to selected track.

vii) HEADING SELECT KNOB

Positions headings bug on compass card. The bug rotates with the compass card.

viii) COMPASS CARD

Rotates to display heading of airplane with reference to lubber line on HIS.

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 ix COURSE SELECT KNOB
Positions course bearing pointer on the compass card by rotating the bearing selector
knob.

x) VOR & LOCALIZR DEVIATION BAR (D-BAR)

The center portion of bearing select pointer moves laterally to pictorially indicate the
relationship of aircraft to the selected bearing. It indicates degrees of angular
displacement from VOR radials and localizer beams, or displacement in nautical miles
from RNAV track.

xi) COURSE DEVIATION SCALE

A track deviation bar displacement of 5 dots represent full scale (VOR= + 100, LOC = ±
2 o ,) RNAV – varies according to equipment.

xii) HEADING SELECT KNOB

Heading bug to be moved by knob to select desired heading. Used to indicate a
heading to be memorized.

4.3.4 RADIO MAGNETIC INDICATOR (RMI)

The radio magnetic indicator (the RMI) combines the information from the radio navigation
instruments with the directional information on from the directional gyro. The RMI has two
needles, which can indicate both ADF and VOR information.

Fig 4.11 RMI

The two needles are usually marked with single and double lies to make it easier for the pilot
to identify the stations.

There are two small buttons at the lower part of the instrument. These enable the pilot to
select either VOR or ADF as the information displayed by the needles. The RMI card is
slaved to the directionally gyro, so that the heading of the aircraft can be read directly. In this

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 way, the needles will show the bearing to the ground stations continuously. When tuned to a
VOR, the tail of the needle indicates the actual VOR radial. In our example, the signal needle
points to a VOR station, indicating that the aircraft is on radial 0440 and the magnetic tracks
to the VOR is 2240 .The double needle is connected to the ADF and indicates bearing to an
NDB. The compass bearing to the NDB is indicated to be 3140.

The present aircraft magnetic heading is indicated on the top inside the index mark, and is
3600. Without observing the exact numerical values, the pilot will at a glance see that he has
to turn 450 left in order to fly towards the NDB, and turn 135o left to fly toward the VOR
tuned in. You should be aware that the bearing registered from the ADF is showing magnetic
meridian passing through the airplane.

4.4 VOR NAVIGATION

The V OR is a very versatile navigational aid, which forms the basis of the Airway routes
structure. It can be used to assist VFR pilots, as the main navigational aid for enroute
navigation, a holding aid or also as an approach to landing aid. VOR also plays a part in Area
Navigation systems (RNAV).

Let us have a look at the different ways of using the VOR, but first we point out a few
important things that we have to do before using the information indicated by the instrument.

Always make sure you are within the designated operational coverage area (DOC) of the
VOR stations you plan to use. This can be done by checking the AIP (Aeronautical
information published enoute manuals.
After having turned the receiver on, dial the frequency of the aid, then listen to the
identification signal to make sure you are receiving the correct and desired station and that it
is “on the air”.

Make sure that the warning flag (NAV or OFF) is not visible, indicating that a satisfactory
signal is being received and that the aircraft installation is working properly.

1. ESTABLISHING POSITION
Using the VOR to find our present position. We need either a VOR in combination with
DME, or we can use two VOR stations. By turning the OBS to center the needle with a
FROM indication, we determine the radials on which the aircraft is located. This procedure
gives us two crossing position lines, good enough to determine a fix position.

2. TRACKING A RADIAL INBOUND FROM A PRESENT POSITION

If we want to fly towards a VOR station from our present position, all we have to do is to turn
the OBS to center the CDI needle with a TO indication and fly a heading equal to the
indicated value in the selected bearing window. The inbound track will be the reciprocal of
the radial on which you are positioned. In a no wind condition, a heading equal to the inbound
track will take you to the VOR with the CDI needle centered.

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 Fig. 4.12 illustrates an examples of tracking a track of 080o, joining on radial 260o.

Fig. 4.12 Tracking the VOR

If wind is present, we have to make heading corrections in order to keep the CDI needle
centered. Make a small heading correction and, if the needle drifts to one side, turn towards
the needle, since the needle actually indicates the position of the desired track. Use only small
changes in heading at a time and wait for the needle to move back to center position. This
procedure of changing the heading to stabilize the needle in center is called “bracketing”.

Use small changes of heading and keep the new heading for a while to await needle
movement. If we see that the needle remains still at a position off center, we have found the
correct WCA, but we need to correct a bit more in order to get back on track.

3. INTERCEPTING A RADIAL
When you are planning to intercept and follow a specific radial, you first have to determine
your position in relation to the desired track. If you are supposed to track a radial outbound,
set the CDI to desired radial. CDI deflection will now tell you which way to turn in order to
make an intercept. The intercept angle will depend on different factors. If ATC wants you to
join the new track as soon as possible, you can make an initial intercept of up to 90 0 and,
when the CDI starts to move, you need to start leading the turn to establish on the new radial.
If you are close to the VOR station, the needle will move quite fast. Conversely, if you are far
from the station the needle will move more slowly. Aircraft speed will also affect the needle
movement.
If there are no restrictions regarding the intercept, an intercept angle of 30 0 or 450 is normally
a good alternative. Your flight instructors will teach you more about this during practical
navigation training.

If you are tracking TO a VOR station and you are to continue on the same course after you
have passed the station, you will see that when getting close to the station, needle movement
becomes very erratic. The TO/ FROM flags will flicker during the passage of the station and
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 the warning flag (NAV/OFF) will appear momentarily. This is due to the so- called “cone of
confusion” which is directly overhead the VOR.

4 VOR APPROACH
A VOR approach may be a published procedure at an airport. An example of such a procedure
is illustrated at fig. 4.13. This procedure is for illustration only and must not be used in
conducting an actual approach.

4.5 LIMITATIONS AND ACCURARY

Since the VOR operates in the VHF band, the range is “line of sight” but it will be limited by
factors such as terrain and output power.
The formula for range of a line of sight transmission is:

Range (NM) = 1.25 HT x + 1.25 HTRx

When HT is the elevation of the transmitter in ft and HRx is the height of receiver in feet.
The following example indicates the maximum range of a VOR if the aircraft is at 22,500ft
and the VOR transmitter is at 400ft.

Range = 1.25 HTx + 1.25 HTr NM

= 1.25 400 + 1.25 22500 NM

= 25 + 187.5 = 212.5 NM
This formula result in a maximum range for an en-route VOR approximately 200 NM for an
aircraft flying at 22,500ft, but it must be noted that the higher the aircraft altitude, the more
subject to interference is the reception of the signals. A geographically distant VOR, which is
close with regard to frequency, can cause interference. VOR installations will have a
maximum range and altitude published in the AIP and other documentation. If used within
these limits, the transmissions are protected from harmful interference from the other known
transmissions on the same or near frequencies. This is sometime known as a Designated
Operational Coverage (DOC).

The accuracy of the system depends on four main elements:

 SITE ERROR, which is caused by terrain or obstacles in the immediate vicinity of

the VOR transmitter. These errors are called VOR bearing displacement errors and
they are limited to ± 1o. Since the ground stations are monitored, the transmitter will
be shut down if the error exceeds this limit.

 PROPAGATION ERROR, which is caused by terrain or other obstructions as the

signal travels from the transmitter to the receiver. Site error and propagation error
shall not exceed ± 30 together. We call them ground errors.

 AIRBORNE EQUIPMENT ERROR, is caused by tolerances within the receiver

and indicator. They are typically ± 10 to ± 30, depending on installation.

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 A FOURTH ERROR, induced by the pilot operating the system, called pilotage
error, is typically said to be ± 2.50

All in all, this results in an average total system error of about ±50. This means that if the
indications say that you are on radial 090o, you can be radial 085o or 095o radials.

Note
The geographical error resulting from this, increases the further you are from the VOR.

4.6 VOR Test Transmitter (VOT)

At some airports they have installed VOR test transmitters, referred to as VOT. These enable
pilots to check their airborne equipment. The test can be conducted at any position on the
aerodrome and the procedure is to tune the published frequency, turn the OBS to center the
CDI needle. The needle shall indicates 1800 with a TO flag, 360 with a FROM flag. If the
indications are not within ±40, the aircraft installation should be repaired.

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 Intentionally left blank

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5. INSTRUMENT LANDING SYSTEM (ILS)
5.1 PRINCIPLE OF OPERATION
The instrument handing system is the primary precision approach facility for civil aviation, a
precision approach being one in which both glideslope and track guidance are provided. The
ILS signals are transmitted continuously and provide pilot interpreted approach guidance.
When flying the ILS approach, the pilot descends with approach guidance to the decision
height (DH), at which point he takes the final decision to land or go around. Any installation
must conform to the standards laid down in ICAO Annex 10 and an appropriate performance
category will be allocated to it. Any exception to these standards will be published in
NOTAMs. Fig 5.1 shows a typical ILS system.

Fig. 5.1 ILS System

The ILS consists of three main components: localizer, glide path and maker beacons. Fig. 5.2
shows a typical ILS installation.

Fig 5.2 A typical ILS Installation

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 The localizer transmitter supplies approach guidance in azimuth along the extended runway
center line. The glide path transmitter provides approach guidance in the vertical plane.
Marker beacons provide accurate range fixes along the approach. Many ILS installation use
an associated DME to provide a more accurate and continuous ranging facility than that
provided by the markers. ILS installations may also be complemented with a low power
NDB, known as locator beacon, the function of which is to provide guidance, during
intermediate approach, into the final approach path, which is marked by the ILS. The ideal
flight path on an ILS approach is where the localizer plane and the glide slope plane intersect.
To fly this flight path, the pilot follows the ILS cockpit indications.

5.2 LOCALIZER
The localizer provides directional guidance along the extended center line of the landing
runway. It transmits on a frequency between 108.10 and 111.95 MHz, in the VHF band, thus
sharing this band with terminal VORs. Localizers transmit on frequencies with ODD first
decimals only. Therefore, 108.30 MHz would be a localizer frequency, whereas 108.40 MHz
would not. The localizer transmitter aerial is located in line with the runway center line, at a
distance of approximately 300 m from the “up – wind‟ end of the runway. Fig 5.3 shows a
typical localizer aerial.

Fig. 5.3 Localizer aerial

This aerial may be 20m wide and 3m high, and consists of a number of dipole and reflector
elements. The radio signal transmitted by the localizer aerial, (see fig. 5.4), produces a
composite field patter consisting of two overlapping lobes. The two lobes are transmitted on a
single ILS frequency and, in order to make the receiver distinguish between them, they are
modulated differently. The lobe on the left – hand side is modulated by a 90Hz tone and the
sector formed by it is called the YELLOW sector. The lobe on the right hand side as seen by
the Pilot making an approach is modulated by a 150Hz tone and the sector it forms is called
the BLUE sector.

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 Fig. 5.4 Modulation Pattern - localizer

A receiver located to the left of the center line will detect more of the 90Hz modulation tone
and relatively less of the 150Hz modulation. This difference is called DDM (Difference in
Depth of Modulation) and it causes the vertical indicator needle to indicate that a correction to
the right is necessary. Conversely, a receiver right of the center line receives more of 150Hz
than 90Hz modulation and therefore, the needle will indicate that a correction to the left is
necessary. The line along which the DDM is zero, defines the localizer center line. When
flying along this line, there will be no deflection of the needle, indicating that the aircraft is on
the center line.

On each side of this line the DDM increases in a linear fashion. The localizer coverage should
provide adequate signals to distances of 25NM within 100 on either side of the center line.
Further coverage must be provided to distances of 17NM between 100 and 30 on either side of
the center line.

Finally, coverage must be provided to distances of 10NM at angles greater than 350 from the
center line, for those installations in which all round coverage is provided. Where
topographical features dictate or operational requirements permit, the limit may be reduced to
18NM within the ± 100 sector, and within 10NM within the remaining coverage. Fig. 5.5
shows the required localizer horizontal coverage.

Fig. 5.5 Required localizer horizontal coverage

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5.3 GLIDE PATH
The glide path transmitting aerial is usually placed about 300 m upwind from the threshold
and 150 m from the center line. The transmitter aerial is placed 300 m up wind from threshold
because this is the optimum touch down point at which the extension of the glide path
intersects the runway. This ensures adequate wheel clearance over the threshold and over any
other object or terrain during landing approach.

Glide path transmission takes place in the UHF band on 40 spot frequencies from 329.15 to
335 MHz. UHF are used to produce more accurate beams. The transmission is beamed in the
vertical plane in two lobes similar to the localizer transmission. The upper lobe has a 90Hz
modulation, while the lower lobe has a 150 Hz modulation.

The DDM (Difference in Depth of Modulation) will energize, the horizontal needle of the
instrument, so as to indicate whether the aircraft is in the 90Hz lobe or in the 150Hz lobe. In
this way, it gives the position of the center line of the glide path. The lines, along which the
two modulations are equal in depth, define the center line of the glide path. It is generally 30
from the horizontal, but it could be adjusted to between 20 and 40 to suit the particular local
conditions. A glide slope much in excess of 30 requires a high rate of descent for the normal
turbo jet aeroplane. The sitting of the glide path aerial and the choice of the glide path angle
are dependent upon many interrelated factors:

 Acceptable rate of descent and approach speeds for aircraft using the airfield.
 Position of obstacle and obstacle clearance limits resulting there from
 Horizontal coverage
 Technical sitting problems
 The desirability of attaining the ILS reference datum 50 ft above the threshold on
the center line
 Runway length

5.3.1 GLIDE PATH COVERAGE

The coverage in azimuth extends 80 on either side of the localizer center line, to a distance of
10NM the converge in the vertical plane extends from 0.45 times the nominal glide path angle
(Ө) to 1.75 times the nominal glide path angle above the surface. Remember that correct
signals are guaranteed only within the approved coverage zones and you can never trust
signals received outside these zones, see fig. 5.6.

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 Fig 5.6 GP coverage

5.3.2 CRITICAL AND SENSITIVE AREAS

The ILS critical area is an area of defined dimensions about the localizer and glide path
antennae, where vehicles, including aircraft, are excluded during all ILS operations,
(see fig. 5.7). The area is protected to prevent aircraft or vehicles causing unacceptable
disturbances to the signal –in space.

The ILS sensitive area extends beyond the critical area, where movement or parking of
aircraft and vehicles is controlled to prevent interference to the ILS signals.

Fig 5.7 ILS critical/sensitive areas

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5.3.3 MARKER BEACONS
The purpose of the markers is to provide range information while on the approach.
They transmit an almost vertical beam. Almost all installations are equipped with
an outer marker and a middle marker. Category 2 or 3 ILS may be equipped with an inner
marker as well. Audio and visual signals in the cockpit will indicate when the aircraft is
passing overhead.
All marker beacons transmit on the same frequency 75 MHz and thus no
frequency selections are necessary for the pilot. In many installations, marker beacons
are being replaced or supplemented by the use of a DME paired with the localizer.

PRINCIPLE OF OPERATION
Marker beacons are radio beacons transmitting their power vertically towards the sky, all
transmitting on the same frequency, 75MHz in the VHF band. The polar diagram of the
transmitted signal is a vertical fan or funnel – shaped lobe. Quit opposite to the NDB, the
marker beacon can only be received when directly overhead it.

Therefore the marker beacon cannot be used as navaid to track to. The carrier wave on 75
MHz is amplitude modulated, A2A emission, with different audio frequencies in order to
distinguish between the different types of markers. There are four types of markers:
 Airway marker (fan marker)
 Outer marker
 Middle marker
 Inner marker

The airway marker is, as the name indicates, used while route flying along airways.
It is used:

 To identify certain fixes along routes where there are no other means of establishing fixes.

 It can be used over mountainous areas where it is difficult or impossible to receive other
navaids than the one being tracked.

 To supplement an NDB providing vertical cover above the “cone of silence”.

Outer, Middle and Inner markers are parts of the ILS installation and are used when
conducting an ILS approach to an airport.

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 AIRBORNE EQUIPMENT
Since all marker beacons transmit on the same frequency, there is no need for a frequency
selector in the receiver. Besides the receiver unit, the airborne equipment consists of three
coloured lamps and a sensitivity switch, (see fig. 5.8). Audio – output is channeled through
the audio panel of the nav/com installation of the aircraft.

Fig. 5.8 Maker Receiver Control and Indicator

The passage of the different markers is indicated as follows:

(i) AIR WAY MARKER /INNER MARKER

If the frequency of the modulating signal is 3000Hz, the white indicator lamp will
flash, a high pitched 3000 Hz tone keyed to form dots (“……..”) at a rate of 6per
second will be heard. Airway markers are becoming rare, as is the inner marker of
ILS.

(ii) MIDDLE MARKER

If the frequency of the modulating signal is 1300 Hz, the amber indicator light flashes,
a medium pitched 1300 Hz tone keyed to form alternating dots and dashes (“. - . -. -. “)
will be heard. The rate being 2 dashes per second.

(iii) OUTER MARKER

If the frequency of the modulating signal is 400 Hz, the blue indicator lamp will flash
and a low – pitched 400Hz tone, keyed to form dashes (“---------“) at a rate of 2 per
second will be heard.

GROUND INSTALLAION
Airway markers are gradually being phased out but can be found along airways in order to
establish accurate reporting points. In areas with poor radio – coverage, such as mountains
areas, airway markers can provide a point source fix. With the sensitivity switch set to high;
these markers can be received at 50,000ft.

As mentioned earlier, the outer, middle and inner markers are parts of the system. If fitted,
they are all installed in the approach end of the runway along the extended center line.

ICAO recommends that the outer marker should be located 3.9 NM from threshold. If for
topographical reasons this is not possible, it can be located anywhere between 3.5 and 6NM
from the threshold. It should be installed within 75m of the extended center line of the
runway.

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 The middle marker is recommended to be located 1,050 ±150 m from the runway threshold
and not more than 75m off the extended center line of the runway.

The inner marker is recommended to be located between 75m and 450 m from the runway
threshold and not more than 30m from the extended center line of the runway.

Normally, when on the glide slope, outer marker crossing height will be approximately 1500
to 2000ft. The purpose of the outer marker is to provide height, distance and „equipment
functioning‟ checks aircraft on intermediate or final approach.

The middle marker will be crossed at around 200ft above aerodrome level (AAL), which is
close to decision height for a normal ILS approach. The purpose of the middle marker is to
indicate the imminence, in low visibility conditions, of visual approach guidance.

The purpose of the inner marker is to indicate to the pilot that the threshold is about to be
passed, and the height will be the lowest decision height applicable in Category II operations.

5.3.4 AIRBORNE EQUIPMENT

The ILS airborne equipment consists of a frequency control box, a VHF localizer receiver, a
UHF glide path receiver, a 75 MHz marker beacon receiver and an ILS indicator. Naturally,
three separate aerials are included in the installation – one for each receiver.

Since all the marker beacons transmit on the same frequency, there is no need for a marker
beacon control box. Markers are automatically identified by an audio coded signal, by the
related transmission audio tone and a coloured light.

5.3.5 FREQUENCY PAIRING

Both localizer and glide path tuning are effected from a single control unit. This is possible
because an international agreement under ICAO standards has been reached on the pairing of
frequencies within the ranges allotted to the two parts of the system. This means that every
localizer frequency has a particular glide path frequency paired to it. Since the frequencies are
paired, it is only necessary to select the correct localizer frequency and the glide path receiver
will then be automatically tuned to the appropriate UHF channel. The VHF navigation
receiver panel is used to tune the ILS frequency.

5.3.6 LOCALIZER AND GLIDE PATH RECEIVERS

Once the localizer frequency has been tuned, both the localizer and the glide path receivers
are activated, and they send the received signal to the indicator. The two receivers are similar
to each other in that they both detect the modulations on the carrier wave. The modulations
(90 Hz and 150 Hz) are compared and the Difference in Depth of Modulation (DDM) is
measured. This output, which is in the form of a DC electrical signal, is used to drive the
pointer on the display. If the aeroplane is on the center line, the 90Hz and the 150 Hz signals
will have the same amplitude and the indicator needle will be centered. If the aircraft is not on
the center line, one signal will be stronger than the other, dependant upon the position of the
aircraft and the resultant DC output energizes the needle displacement. To indicate whether
the received signals are adequate or not, a warning system is incorporated into the receivers.
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 A red warning flag appears on the ILS indicators if the sum of the two depths modulation is
not sufficient.

5.3.7 ILS INDICATOR

The indicator consists of a CDI, similar to the simple VOR indicator, but with an additional
needle, as illustrated in fig. 5.10. Localizer signals displace the vertical needle, while glide
path signals displace the horizontal needle. The same indicator is normally used both for ILS
and VOR guidance. A combined ILS/VOR indicator is shown in fig. 5.9, called Horizontal
Situation Indicator (HIS).

Fig. 5.9 HIS in fail mode and HIS showing ILS indications

When the localizer receiver detects that the 150Hz signal is stronger, then a voltage is fed to
the localizer needle that moves it to the left. This indicates that the localizer center line is to
the left of the aircraft on approach. If the 90Hz signal predominates, as in the case of the
illustration, then the voltage fed to the localizer needle moves it to the right indicating that the
pilot has to turn right to get back on center line.

Full – scale deflection (the needle at the outer dot) will occur when the aircraft is displaced
2.50 from the center line. In other words, when tuned to a localizer frequency, the indicator is
four times more sensitive than it is when being tuned to a VOR (full scale deflection for a
VOR corresponds to 100).

Unlike the VOR, which gives the pilot a choice of 360 radials using the omni – bearing
selector (OBS), the localizer course is a single fixed beam. Once a localizer frequency is
selected, all the needle indications will refer exclusively to the localizer center line.
Consequently, the fact that the instrument is fitted with the OBS has absolutely no
significance and rotating it will have no effect on the ILS indications. However, you should
always turn the OBS to the correct inbound course when flying the ILS. The localizer
indicator does not give any heading information. It only gives information regarding the
geographical position of the aircraft. It displays how many degrees the aircraft is displaced
from the localizer center line.

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 The position of the glide path, relative to the aircraft, is indicated by the horizontal needle of
the indicator. The vertical glide path scale on the usual cockpit indicator consists of
5 dots above and below the center position, (see fig. 5.10).

Fig. 5.10 ILS instrument showing a fly up and right indication

If the 90Hz signal is the stronger one, the aircraft is above the glide path and the indicator
needle is deflected down. This indicates that the aircraft must FLY DOWN to capture the
glide path. Conversely, if the receiver detects a stronger 150 Hz signal, the needle will be
made to move up. This is known as a FLY UP indication. The glide path has a total depth of
about one and a half degrees, making the glide path indicator considerably more sensitive than
the localizer indicator. This means that for a full – scale deflection of the needle the aircraft
will be at least 0.70 above or below the glide path. A “half scale” Fly up indications should be
considered to indicate the maximum safe deviation below the glide path. This is
approximately what is shown on fig. 5.10.

NOTE: The center ring is the dot.

Deviation from the glide path is referred to in terms of dots instead of degrees, in that there
are 5 dots above it and 5 dots below it on the instrument. Very accurate control is required
when flying down a glide path. A more sophisticated instrument, used to fly an ILS approach,
is the horizontal situation indicator. With the HSI, the course arrow must be manually aligned
with the localizer inbound course and the deviation bar is used for localizer guidance. A scale
alongside the instrument provides the glide path position. On the HSI there are often only
marking for full scale deflection and half scale deflection on the localizer and the glide path
needle.

5.3.8 ILS ACCURACY

Up till now we have looked at the ILS as an instrument that provides assistance in approaches
to landing. This means that the ILS provides guidance down to a specified height above the
threshold. If the visibility at this point is good enough for landing, then the pilot may legally
land the aircraft. It is clear that if the existing weather does not permit the pilot to see the
visual references at the prescribed minima, the aircraft cannot land.

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5.3.9 FALSE BEAMS
Even if all the ILS ground equipment is strictly monitored, there are unavoidable factors to
consider. The first of these is the false signals. This problem is particularly associated with the
glide path transmission, and it consists in the reflection of the 90Hz and 150 Hz lobes that
produce false glide paths. This reflection is due to the height of the aerial above the reflecting
surface of the ground and the aerial‟s propagation characteristics. The number of such false
glide paths procedure at any ILS site depends on several factors, such as the design of the
aerial, transmission power, obstacle and other such factors. These false glide paths occur at
multiples of the nominal glide path and thus the first occurs at approximately 60 above the
horizontal for a glide path of 3o. There will never be any false glide path below the true one.
For this reason it is recommended practice, when intending to carry out an ILS approach, to
lock onto the localizer first and then intercept the glide path from below, (see fig. 5.11)

Fig 5.11 Simplified diagram of glide path radiation pattern

Outside the localizer „protected area‟, it is possible to encounter false localizer beams. The
angle from the actual center line to the false beams will vary with the number of aerial
elements. Six elements produce a false beam at approximately 40° and 12 elements at 500 to
600 .

5.3.10 LOCALIZER BACK BEAM (BACK COURSE)

Some localizers also transmit in the opposite direction of the ILS inbound course and the
signal can be received when flying behind the aerial. This signal is call the back beam and
should normally not be used, (see fig 5.12).

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 Fig 5.12 ILS Localizer Beams

5.3.10 ILS CATEGORIES

A system of facilities performance categories has been established to defined the capability of
a particular ILS system. These categories state that the ILS must be capable of providing
guidance from the coverage limit and as follows, for:

 Category I – to a height of 60 m above the horizontal plane containing the threshold.

 Category II – to a height of 15m above the horizontal plane containing the threshold.

 Category III – with the aid of ancillary equipment when necessary, down to and along
the runway.

Similar categorization exists for operational purposes, that is, to establish practical weather
minima for an approach. As a pilot you must be familiar with the following ILS operational
minima, (see table 5.1).

Table 5.1

In a category J ILS approach, also called “CATI”, the pilot may manually follow the ILS
indications down to the decision height (DH), which is not less than 200 ft. At that point, if
visual contact has been established, the landing can be made. If not a go around has to be
initiated. Note that the ILS coverage, which is described earlier in this chapter, refers to ILS
category I. Cat II and III requirements are more stringent. ILS CAT I, although still widely
used, is gradually being replaced by CAT II & CAT III facilities. On a CAT II approach, the
aircraft must be flown by the autopilot down to the DH. From there, if visual contact has been
made, the pilot can make the landing. Otherwise, a go-around must be initiated. A CAT II
approach can only be made at an airport that is category II certified.

5.3.12 PROTECTION RANGE AND MONITORING

National and regional frequency plans have been established by the ICAO and are adhered to
by contracting states. These plans take many factors into account, such as the sensitivity and
selectivity of receivers, the channel spacing and the geographical proximity of transmitters. In
this way, interference between facilities is reduced to negligible proportions.
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 Within Europe, the congested radio frequencies have resulted in FM transmissions from
aerials that are close enough to allow side band interference to spill over into the ILS
frequencies. These can cause random displacement of the localizer, so be aware! Monitoring
equipment automatically and continuously checks both localizer and glide path transmitters.

Whenever a shift or change in the basic transmission is sensed the monitors will take action. If
the ILS is category II or III the transmissions must be stopped within 2 seconds. If category I,
the transmissions will be stopped within 6 seconds.

5.4 USE OF ILS

5.4.1 ILS INDENTIFICATION
Since the localizer and glide path frequencies are paired, selecting a localizer frequency
automatically activates the glide path receiver so that the corresponding glide path frequency
is automatically tuned.

As with most of the radio aids, the ILS must be identified before we can use it. The IDENT is
transmitted on the localizer frequency. The localizer carrier is amplitude modulated by a 1020
Hz tone to give the identification.

The IDENT itself is a two or three letter morse code transmitted at a rate of seven words per
minute. The letter “I” may precede the IDENT. Voice identification is also in use. The glide
path transmitter has no identification

5.4.2 FLYING THE LOCALIZER

When initiating the approach, remember that the localizer indicator shows only the position of
the aircraft in relation to be center line and that no heading information is provided. Thus the
term “follow the needle” is only valid when flying inbound within the coverage area.

For an aircraft on approach, the localizer needle indicates which way the aeroplane should
move to regain the center line. If the localizer needle is to the right, then the aircraft should be
moved to the right. To regain the center line, fly towards the needle.

The aim is to fly a heading that will maintain the aircraft on the center line. If a crosswind
exists, a wind correction angle (WCA) will be required and the aircraft heading will differ
slightly from the published inbound course.
The localizer beam narrows as the runway is approached. Therefore, corrections should
become smaller and smaller.

5.4.3 FLYING THE GLIDE PAATH

The horizontal glide path should be flown in the same way as the localizer needle.
To regain the glide path just fly towards the needle as your glide path. You have to think of
the needle as your glide path. If the glide path needle is below the center, you are too high; a

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 steeper descent must be initiated. Remember that as you get closer to threshold hi high
descent rates are dangerous.

With an angular depth of only 0.70 above and 0.70 below, the glide path needle is three times
more sensitive than the localizer is and 15times more sensitive than the VOR. When
following a glide path, the rate of descent is your reference, so the vertical speed indicator
becomes important. The vertical speed should be determined before starting the decent on the
glide path.

The rate of (descent in feet per minute) may be calculated as follows:

ROD (in feet per minutes) = Tan Ө x GS x 6080

60

Or ROD (in feet per minute) = Ө x GS x 6080 (1:60, less accurate)

60 60
And Height (ft) from touch down = Tan Ө x distance from touch down (nm) x 6080)

Or Height (ft) from touch down (or threshold) = Ө distance (nm) x 6080 (+ 50 ft)
60
N.B: if distance from the threshold is given, then add 50ft to height.

Where ROD = Rate of descent

Ө = Glideslope Angle
GS = Groundspeed.

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6 MICROWAVE LANDING SYSTEM (MLS)

6.1 INTRODUCTION
The ILS has served as the primary precision approach and landing aid for the last decades.

Since the mid – 60s, the limitations of the system have prompted ICAO to look for a system
that should replace the ILS, and fulfill the developing needs for future aviation. The most
pronounced limitations of the ILS – system are:

 Procedural, ILS limits aircraft to long straight in final approaches, at least 7 miles,
creating potential airspace conflicts in multi airport environments and constraining the
number of approach paths that can be provided. Each ILS provides only one approach
path.

 Part of the ILS guidance signal is formed by a direct and ground reflected signal
requiring a significant level of site preparation. ILS can only be installed at locations
where preparation is practical.

 ILS is limited to 40 frequency channels constraining the number of sites that can be
allocated a frequency in a given geographical area. In addition, the ILS frequency
band suffers from interference from high power FM transmitters operating in adjacent
bands. Aircraft receivers are equipped with FM filters, which narrow the band of
reception and reduce this „noise‟ interference but it is still a limitation.

 ILS is sensitive to signal diffraction and blockages caused by ground traffic,

necessitating the use of large protected areas on the airport surface. Within these areas
the ground movement of vehicles and aeroplanes must be prohibited. This reduces the
effective capacity of the airfield when low visibility operations are being conducted.

 These limitations called for a new system, and the development of a microwave
landing system began. Parallel to the development of MLS, the civilian use of satellite
based „GPS – Global positioning system‟ was also under development, both as en –
rout navigation aids and, with augmentation systems, as an approach aid. By the time
MLS was fully developed and tested, the development of GPS was so advanced that,
in some countries, further development and installation of MLS was abandoned, in
favour of GPS. In practice, this chapter describes a system that you may not often
encounter in your career as a pilot.

The MLS system is a precision approach system that provides the plot with highly accurate
azimuth, elevation information. It also utilizes a precision DME (DME/P) which provides
highly accurate ranging information. The system is also capable of transmitting other types of

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 information to the aircraft such as station identification, system status, runway information
and weather.

It operates on 200 channels on frequencies between 5.03 and 5.09 GHz. It is a completely
digital system, and is less influenced by weather and other common sources of disturbances.
The system allows for several approach paths, both in azimuth and elevation. As with visual
approaches, MLS lets the air traffic controller clear the aircraft for curved approach paths,
with a straight – in final segment being as short as 1.5 NM. This will lead to a significant
reduction in air traffic delays.

6.2 GROUND INSTALLATION

The ground installation consists of the following three main elements:
 Azimuth (AZ), see 6.1
 Elevation (EL), see fig. 6.2
 Precision DME (DME/ P), see fig 6.1

Fig. 6.1 Azimuth and DME-P antenna

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 Fig. 6.2 Elevation antenna

Some installations calls also have back Azimuth (BAZ), flare element.

The azimuth (AZ) part of the installation can be compared with the localizer of the ILS but it
provides a much wider area of proportional nav information; up to 400 on each side of the
extended center line. The AZ is provided out to 20NM while the BAZ is provided to 5NM
(ICAO minimum),(see fig 6.3).

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 Fig. 6.3 Time reference scanning beams

The elevation (EL) part can be compared with the glide path of the ILS but the main
difference being that the pilot himself can choose the desired glide path angle; up to 15 0 . the
coverage is as for the AZ, out to 20 NM, and in the horizontal plane also like the AZ. The
transponder serving the DME/ P (precision DME) can also be used as a regular DME; it
operates on the same frequencies, but provides more accurate information. It is normally
placed together with the AZ part, and provides accuracy at ± 30m in final “approach mode”.
The DME/P normally transmits equally in all directions.

The back azimuth (BAZ) is installed to provide navigational guidance for precision departures
and for missed approach procedures.

In practical installations, the coverage in the horizontal plane can vary according to local
conditions and needs, and it does not have to be symmetrical on each side of the center line.

6.2.1 SIGNAL TRANSMISSION FORMAT

The AZ and EL elements transmit on the same frequency while the DME uses a paired
channel in the UHF band. The format of the digital signal is very flexible and the information
from the different elements can be sent in any desired order. Each group is started by a
preamble, which tells the processor in the receiver which functions are being sent. As soon as
one group has been decoded, the processor is ready and waiting for the next element.

There are two types of signals sent; basic data and auxiliary data. Basic data are associated
directly to the operation of the landing guidance system. Station identification is a part of the
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 basic data. Auxiliary data is other data used for siting information not directly related to the
guidance system.

6.2.2 ANGUALAR MEASUREMENT IN AZIMUTH AND ELEVATION

The aerial transmitting the AZ beam, form a vertical narrow fan shaped beam, which is
scanned from one side to the other and back at a constant angular velocity. First, the
preambles for AZ is transmitted, then the “TO” scan starts, when the beam reaches the other
limit, there is a short delay before the “FRO” scan starts. The time that elapses between the
passage of the “TO” scan and the “FRO” scan at the aircraft position is directly proportional
to the angular position of the aircraft. This principle is called “Time Reference Scan Bean”
system, or TRSB all elements operate in sequence because they are all using the same
frequency. The elevation part, EL, works exactly in the same way, except for the scanning
beam moving in the vertical plane, up then down. Vertical position is calculated exactly in the
same way as the horizontal. Normally the horizontal AZ – scan is repeated 13 times per
second, while the vertical EL scan is repeated 39 times per second. This is because the control
system of the aircraft is more sensitive to changes in the elevation.

SITE ERRORS
Site errors are being prevented by interruptions of the beam when passing a reflecting
object.

6.3 AIRBORNE EQUIPMENT

The aircraft receiver measures the time between the passing of the “TO” and “FRO” scans
of both the AZ and the EL elements.

From these times both azimuth and elevation angles can be determined and, when coupled
with a range measurement from the DEM/P, a three dimensional aircraft position can be
determined.

In its simplest form this position can be compared with a planned approach path and , if not
on that path, can be used to create an error signal, which can be used to drive the conventional
ILS indicator to show displacement from the selected azimuth and glide path approach.

The conventional ILS indicator is used since it is also required for conventional ILS
approaches. The indicator is, therefore, multi –mode.

More sophisticated, computerized systems would allow the full potential of MLS to be
realized, making it possible to follow curved and segmented approaches.

If the DME/P is not available, the system still provides an ILS look alike approach.

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 RADAR PRINCIPLES

7.1 Pulse Techniques and Associated Terms

For you to be able to understand how the various types of radar operate you will need to
refresh and expand some fundamentals and expressions related to radar. Some of these are
common to other electronic equipment in the aircraft, others will more specifically concern
the radar. You should review the following expressions.

Frequency f (Hz)
Wavelength  (m)
Speed of light c (m/s)
Time intervals –(microseconds) s (s)
Pulsewidth PW (s)
Pulse interval PI (s)
Pulse repetition freq./rate PRF/PRR (Hz)
Radar mile (NM) 12.35 (nm)

The components of a RADAR Unit will consist of:

 A transmitter
 A TR switch
 An aerial
 A receiver (and possible a receiver aerial)
 A timebase
 A display

We will take a look at each of those elements in turn – in simple terms.

The components of a typical radar set are shown in Fig. 7.1. This type of radar is called
primary radar. In a primary radar all radio frequency energy is produced at the radar site and
the target (aircraft) is passive; it is just randomly reflecting the energy in the radar signal
hitting it.

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 Fig. 7.1. Components of a typical radar set

The master timer initiates the process by sending a trigger signal simultaneously to the Time
base generator and the Modulator. The modulator forms a high energy pulse, having the pulse
length required. This pulse is fed directly to the oscillator which produces high RF energy for
the duration of the pulse. The RF pulse is fed from oscillator to the directional aerial. The
Transmit/Receive switch (T/R switch) is operated automatically by the transmitter signal, in
such a way that the receiver signal, is completely isolated from the transited signal while it is
being transmitted, but connected directly to the aerial at all other times. A waveguide is used
to connect the T/R switch and the aerial.

When the reflected signal is received by the aerial it is fed to the receiver and amplified. After
amplification and detection the receiver signal will be a pulse having shape similar to the
pulse originally transmitted. The pulse representing the received signal is fed from the
receiver to the display unit. At this time the display unit has already been fed information on
the time the pulse has been sent from the oscillator to the aerial, and a time base has started.
The received pulse is now shown on this time base, and the distance from the radar unit to the
target may be calculated by using the time from the time base and c, the speed of propagation.

7.1.1. Continuous Wave versus Pulse Modulation

Some applications, for example the Radio Altimeter, use a continuous wave transmission
instead of pulses. This method is preferred where the range to be measured is very shot. In the
radio altimeter the radar is measuring height above ground level during the final stages of a
precision approach; thus the range being measured is in terms of feet rather than miles.
However, separate transmit and receive aerials are required in this type of system.

7.1.2 The Display

In simple radar systems, the timer and display is a single unit known as a „plan position
indicator‟ or PPI. In many modern applications however, the information from this timer is

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 processed and sent, with other information, to a video display. This is particularly evident in
modern Air Traffic Control Systems.

7.1.3 The Transmitter

The purpose of the transmitter is to produce a short pulse of high RF power. The duration of
the pulse may be a few micro seconds (ms) and the power several megawatts. The modulator
gives sharp and DC power to the pulse, the magnetron transforms this power into RF energy
at the frequency the set is going to operate on.

Choice of frequency (wavelength) is governed by a number of factors as follows:

Attenuation
If the wavelength is less than 10cm attenuation due to intervening weather is very high, so
return signals will be weak.

Target Size
The relative size of desired target must be considered. Smaller targets will require shorter
wavelength in order to create reflection of energy.

Aerial Size
If space is limited and aerial sizes are restricted, shorter wavelengths will be used which will
give narrower beams. The following frequencies are commonly used:

1000 MHz,  = 33 cm long range surveillance

3000 MHz,  = 10 cm surveillance Radar & Approach Radar
10,000 MHz,  = 3 cm Approach Radar

Pulse Length/Width
The energy content of a pulse is increased if the number of cycles transmitted during the pulse
is increased.

The modulator creates pulses of the desired length and at the desired rate see fig 7.2

Fig. 7.2 Pulses sent per second = Pulse repetition frequency

Factors affecting the choice of pulse length include:

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 Minimum Detection Range
Since the aerial is common to both transmitter and receiver, it is important to protect the
sensitive receiver from the high power pulse. The receiver is therefore „disconnected‟ from
the aerial during the transmission of the pulse (and for a short interval after)

Because any returning reflected energy is swamped by the pulse transmitted no returned pulse
can be detected while the transmitter is operating. Thus, the pulse width, or length, determines
the minimum range that can be detected by the radar.

In practice this is not a problem for navigation radars because pulse widths are in the order of
2 or 3 microseconds (s) (300 to 450 m in range).

In theory, a target that is at least half-a-pulse-width away from the antenna will be detected at
the receiver.

PRF and PI
The PRF and PI govern the maximum range of radar, (see fig 7.2) to avoid ambiguities, the
radar waits for a pulse to go out and back from a target at the maximum range before the next
pulse is transmitted. For example, given a PRF of 500 Hz and ignoring pulse width and fly-
back, the PI is:

1/500s = 2000 s

At the speed of radio waves (300 000 km/s) the distance traveled during the PI is 600 km, and
therefore the maximum theoretical range (MTR) is 300 km (there and back).

Range Discrimination
Te ability to detect separate targets, which are on the same bearing (azimuth) and are close
together, is dependent on pulse length. For example, if a pulse length is 4 microseconds (s).
Its physical length will be 1200m. If two targets lie on the same bearing and are within 1200m
of each other, they will both be illuminated at the same time and their echoes would be
merged at the receiver.

Distance = Speed x time

2

300 x 106 x 4 x106 = 600m

2

Reflected Radio Energy

Radar emissions are generally in frequency bands above VHF because:
a) Direct waves make calculation of echo ranges simple;
b) Wavelengths have to be short to detect small targets;
c) Wavelengths have to be short to keep beam widths narrow;

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 d) Short wavelengths mean aerials can be smaller and therefore be more efficient in
terms of the power in/power out ratio.

Choice of PRF is also affected by a number of factors including:

Design Maximum Range

The transmitter must remain „silent‟ while the receiver is „listening‟ for echoes. If the design
maximum range is 200 NM the receiver must be allowed to „listen‟ for the period of time
from when a pulse has been transmitted until it can go 200 NM and then return. That is a
round trip of 400 NM, which would be the minimum PRI (pulse repetition interval) but in
practice the minimum PRI would require a silent period of 2473.3 s. This would be
increased to allow for receiver recovery time. The PRF is the inverse of the PRI so that, if the
PI was 2500 s the PRF will be 400 Hz.

Range (m) = (speed x PI)

2
= 300 x 106 x 2500 x 106 = 375km (202nm)
2

or Range (NM) = PI (s) = 2500 = 202 NM

12.35 12.35

The formula for calculating MTR, PRF and PI are:

MTR = c
2 x PRF

PRF = c
2 x MTR

PI = 1 or 2 x MTR
PRF c

(c can be taken as 300 x 103 km/s or 161 800 NM/s)

Calculation examples
a) A radar has a PRF of 300 pps. What is its maximum theoretical range in km?

MTR = c = 300 x 10 km/sec = 500km

2 PRF 600 pps

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 b) A radar has a PRF of 450 pps. What is the theoretical range in nms?

MRT = c = 161 800 NM/sec = 179.9NM

2 PRF 900 pps

c) The MTR of radar is 200 km. what is its theoretical PRF and pulse interval in
mocroseconds?

PRF = c = 300 x 103 km/sc = 750 pps

2MTR 2 x 200 km

PI = 1s or 1 000 000 s = 1000000 s = 1,333.3 s

PRF PRF 750

Data Acquisition
If the pulses are of short duration (1 s or less) it may take up to six echoes to cause the radar
display to show the target echo, due to elimination properties of the screen. The PRF must be
sufficiently high to allow for this to be achieved in the period of time that the effective part of
the aerial is pointing towards the target.

7.1.4 The Aerial

A highly directive aerial system is necessary in order to:
 Concentrate the transmitter power and increase the effective pulse power.
 Provide azimuth information

The system most commonly used consist either of a wave-guide horn and parabolic dish, see
fig. 7.3 or a flat plate slotted waveguide system. Either system is designed to focus the
radiated energy into narrow beam.

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 Fig. 7.3 Directive aerial transmitter

We will look at the most easily understood „parabolic dish‟ reflector.

This radar pulses are sent through the wave-guide horn and reflect from the parabolic dish.
This acts very much as the reflector in a car headlamp.

The polar diagram from such an aerial assembly will appear, as illustrated in fig 7.4 with a
main lobe and a number of smaller „side-lobes‟. The side-lobes are not desirable and may, on
some systems, be suppressed by modification of the reflector.

Fig. 7.4 Polar Diagram

The beam width, i.e. the angle contained between the ½ power points on the polar diagram,
will determine the ability to discriminate between targets that are close together and at the
same range. If, for example, two targets are at a range of 60 NM and are separated by 1 NM, a
beam of more than 1o will allow both targets to be „illuminated‟ at the same time.

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 The reflections from these targets will merge at the receiver and they will appear as one
„echo‟ on the display. It should also be noted that, as long as a target is within the beam, it
will be illuminated and will continue to Paint an echo. All targets will therefore show as
having an azimuth dimension equal to the beam width irrespective of the physical size of the
target.

For a parabolic reflector, the beam width can be calculated from the following relationship:

Beam width (in degrees) = 70 /D

Where  is wavelength (cm) of transmission.

D is dish diameter in cm.

Thus, to get a 10 beam width at a 25cm wavelength requires a dish diameter of 17.5m. On
10cm radar the same beam width is possible with a 7m dish.

In order to provide coverage, the aerial is rotated in azimuth so that the beam is also rotated.
The direction in which the aerial is pointed when a target is „illuminated‟ provides the
azimuth (or bearing) information. This direction is relayed to the display by an electronic link,
which may be either analogue or digital; the rate of rotation of the aerial must be matched to
the criteria that affect the selection of the PRF. It must be sufficiently high to allow for target
information to be renewed at short enough intervals of time, while at the same time it must be
slow enough to allow sufficient echoes to be detected in order to allow the display to show the
target.

7.1.5 The Receiver

This unit is designed to detect the extremely low energy signals reflected from, a target. The
receiver must therefore be extremely sensitive and provide very high amplification. It must be
protected from the very high energy of the transmissions from the same aerial and is therefore
electronically „switched of‟ during transmission.. This means that the receiver is dead during
transmission and for a short period afterwards (known as receiver recovery time).

Echoes from the target, after being detected and amplified, are sent to the display. In the case
of ATC radar, the echoes will include returns from fixed objects on the ground. These may
well hide the returns from aeroplane targets.

In order to remove these targets, a circuit known as “Moving Target Indicator‟ (MTI) is
introduced. If the target is moving radially towards or away from the aerial, the echo pulse
will be different in that the radio frequencies within the pulses will be increased if the target is
moving towards the aerial and decreased if moving way, this is known as a „Doppler‟ effect.

Stationary target will not be affected by this phenomenon and the receiver circuit can be
designed to reject all signals that do not exhibit a change in frequency. Unfortunately, if a
target is moving at a constant range from the radar, there will be no motion towards or away
from the receiver, hence no Doppler effect. The MTI circuit could reject such targets unless

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 measures are taken to counteract this problem. In modern units this is achieved by varying the
transmissions.

7.1.6 Timer
In its simplest form, the timer is combined with the display and consists of an electron gun
and screen assembled into a unit known as a cathode Ray Tube (CRT). This is illustrated in
fig 7.5.

Fig.7.5 Cathode Ray Tube (CRT)

The elements of this unit have the following functions.

 Cathode when heated, emits electrons
 Grid Admits electrons leaving the cathode to pass the grid and continue on the scene
 1st and 3rd anodes accelerate electrons away from the grid
 Focus coil (second anode) focus the stream of electrons to a point on the fluorescent
screen
 Deflection plates deflect the electron beam away from the centre of the screen. There
are two sets of plates: 2 vertical and 2 horizontal plates giving electrostatic deflection.
Two sets of coils may also be used for this purpose, giving magnetic deflection.
 Conducting anode drains spent electrons from the vicinity of the screen

In the CRT, the stream, of electrons, striking the interior surface of the screen, causes the
screen to fluorescence at the point of impact, if the electron stream is deflected, the spot will
leave a trace image, consider the schematic view of the face of a CRT as shown in fig 7.6.

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 Fig 7.6 Schematic view of the CRT front
When a positive potential is applied to X2, the negative electron beam will be deflected to the
left and reach it‟s starting position. If the negative potential is now applied to X2 and a similar
positive potential to X1, the beam will move from left to right at a constant rate, and light up a
spot where it hits the screen. If the movement of the spot is made fast enough, the eye will
observe the spot as a straight line across the screen.

When the spot reaches the right hand side, the flow of electrons is interrupted while the
reflector potential are instantly reversed, this is called the fly back while, for obvious reasons,
the potential illustrated in fig 7.7. is known as a „saw-tooth‟ voltage.

Fig. 7.7 “Saw-tooth” voltage

If the frequency at which the potential applied to the plate is made to match the PRF then the
time taken for a single movement of the spot from X1 will match the interval between pulses
and is therefore compatible with the maximum range.

The trace left by the spot is called a time-base.

The receiver output is connected to the Y plates. When a target echo is received, a potential is
applied to the „Y‟ plates and the spot is displaced vertically. The distance from the origin of

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 the time base to the spot deflection is proportional to the time taken from transmission of the
pulse to reception of the target echo and is therefore a measure of range.

This type of display is called an „A‟ scope it is seldom seen now but is useful for illustrating
the principle of a „time base‟.

Most radar timer/display CRTs use a rotating time base. In this time base is rotated by varying
the potential, in sequence, to the deflector arrangement. Targets are now shown by causing the
spot to become brighter. This is achieved by connecting the receiver output to the grid and
causing the flow of electrons to be increased momentarily.

If the time base is made to rotate in sympathy with the aerial then bearing information can be
derived.

Radar Performance
Radar uses frequencies that are normally in the higher bands, where propagation follows the
direct wave path.

In general, the range of radar is therefore „line of sight‟.

Radar is subject to factors, other than those of design, which will affect its performance as
follows:

(i) Atmospheric Conditions

At the very high frequencies used, super refraction caused by inversions of
temperature and/or humidity, may cause the direct wave range to be considerably
increased. It is possible for echoes to return from a range greater than eh design
maximum range and to appear on the screen as false targets at any range.

Sub refraction will cause poor radar performance at the upper range limits.

(ii) Weather
Rain and snow will attenuate the radar signal and will cause at least reduced
performance and possible even blind sectors.

(iii) Size, Shape and Aspect

As previously mentioned, the size and shape of the target will have a tremendous
effect on its ability to reflect radar signals, how it appear to the radar is also important.

By the same reasoning, a flat surface at right angles (to the direction to the radar
aerial) will produce a much stronger return than a similar sized curved surface.

7.3 Secondary Radar

We have looked at the principles of operation of primary radar and at some of the factors
which affect its performance. Some of those effects can be minimized by using secondary

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 radar techniques. The secondary radar still uses calculation of time to give distance and
direction of the aerial to give direction, but the target plays an active role.

The interrogating radar unit sends out a pulse (interrogation pulse) when this pulse is detected
at the target, it triggers a transmitter to respond, sending a signal back to the interrogator, this
signal will be stronger than an echo, will not be dependent on how well the target has
reflected the energy and could be coded with additional information.

7.4 Improvement of Reflected Energy

The amount of reflected energy received will vary with such things as target size, target
aspect, and the construction materials of the target. There are also methods which can be used
to increase the amount of reflected energy returned to the receiver.

a) Increasing the PRF

More pulses transmitted per second means more energy returned. There is, however, a
limit on this imposed by the expected range of the system. If a pulse is transmitted
before all echoes from the previous pulse have been received, false returns will appear
on its time base to confuse the radar. To avoid this ambiguity the transmission power
must be reduced as the PRF is increased.
b) Increasing the Transmission Power
Increasing the transmission power would require a reduction in the PRF to avoid
ambiguity and this would be counter-productive.

c) Increasing the Beam Width

Increasing the beam width so that the target stays longer in the beam will increase the
detection probability but will reduce the resolution of targets in azimuth. As the target
is swept by the beam it is successively illuminated by the leading edge, centre and
trailing edges. The results in “stretching” the target size by approximately one beam
width, (see fig, 7.8). Target size in azimuth is therefore, approximately, 2x beam
width.

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 Fig. 7.8 Radar Resolutions

d) Increasing the Pulse Length

Increasing the pulse width will put more energy on the target but range, or radial
resolution of targets will suffer. Considering the diagram below, (see fig 7.9)

Fig 7.9 Radar Resolutions

a flat plane reflector will continue to reflect the pulse throughout its duration. Thus,
the apparent target depth is equivalent to the pulse width, or length. Since range
measurement is calculated from the time to get to and from the target, the apparent
depth of the target (radial resolution) will be:

C x Pulse Length
2

Both beam width and pulse length therefore result in target „stretching” in azimuth and
range (see fig 7.10).

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 Fig. 7.10 Radar Resolutions

e) Slowing scan Rate

Slowing the scan rate will increase the period of target illumination and guarantee
increased reflected energy, but will also increase the time period between successive
scans which could led to identification and tracking difficulties. The difficulties can be
overcome by using a “Janus” (2 beam) or “Hydra” (multi-beam) system, but these
systems are not generally used in civil aviation applications.

Radar design is very much “horses for courses”, in other words, the selection of rotary
speed of the antenna, pulse duration and PRF for optimum scanning rate, transmission
power and focusing of the beam depend on the purpose of the radar and the
installation requirements.

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 Distance Measuring Equipment (DME)

8.1 The Purpose of the Equipment

DME provides navigational assistance in the form of range from a ground station. Whilst
useful for en-route navigation, it is particularly advantageous at terminal areas during the
climb and descent procedures. Used in conjunction with an aid which gives bearing e.g. VOR,
it gives a simultaneous fix.

Fig. 8.1 DME System

8.2 Basic Principles

DME is a secondary radar system. A short pulse pair is transmitted from an aircraft
(interrogator) to a responder beacon at a ground station (transponder). After a 50 microsecond
delay, called station delay, the ground station replies with a similar pulse pair, on a different
frequency. The time taken for the round trip, less an allowance for the delay at the station, is a
measure of the distance from the aircraft to the ground station and back. Half of this distance
is the slant range of the aircraft from the station, (see fig 8.1)

Range nm = 161 800 NM/s x (Time – Delays)

2

or Range = Time corrected for station delay s

12.35 s /NM

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 Type of pulse
The DME “pulse” is actually a double pulse, or pulse pair. This gives DME pulses a
distinctive character and protects the system from pulses produced by other equipment and
from random noise.

Radio Frequencies
DME operates in the UHF band between frequencies of 960 and 1215 MHz at one MHz
intervals. There are gaps in this coverage to allow for SSR frequencies. The aircraft transmits
on a frequency within the DME band and the ground station replies on a frequency which
differs from the interrogation frequency by 63 MHz, (see table 8.1).

VHF Nav Frequency DME channel DME interr. DME reply

(MHz) (MHz) (MHz)
108.00 17 X 1041 978
108.05 17 Y 1041 1104
108.10 (ILS) 18 X 1042 979
108.15 (ILS) 18 Y 1042 1105
108.20 19 X 1043 980
108.25 19 Y 1043 1106
108.30 (ILS) 20 X 1044 981
108.35 (ILS) 20 Y 1044 1107
108.40 21 X 1045 982
108.45 21 Y 1045 1108
Table 8.1 Channel Pairing

The propagation is by direct wave radiation and abnormal atmospheric conditions may cause
a slight delay to the signal, due to refraction.

Aircraft Antenna
This is a simple „blade‟ type omni-directional aerial, usually located underneath the aircraft. A
large bank angle may obscure the antenna momentarily, during turns.

Reasons for Change of Frequency

The reasons for using different frequencies for the air/ground and ground/.air transmissions
are so that:

 Transmissions of different aircraft do not trigger the aircrafts‟ receivers

 Reflections from objects around the ground station do not re-trigger the ground
transmitter when it replies to an aircraft.

Selection of Correct Replies

The ground station replies to as many aircraft as it can, using similar transmission pulses on
the same frequency. Each aircraft receiver must identify its own reply among all the replies
sent to other aircraft. It does this using a procedure known as “jittering the PRF”, (see fig 8.2).

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 Fig. 8.2 Transmission sequence – “jittering” the PRF

Each aircraft‟s PRF averages 30 pulse-pairs per second (pps) but the actual PRF used is made
to “jitter” a little either side of this figure in a random way from one pulse to the next. A
modern computer-controlled DME receiver can memorize all the replies received between
successive interrogations pulses from its transmitter. By employing a stroboscope technique,
the aircraft‟s pulses, received back via the ground station, are synchronized with the original
transmitted sequence in order to identify its own signal. During the search phase (when
locking on) the aircraft DME compares the stored time intervals between the interrogation and
the replies over a series of interrogation pulses until it finds a reply which is being received at
a near-constant time interval.

This procedure is normally completed in less than 2 seconds. The receiver logic or computer
circuits adjust the time of the expected reply to compensate for the aircraft‟s movement. The
receiver will then be on the “track” mode and is said to have “locked on” and slant range will
be measured or computed continuously.

Station Capability
The ground beacon is capable of replying at 2700 pps and so, on average, is able to reply to
about 100 aircraft. If the station is saturated by aircraft interrogations, it replies to the
strongest aircraft signals up to the maximum number of aircraft that it can accept; the strength
of the signals being determined by receiver gain or sensitivity, height, range distance
transmission power of aircraft.

Signal Controlled Search (SCS)

After switch on, a special circuit in the airborne DME checks if pulse pairs of the tuned DME
station are received, if not then the DME will automatically switch to the standby mode. This
may be because the antenna of the DME is obscured of the aircraft is outside the range of the
DME station. Interrogation of the DME ground stations starts when pulse pairs are received.
The airborne DME will then switch to the search mode.

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 Constant Duty Cycle
The DME station will always send pulse pairs. The constant duty cycle required by ICAO is a
minimum of 700 pulse pairs per second but recommended 2700 PPPs. If there are not enough
interrogating aircraft then the DME station will send squitter pulse pairs to maintain the
constant duty cycle.

Interruption of Signal
Should the ground station fail, or the signal be interrupted for any other reason, the aircraft
receiver will continue to compute slant range for about 10 seconds (less if lock-on has only
just taken place). If contact is regained within this period the equipment stays locked on and
re-adjusts as necessary to the returned signal. If the beacon is off the air for longer than about
10 seconds, all aircraft must repeat the locking-on procedure when transmissions
recommence. With older DMEs the maximum number of aircraft to which the station can
reply will be reduced.

Echo Protection
Multi-path signals may cause an “echo” effect, either to the ground station or aircraft. To
protect against this, there is a suppression period of 60 to 150 s after interrogation of the
ground station. In the air, the aircraft receiver will, receive only the first set of replies.

Accuracy
The accuracy of older DME is within 0.25 NM + 1.25% of slant range whichever is the
greater. (Modern equipment have an accuracy of 0.2 NM).

Range
DME uses direct wave propagation so its maximum theoretical range is given by the direct
wave range formula. The minimum range to a beacon occurs when the aircraft is directly
overhead, the DME will indicate the height of the aircraft converted from feet to nautical
miles. The slant range when not overhead can be converted to plan range using the
Pythagoras‟ Theorem formula:

Plan range = (Slant range)2 – (Height) 2

Identification
Identification is by an aural three-letter morse group transmitted at least once every 40
seconds.

Fig. 8.3 Finding plan range using Pythagoras‟ theorem, where height is in NM

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 In the case of a co-located or associated VOR/DME, the DME ident is transmitted every 30
seconds at a higher pitch (1350 Hz) than the VOR ident (1020 Hz), which is sent three times
in each half-minute.

Grounds Stations
Fig 8.4 shows the types of ground station that can be used with DME

Fig. 8.4 Types of ground station that can be used with DME

Frequency Pairing
DME was designed to be co-located with a VOR. To ease the workload of pilots and to
simplify the planning of frequency allocations by radio engineers the VOR and DME
frequencies are “ganged” together so that by selecting a VOR frequency, the pilot
automatically selects the associated DME frequency.

Military aircraft use TACAN, which is a purely UHF aid giving both bearing and range, like
VOR/DME. The pilot just has to select a channel number (e.g. 81X or 90Y) to access the
station, see table 8.1 or civil VOR equipment selecting the equivalent (paired) frequency, the
range element of TACAN can be received, but not the bearing.

Unfortunately, not all VORs and DMEs are co-located and so selection of a given VOR will
cause one of three things to happen if there is a DME on the “ganged” frequency.

 Co-located and Association VOR/DME or VORTAC

DME or TACAN will give slant range from a beacon co-located and associated with
the VOR. This is the ideal case and so both facilities transmit the same call sign. The
term co-located means that the VOR and DME/TACAN transmitters are within 2000
ft (600 m) of each other if on an airway and 100 ft (30m) of each other if on an
airfield.

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  Not Co-located but Associated VOR/DME or VORTAC
If the two transmitters are over the co-located distance but are still close enough for
normal navigation (up to a few miles apart), the last letter of one of the call signs is
changed to a „Z” (normally the UHF call sign), e.g. VOR = “BEN”, DME = “BEZ”

 Not Co-located or Associated

If the two transmitters are many miles apart (more than 6 NM) the two call signs will
be totally different but the DME or TACAN beacon will be triggered if within radio
range of the aircraft.

ILS/DME
Some airfields have a DME beacon whose frequency is ganged with the ILS VHF, when the
ILS localizer frequency is selected, the glidepath UHF and the DME UHF are automatically
selected and the DME shows the slant range to the threshold of the runway concerned.

The station delay is modified to indicate zero range at the threshold and accuracy 100 ft in
line with the runway centerline.

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 Ground Radar
9.1 Introduction
For Today‟s Air Traffic Control systems, the role of radar is crucial in allowing for the safe
and efficient controlling of an ever-intensifying air traffic density.

To provide for the needs of this task a number of different Air Traffic environments demand
different performance parameters from the radar. The following units provide these:

9.2 Surveillance Radar

There are three levels of surveillance radar, one for en-route, one for terminal area and one for
approach.

9.2.1 En-route Surveillance

This is normally provided by a radar having the following properties:
 A range capability of 200 NM to 300 NM
 A ability to penetrate intervening weather
 The ability to detect small targets out to maximum range.
 Moderate target discrimination capability in range and bearing.

These needs are generally fulfilled by using two radar systems, primary and secondary.

A primary radar with a wavelength of 20 – 50 cm, pulse length 4 s, PRF 270 pps horizontal
beam 1.780 beamwidth, aerial rotation 5 RPM

Secondary radar, which will provide the complement to the primary radar, improving the
possibility of detecting targets at long ranges and allowing for the identification of
cooperating targets.

9.2.2 Terminal Approach Surveillance Radar (TAR)

This provides separation between aircraft within the terminal area during transit, approach and
departure. It may be used to provide a radar approach.

The service is provided by primary radar with the following characteristics:

 Ranges up to about 60 or 80 NM
 Ability to refresh the target information at short intervals.
 Ability to penetrate intervening weather
 Good target discrimination properties
 Good accuracy

These needs may be provided by radar having a wavelength of 10cm, pulse length 3.9 s,
PRF 350 pps, beam width 1.20, aerial rotation rate 8
An SSR element is also normally used in the terminal surveillance radar environment.

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 Surveillance radar displays for en-route and terminal are normally to a high degree processed
by computers, and combine information from both primary and secondary radar, this
information is superimposed on an airspace map to show the controller a complete situational
picture on an easily viewed screen.

This type of radar may be used to provide a „surveillance radar approach‟ (SRA).

9.2.3 Range, Accuracy and Limitations of Surveillance Radar

Surveillance radar is capable of providing coverage to over 200 NM. however, both primary
and secondary elements are strictly „line of sight‟ so if you are below the radio horizon, you
will not be detected.

You must also remember that, even if above the horizon, if your aircraft is small, or are
heading straight towards or away from the radar aerial, or your aeroplane is made of Glass
Fibre, you will be a poor target for primary radar and your only hope of being detected often
lies with the secondary radar element. This, of course, depends on:

 The ground unit being equipped with an interrogator

 Your aeroplane being fitted with a transponder
 You using the transponder correctly
The accuracy of surveillance radar is dependent on the type of unit used but will be
sufficiently effective to allow for a traffic separation of 5 miles and this may be reduced to 3
NM within a range of 40 NM of the radar aerial.

9.2.4 Surveillance Radar procedures

En-route Procedures are limited to those required for identification. This may involve carrying
out turns as directed by the controller or at the request of the controller identifying your
position as a radial and range from a VOR/DME station.

In European regions, identification is more frequently carried out by use of the secondary
surveillance radar (SSR) element.

Approach (SRA)
As mentioned (9.2.2) surveillance radar may be sued to provide the pilot with approach
guidance including azimuth information and altitude advisories.

The success of such an approach is dependent upon:

 The skill of the controller

 The ability/willingness of the pilot to carry out the controller‟s instructions.

You must remember that this type of radar has no height finding capability so that all height
information is advisory and is the height you should be at the range and bearing that the
controller observes.

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9.3 Precision Approach Radar (PAR)
Many military airfields have PAR installations. These are primary radar units that are
designed to provide guidance during final approach to landing.

A PAR consists of two elements: one providing azimuth and range information and the other
providing elevation and range information.

Each element utilitises 3-cm wavelength (10 GHz) radar with high PRF and short pulse length
(less than 1s).

The two elements are sited at the approach end of runway to the side of the landing threshold.
The azimuth element scans a very narrow beam (0.60) from side to side over a sector which
covers the required minimum azimuth sector.

The elevation element has a narrow vertical beam width (0.60) but a broader azimuth beam
width (up to 300). It sector scans vertically from an elevation of about 0.50 up to 80. Both
scans are at a rapid rate in order to ensure that the target information is refreshed quickly.

Target information is presented to the controller on two screens mounted one above the other.
The upper screen shows the range to the target and its position relative to the nominal glide
path.

The lower screen shows the range and the position relative to the extended runway centre line.

Procedure
The procedures are fully detailed in the Air law syllabus but the following is a brief resume.

Prior to commencing the approach, the controller will advise you of Aerodrome QNH. All
heights will be referred to this datum. You will then be given instructions designed to help
you on the glide path and centre line and, in addition, the following calls will be made:

At 4 miles from threshold: “Clear to land surface W/V is…”

At 2 miles from threshold: “Check decision height”

From a range of 4 miles, distance to go will be passed at ½ or ¼ miles intervals.

At ¼ mile you will be advised “Approach completed”

You must remember that it is the pilot‟s responsibility to ensure that the runway is in sight
before DH. If the runway is not in Sight at DH a missed approach procedure must be initiated.
In some installations the controller will have radar information to aid navigation and give
advice also after the aircraft has passed DH; in some installations even when the aircraft is
rolling out on the runway.

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9.4 Airfield Surface Movement radar (SMR)
This is a highly specialized primary radar unit that is designed to assist controllers in
maintaining safe separation between aircraft and vehicles on the ground and to monitor all
ground movements.

It requires only short range but must be capable of:

 Very low minimum range
 3600 coverage
 Very high level of accuracy
 Excellent target discrimination

The following are the typical specifications for an SMR:

Frequency 10,000 MHz
PRF 15,000 PPS
Pulse length 0.5s
Beam width 0.480
Aerial rotation 60 – 75 RPM

Primary radar used as SMR will detect all vehicles and aircraft, but will not provide an
automatic identification of every vehicle and aircraft.

If a secondary radar is used as SMR, and all aircraft and authorized vehicles are equipped
with transponders, all authorized vehicles and aircraft may be localized and identified.

The problem is the unauthorized vehicles (and other traffic). This traffic can only be detected
and localized, but not identified by any of the radars.

Fig. 9.1 Surface Movement Radar

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 Fig. 9.2 Surface Movement Radar

Fig. 9.3 Applications of the Radar Bands

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10.0 Secondary Surveillance Radar (SSR)

10.1 Introductions
The primary radar element of the ATC Surveillance radar System provides detection of
suitable targets with good accuracy in bearing and range measurement but with the following
limitations:

 Targets that are too small, are built of a poor radar reflecting material or have a poor
aspect may not be detected.
 Targets cannot be identified directly
 Radar energy suffers from attenuation (losses) both on the path out to the target and on
the return path of the reflections.

To overcome these problems, a surveillance radar installation will often consist of both a
primary radar and a secondary radar, the latter being known as a secondary surveillance radar
(SSR). The role of the SSR is to complement the primary radar element. Fig. 10.1 shows the
aerials of a primary radar (the big reflector) and an SSR radar (the long, flat aerial on top).

Fig. 10.1 Primary Radar Aerial with SSR Antenna on top

10.2 Principles
SSR operates on secondary radar principles. An SSR “link” uses one ground-based transmitter
and receiver, called the interrogator and one airborne transmitter and receiver, referred to as
the ATC transponder, or simply „transponder‟. The interrogator transmits pulse pairs. A
receiver within the interrogator‟s beam receives these pulses and decodes them. The
transponder then responds by transmitting a pulse train (many pulses in a stream) back to the
interrogator. The pulse train contains information according to what the interrogator
requested.

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 All interrogations are transmitted at a frequency of 1030 MHz and all transponder responses
are transmitted at a frequency of 1090 MHz. The SSR aerial consists of a radiator and
reflector similar to that used in the primary radar but, because the return is much stronger than
that of a primary radar reflection, it is much smaller. Some early SSR aerials were of a small
size and because of this and the frequencies used, the beam width tended to be large and a
considerable number of side lobes were transmitted. Fig. 10.2 shows the polar diagram, in the
horizontal plane, for such an aerial.

Fig. 10.2 Polar Diagram

The large beam width reduced the bearing accuracy and increased the opportunity for false
interrogations to be generated by reflections from buildings and other obstacles. This could
cause aircraft responses from any direction. This problem has been minimized by aerial
design. The aerial illustrated at fig 10.1 is one of the improved versions and is known as a
large aperture aerial. Side lobes do still create a problem since aeroplanes‟ receivers,
especially at closer range will detect them and this could trigger false responses from
aeroplane outside the main beam. To counteract this, a process known as “side lobe
suppression” (SLS) is introduced.

Fig. 10.3 SSR radar screen

Interrogations are sent in the form of a group of three pulses that we will identify as P1, P2 and
P3 (see fig 10.4). The spacing between P1 and P2 is constant at 2.0 s. Pulse “P2” is used in the
electronic side lobe suppression. The P1 and P3 signals in the main beam are stronger than the
omnidirectional P2 pulse.

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 If the P1 pulse is weaker than the P2 control pulse, then the P1, P3 replies are suppressed. The
spacing between P1 and P3 is set at a value dependent upon the type (mode) of response
required from the aeroplane transponder. There are two current modes and their applications
and „P1 to P3‟ intervals are in table 10.1

Mode Use P1 to P3 spacing

A ATC 8 s
C Auto P. ALT. report 21s
Table 10.1 Modes of Interrogation

Modes B and D are not currently in use and the conventional aeroplane transponder is
designed to use only modes A and C. A typical control panel for the airborne unit is shown in
fig 10.5. This panel controls 2 transponders.

The pilot sets the transponder to the mode and code as instructed by ATC. If the transponder
is set to the “ON” position, the unit will respond to Mode A interrogations. If set to “ALT”,
the transponder will respond to Mode A and C interrogations, sending identification and
automatic altitude information.

Fig 10.4 Interrogation Fig. 10.5 Transponder

A special „ident‟ feature is utilized in order to allow ATC to confirm an aeroplane‟s identity.
This is activated by the pilot only on instruction from ATC. When the IDENT button is
pushed, an additional pulse is transmitted 4.35 s after the second framing pulse. At the
controller‟s display, the „ident‟ pulse will cause the particular aeroplane‟s echo to „fill in‟ or
flash.

10.3 Use of transponder

Pre-departure the transponder should be set to stand by. The test function should then be
activated in order to establish the operational status of the equipment. When instructed, set the
mode and code given by ATC, and when told to “Squawk” set the controller to “ON” or

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 “ALT” as appropriate. In order to avoid causing inference, do not change the code without
first selecting “STBY” on the control panel. However modern transponders automatically
relevant to “STBY” mode for a short period of time when the code is changed, when in an
abnormal situation there are three codes that you may set to alert the controllers. These codes
have their predefined meaning and, with one of these selected, signal indicating a „special
condition” will be triggered on the controller‟s screen. The aircraft symbol may change colour
to attract his attention. On some radar systems, a sound alarm will be triggered together with
the visible alarm.

 Code 7500 Unlawful interference

 Code 7600 radio failure
 Code 7700 emergency
 Code A 2000 – When entering UK airspace from an adjacent
region where the operation of transponders has not been required.
 Code A 7007 - Aircraft observing under the „Open Skies Treaty‟.
 Code A 7004 - Aircraft practicing, or displaying, aerobatic manoeuvres
 Code A 0000 – Transponder malfunction. e.g incorrect altitude
indication
 Code A 0030 - „Lost‟ aircraft
 Code A 0033 - Parachute dropping. Activated 5 minutes before start
of drop until parachutes are on the ground
From the tot time the ATC controller may ask you to “SQUAK IDENT”. By pushing the
“IDENT” button, the transponder is activated to transmit the additional pulse. This is shown
on the radar display as a flashing target. This function, when first enabled, will continue for
approximately 20 seconds. Never press the “IDENT” button unless you are instructed to by
the air traffic controller.

10.4 Presentation and Interpretation

The SSR information is presented together with the primary radar information. The difference
between the two is that the primary information is very accurate in bearing and range, but
doesn‟t consist of any other information. The secondary radar information provides reliable
information that can identify every aircraft and provide altitude information.

The primary radar element provides the necessary bearing and range and the use of computer
generated displays allows calculated information, such as track and ground speed, to be
shown. Fig 10.6 illustrates a common style of displaying combined (primary and secondary
surveillance radar) information on the air traffic controller‟s radar screen.

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 Fig. 10.6 Fig Information on the radar screen

10.5 Limitations
Since all SSR units operate at the same frequency, this can result in an aeroplane‟s response to
one interrogator being detected by other ground units. Such responses will be out of
synchronization and will cause random responses to appear. This is called “Fruiting‟ (FRUIT
– False Replies Unsynchronized to Interrogator Transmission).

10.6 Mode S (Selective Addressing)

This is a development of the basic SSR. The „Mode S‟ ground interrogators and airborne
transponders are fully compatible with the conventional Mode A and C units. However, Mode
S units working together have much greater capabilities.

The increased use of Mode S will have the following benefits over standard SSR
 Elimination of synchronous garbling
 Elimination of „fruiting.
 Increase traffic capacity
 Improve safety

ATC Service
Mode S data link can serve as a back up to many ATC services that are provided today by
VHF „voice communications‟. This data link back up will improve system safety by reducing
communications related errors within the ATC system. Many types of messages are potential
candidates for data link back up and other ATC services. These include:

 Flight identification, altitude clearance confirmation

 Take-off clearance confirmation
 New communication frequency for sector hand-over
 Pilot acknowledgement of ATC clearance
 Transmission to the ground of aircraft flight parameters, and
 Minimum safe altitude warning.

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11.0 Airborne Weather Radar (AWR)
11.1 Use
The first goal of the weather radar is to detect potential turbulent clouds which have to be
avoided. The second use is ground mapping to check aircrafts position. Formerly the weather
radar screen was monochrome, nowadays all modern AWR‟s are equipped with a color
screen.

11.2 Working Principle

The weather radar is a primary radar. That means that it is dependent on reflection of radar
signals as result of remission by the radar itself, for the use of the detection of precipitation
the best wavelength is 3.2 cm, frequency 9375 MHz (UHF). The PRF is 200 Hz, in T-mode
1500 Hz.

The best echoes are obtained from wet hail and heavy rain.
11.3 Antenna Scanner and Beam
There are two types of antennas in use. The traditional parabolic antenna and the flat plate
antenna, (see fig 11.1). The most common size is an 18 inch diameter. It swings from 600 or
900 on one side to the other side of the longitudinal axis. The larger the antenna the narrower
the transmitted beam.

Fig. 11.1 Flat plate antenna of the AWR

The advantage of a flat plate antenna is that it gives a narrower beam and less side lobes. The
antenna can be tilted up and down over plus or minus 150.

The traditional parabolic antenna can transmit two kinds of beams. A weather beam which has
the form of a narrow cone and is transmitted perpendicular to the antenna and a mapping
beam. On the parabolic antenna there is a small shield with small vertical rods. When the
polarization of the transmitted radio waves is changed this shield acts as a reflector and the
pencil beam is altered to a cosecant squared beam. This beam spread out vertically to the
ground and is used for ground mapping. (See Fig.11.2).

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 Fig. 11.2 Radar Beam

For ground mapping with a flat plate antenna the tilt has to be set to a low setting downwards.
For that type the beam remains a pencil beam. Ground echoes will be received to a distance of
about 70 NM.

11.4 Display, Iso Echo Contour and Colours

The echoes of cloud that contain precipitation are shown on a display. The screen is
rectangular in shape, (see fig. 11.4).

The aircraft is at the bottom of the screen. On the CRT bearing lines are depicted with
intervals of 15o or 30o. Range rings are shown on a electronic way in the form of half circles.
The space between the circles depends of the range that is set.

Monochrome radars are equipped with an iso-echo contour circuit. This allows any return
stronger than some pre-determined signal strength will be displayed on the AWR screen.

However, severe turbulence is usually encountered in areas of heavy precipitation, especially

where the horizontal precipitation gradient is steep. Radar echoes vary in strength according
to the rate of precipitation, but a monochrome CRT display is incapable of showing a large
enough of signal intensities for the gradient to be satisfactorily assessed. The purpose of the
Iso-echo Contour Circuit is to show the pilot where areas of severe turbulence can be
expected. When the contour circuit switch on, a second and higher pre-determined signal
level/range curve comes into effect, (see fig 11.3) Signals whose strength exceeds this line are
inverted and amplified and only that part of the signal still lying above the original signal
threshold will be displayed on the screen.

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 Fig. 11.3 Signal returns from a transmitted pulse (contour ON,

Returns from clouds having an area of strong precipitation within them will appear to have
holes in them. Turbulence is likely to be severe where the hole occurs but could be worse
where the precipitation gradient is steep and this will be where the hold is close to the edge of
the cloud return, (see fig 11.4).

Fig. 11.4 Monochrome radar screen

Modern radars have often a raster scan display where the radar picture is build up from the
memory of a computer. The information is displayed on the Navigation Display (ND) of the
EFIS (electronic flight information screen). There are different colours in order to indicate the
rate of precipitation colours that are used from light to heavy rain are green, yellow, red and
magenta. Area coloured red or magenta should be avoided, (see fig 11.5).

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 Fig. 11.5 Colour radar display on EFIS ND

11.5.1 Calculating cloud Height Using Weather Radar

Theoretically it is possible to calculate the eight of a CB cloud using trigonometry. The result
will not be particularly accurate, so the 1 in 60 rule may be used.

An aircraft at 30,000 ft is approaching a cloud formation. At 75 NM range, measured on the

AWR, with tilt angle adjusted to + 30, the radar return disappears, If the beam width is 50,
calculate the height of the cloud top above sea level (see fig 11.6).

Fig. 11.6 Height of Cloud top above sea level

The area between the bottom of the beam and the horizontal trajectory of the aircraft forms a
right-angled triangle, where h is the cloud height above 30,000 ft (side opposite) and the side
adjacent is 75 NM. The angle subtended by h is the difference between the tilt angle and half
the beam width, i.e. 30 – 2.50 = 0.50

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 Using the 1:60 rule:

0.50 = h or h = 0.50 x 75 x 6000 ft

60 75 60

h = 0.5 x 75 x 100  3750ft

3750 + 30,00 = 33,750 ft 34,000ft

The formula for calculating cloud height is, therefore:

Cloud height = (tilt angle – ½ beam width) x
radar distance (Nm) x 100 + A/C altitude

11.6 Precautions
When the aircraft is on the ground the weather radar should be set to standby or off for radar
waves with a high intensity could be harmful for persons in front of the aircraft. Especially in
hangars it should be noted that the radar is not switched on due to the reflection of the signals
to the walls.

11.7 General radar Display Information

The reflection of the radar signals mainly depends of the size and shape of the precipitation in
the cloud. The bigger the rain drops the better the reflection. Snow gives weak reflections.
The best reflection is obtained from precipitation around the 00 level.

11.8 Sensitivity Time Control (STC)

The receiver of the weather radar continuously varies the amplification of the received radio
signals depending on the distance from which the radar signals are reflected. The further the
reflecting objects away the more amplification occurs up to the maximum of the amplifier.
This maximum is reached for signals coming from bout 80 NM. The increasing amplification
is performed to create a same intensity of echo for similar showers close and further away.

11.9 Path Attenuation Correction (PAC)

Heavy precipitation weakens the radar signals strongly behind the cumulonimbus cloud. Path
attenuation correction in modern signals gives extra amplification to reflected signals behind
an area of heavy precipitation. Due to the limitations of the amplifier this can only be done in
the range where STC is possible.

11.10 Ground Clutter Suppression (GCS)

On modern radars GCS can be switched on, (see fig 11.7) Doppler techniques allow to filter
out echoes of which the frequency of the return is not or hardly changed, the relative speed of
the aircraft taken into account. Echoes with a speed change of less than 1 m/s are omitted and
not shown on the screen.

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 Fig 11.7 CDU for Weather Radar
11.11 Control Display Unit
With the mode selector the turbulence mode WX + T can be selected by the pilot, (see fig
11.7) to detect areas with turbulence. The shape of the returned pulse is compared to the
transmitted pulse. If there are many different movements in the cloud with precipitation then
the received pulse will have a flatter shape. The more flattening the more turbulence. Winds
shifts over 5 m/s can be made visible. For this mode the PRF has to be increased to 1500 Hz,
causing a limitation of the range of the TURB mode to 50 NM.

Clear Air Turbulence (CAT) cannot be detected by weather radar. In the mapping mode the
contours of cost lines are clear to be seen, but be careful to interpret the echoes when there is
coastal ice in areas at higher latitudes. Generally, small lakes cannot easily be detected.
Mountain ridges can be interpreted as coastal line. Build up areas do not differ from open
areas.

Test
In the test mode the screen will give a rainbow of the four colours with green below, then
yellow and red, followed by magenta on the outside.

WX (VAR)
In this mode the gain can be adjusted. If the gain is set too low compared to the WX mode
then the “below cal” lamp will light up.

Map
In the mapping mode the gain can be set to non-standard values to obtain a better radar
picture. When the screen is full with white echoes coastal lines can be enhanced when the
gain is set to a lower value. If a coast line is far away the gain should be increased. Care has to
be taken if the coastal area is flat and a higher area if more inland.

SYS ½
Most transport aircraft do have two transmitter and receiver sets. If one fails the other set can
be selected.

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12.0 Ground Proximity Warning Systems
12.1 Introduction
It is estimated that over 50% of transport aircraft losses have been caused by “controlled flight
into terrain” (CFIT), for such reasons as intention, confusion, vertigo, distraction, instrument
reading error, poor visibility and navigation error. The GPWS is designed to prevent this sort
of accident. The flight deck crew is given advanced warnings, both aurally and visually, if the
aircraft enters flight path which could lead to a dangerous situation close to the ground.

The GPWS uses various input signals from other on-board systems. Systems providing
altitude, airspeed, attitude, glide slope and position are required for basic and enhanced
functions. Acceleration, Angle of Attack (AoA) and flap position are required for windshear.
By constantly monitoring these input signals to rate of closure with the terrain immediately
beneath the aircraft is assessed. In a typical GPWS, a red PULL-UP light together with a
WHOOP-WHOOP PULL-UP audible command will give warning of unsafe proximity to the
ground. When the dangerous condition has been corrected, the warnings will cease and the
system will reset itself automatically.

12.2 General Operation

The GPWS is normally activated between 50 ft and 2,450 ft above the surface – this height
obviously being determined by the radio altimeter. The GPWS must never be deactivated (i.e.
by pulling the circuit breakers) except when using approved procedures at those airfields
where GPWS inhibition is specifically required.

12.3 GPWS Modes

The GPWS monitors seven basic MODES of the aircraft‟s operation plus monitors for
windshear:

 MODE 1 Excessive barometric descent rate

 MODE 2 Excessive Terrain Closure rate
 MODE 3 Height loss after take-off or go-around
 MODE 4 Unsafe terrain clearance when not in the landing configuration
 MODE 5 Below glide-slope deviation alert
 MODE 6 Below selected min RA, altitude callouts and bank angle alert.
 MODE 7 Windshear alerting

In addition to the inputs from the ADC and the radio altimeter, information is also fed into the
GPWS computer from the follow:

(i) Main landing gear selector assembly

The position of the landing gear will govern whether or not MODE 3 is activated, and
will also determine the height/barometric rate of descent conditions that would
activate a MODE 4 warning.

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 (ii) Flap Selector Assembly:
The position of the flap again governs whether or not MODE 3 is activated and will
also determine the height/terrain closure rate, which would activate a MODE 2
warning and the height/barometric rate conditions, which would activate a MODE 4
warning.

(iii) ILS Receiver:

The degree of deviation from the glidepath, together with glidepath validity signals, is
used in MODE 5.

(iv) Stall Prevention:

Whatever stalls prevention devices are fitted to the aircraft, e.g. stall warners, stick
shakers and stick pushers, will normally feed a signal to the GPWS computer to inhibit
to the GPWS warnings during the incipient stall and/or stalled condition.

Finally, the GPWS system has a fully integrated self-test function, which is capable of
checking the signal path from all of the inputs described above. If the system checks out
satisfactorily when the test switch is depressed, the normal indication to the pilot is that both
the visual and the aural warnings are activated simultaneously. System testing by this means
is normally prohibited when airborne.

12.4 System Operation

MODE 1 – Excessive Descent Rate
MODE 1 is activated when the barometric descent rate is excessive with respect to the aircraft
height above the terrain, as determined by the radio altimeter. The barometric rate signal is
obtained from the ADC. The warning envelope for MODE 1 has an upper limit of 2,450 ft
above the ground, and at this height a warning will be given if the barometric rate of descent
exceeds 7,350 ft/min. At the lower limit of the envelope, which is 50 ft above the ground, a
barometric descent rate of 1,500 ft/min or more will cause MDOE 1 activation.

The full operating parameters for MODE 1 are shown in fig 12.1

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 Fig. 12.1 Mode 1: Excessive sink rate

MODE 2 – Excessive Closure to Terrain

MODE 2 activation occurs when the aircraft is flying into rising terrain. This is achieved by
measuring the terrain closure rate as determined by the radio altimeter. MODE 2 exists in two
forms, 2A and 2B.

MODE 2A
MODE 2A is active during climb out, cruise and initial appreach – flap NOT in the landing
configuration and the aircraft not on the glideslope centerline.
If the terrain closure rate is equal to or in excess of 6,000 ft per minute, MODE 2A will be
activated. Again, the lower limit of MODE 2 operation is 50 ft above the ground, and at this
height a warning will be given if the terrain closure rate exceeds 2,063 ft per minute with the
flaps NOT in the landing configuration.

The “Speed Expansion” alert envelope varies as a function of the aircraft speed. As the speed
increases, the boundary expands to provide increased alert times at higher speeds.

The full operating parameters for MODE 2A are shown in fig 12.2.

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 Fig. 12.2: Mode 2A Excessive terrain closure rate, flaps not in landing position

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 MODE 2B
MODE 2B provides a desensitized envelope to permit normal landing approach manoeuvres
close to terrain without unwanted alerts. MODE 2B is automatically selected with flaps in the
landing configuration or when the ILS deviation is less than 2 dots. It is also active during the
first 60 seconds after take-off.

With the flaps in the landing configuration, MODE 2B will be activated at the upper
parameter of 790 ft AGL if the terrain closure rate is equal to, or exceeds, 3,000 ft per minute,
or at the lower paramerter of 220ft AGL if the terrain closure rate is equal to, or exceeds,
2,250 ft per minute.

The full operating parameters for MODE 2B are shown in fig 12.3

Fig. 12.3 Mode 2B: Excessive terrain closure rate, in landing configuration

MODE 3 – Altitude Loss After Take-Off

If an excessive height loss is experienced after take-off or during a go-around procedure,
when the aircraft is between 50 ft and 700 ft above the ground as determined by the radio
altimeter, MODE 3 is activated. With the aircraft at 700 ft. AGL, MODE 3 will be activated if
an accumulated barometric height loss of 70 ft or more is sensed by the ADC. With the
aircraft at 50 ft. AGL, MODE 3 will be activated if the accumulated barometric height loss
exceeds 40 ft. MODE 3 is inactive when the landing gear and flaps are both in the full landing
configuration.

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 The full operation parameters for MODE 3 are shown in fig 12.4

Fig. 12.4 Mode 3: Altitude loss after take-off or go around

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 MODE 4 – Unsafe Terrain Clearance
When an unsafe terrain clearance situation is experienced, with the aircraft NOT in the
landing configuration, MODE 4 which consist of 3 sub-modes, will be activated.

MODE 4A
Regardless of the barometric rate, MODE 4A will be activated when the terrain clearance
reduces to 500 ft AGL, unless the landing gear is fully down. At 500 ft AGL, as determined
by the radio altimeter, and a barometric rate of descent of 2,000 ft per minute or more, MODE
4A will be activated unless the flaps are also in the landing position.

The full operating parameters for MODE 4A are shown in fig 12.5

Fig. 12.5 Mode 4A: Descent in wrong configuration – gear up

MODE 4B
If descending in the wrong configuration, i.e. with gear down and flaps not in landing
configuration, and the warning envelope is penetrated at too low a speed (159kts or less),
MODE 4B will be activated and “TOO LOW FLAP” is announced. At higher airspeed “TOO
LOW, TERRAIN” is announced. The voice message will be repeated if penetration of the
envelope increases significantly. The operating parameters are shown in fig 12.6.

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 Fig. 12.6 Mode 4B: Descent in wrong configuration –gear down but flops not in landing position

MODE 4C
After passing 100ft on takeoff, unless gear is down or flaps are in the landing range, MODE
4C will be activated. For a go-around the warning floor is enabled at 245ft. The Minimum
Terrain Clearance (MTC) upper limit increases with speed above 190 kts. When penetrating
the boundary the “TOO LOW TERRAIN” voice warning is given. The voice message will be
repeated if penetration of the envelope increases significantly.

The operating parameters are shown in fig 12.7

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 Fig 12.7 Mode AC: Minimum terrain clearance (MTC)

MODE 5 – Excessive Deviation below Glideslope

MODE 5 is activated when the aircraft is significantly below the ILS glidepath, 1.3 dots, with
the aircraft between 1000 ft and 50 ft AGL, as determined by the radio altimeter, and with the
landing gear down. The glidesplope warnings may occur at the same time as pull-up warnings
on the occasions when the pull-up alert is due to an active MODE 1, 2, or 4 but not MODE 3.
If MODE 3 is activated, MODE 5 is automatically inhibited.

The operating parameters are shown in fig 12.8

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 Fig. 12.8 Mode 5: Descent below glideslope

MODE 6 – Advisory Callouts

MODE 6 is activated when the aircraft descends below selected altitudes, minimum selected
radio altitude, and also for excessive bank-angle.

In mode 6 callouts of selected altitudes and minimums are available.

Where the callouts occur are a customer option. These callouts consist of predefined radio
altitude based voice callouts or tones, and an excessive bank angle warning, no visual alerting
is provided. Table 12.1 covers the possible options available.

The decision height callouts require the landing gear to be lowered, and will occur when
descending through the radio altitude corresponding to the selected decision height. The
minimums callouts will take priority over all other height callouts.

In MODE 6, callouts of selected altitudes and minimums are available. (See fig 12.9.

Fig 12.9 “MINIMUMS, MINIMUMS”

A graphic presentation is given in fig 12.10.

Fig.12.10 Mode 6: Altitude call-outs

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 Callouts Occurs at (ft AGL)
Radioaltimer 2500
Twenty five hundred 2500
One thousand 15000
Five hundred 500
Five hundred tone (2 second 960 hz) 400
Four hundred 300
Three hundred 200
Two hundred DH + 80
Approaching minimums DH + 100
Approaching decision height DH + 100
Plus hundred DH + 50
Fifty above DH
Minimum DH
Minimum/minimums DH
Decision height DH
Decide DH
One hundred 100
One hundred tone (2 second 700 hz) 100
Eighty 80
Sixty 60
Fifty 50
Forty 40
Thirty five 35
Thirty five tone (1 second 1400 hz) 35
Thirty 30
Twenty 20
Twenty tone (1/2 second 2800 hz) 20
Ten 10
Five 5
Table 12.1

MODE 6 is also used to alert crew of excessive roll angles. The alert, which is aural only,
consists of the words „BANK ANGLE‟. The bank angle at which the alert is sounded will be
specific to the particular type of aircraft.

With proximity to the ground, the bank angle limit reduces, and at low altitudes it will be
sufficiently low to prevent wing tip or nacelle/engine damage during take-off and landing.

One envelope is defined for turbo-prop and jet business aircraft (see fig 12.12). Bank angle in
excess of:
  100 between 5 and 30 ft
  100 to 400 between 30 and 150 ft
  400 to 550 between 150 and 2450 ft
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 Produce the bank angle advisory (shaded area). Bank angle advisories are inhibited below 5
ft.

A graphic presentation is given in fig in 12.11 and fig 12.12

Fig. 12.11 Callout

Fig.12.12 Bank angle degrees

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 MODE 7 – Windshear Alerting
When the several parameters indicate the initial conditions of entering an area of windshear
MODE 7 is activated. The parameters are airspeed, groundspeed, barometric height, rate of
descent and radio altitude. In the basic system there is no scanning beam looking ahead to
avoid the encounter entirely, but it may contribute to an early recognition of the situation and
allow the pilot to initiate an earlier go-around procedure, thereby preventing an accident to
happen.

The aural warning consist of a two tone siren followed by the voice warning: WINDSHEAR –
WINDSHEAR – WINDSHEAR. The arsal warming is activated only once during a wind
shear encounter. The visual warning is provided by a illumination of the windshears. Lights
on the captain‟s and first officer‟s instrument penal, on EFIS equipped aircraft the alert
(message) WINDSHEAR will appear in red in the lower middle of each EADI. The
light/message remain until windshear conditions cease to exist.

The windshear warnings take priority over all other GPWS alerts.

A graphic presentation is given in fig 12.13.

Fig. 12.13 Mode 7: Windshear alerting

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 A summary of the modes is given in table 12.2.

Table 12.2. summary of the modes

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12.5 Alert and Warnings
12.5.1 Alerts
An alert is a caution. For GPWS operations, only MODE 5, excessive deviation beneath
glidepath, gives an alert. This is in the form of an audio “glide slope”. The required action is
to recover to the glideslope and also to attempt to establish the cause of the alert.

12.5.2 Warnings
A warning is a direct command to do something about a situation. In GPWS operations,
warnings are given for MODES 1 – 4 and the crew must immediately level the wings unless a
curved flight path is essential and initiate a maximum gradient climb until the minimum safe
altitude is reached. Where an advanced GPWS is fitted, the cause of the warning should be
verified after the climb has been initiated.

12.5.3 Discretionary Response

Regardless of the type of GPWS, basic or advanced, on receipt of the alert or a warning, a
response must be made. There is a case for a discretionary response to a warning under
specific circumstances when an aircraft is being operated in met conditions of:

 1 NM horizontally clear of cloud; and

 1000ft vertically clear of cloud; and
 Visibility of 5 NM or greater; and
 Where it is obvious to the captain that the aircraft is not in a dangerous situation with
regard to terrain, configuration or present manoeuvre, the response may then be
limited to that of an alert.

The alerts and warnings of basic and advanced equipment are summarized in table 12.3.

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 Fig.12.3

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13.0 RADIO ALTIMETER
13.1 INTRODUCTION

The Radio Altimeter (RA) is a device, which accurately measures the height above the surface
immediately below an aeroplane up to 2500ft, and is particularly suited to low-altitude terrain
clearance measurement. It provides an instantaneous and continuous readout on the flight
deck of the height above mountains, buildings, or other objects on the surface of the earth,
but gives no information regarding high ground immediately ahead of the aeroplane. This
information is also supplied to the:
 Automatic Flight Control System (AFCS) to facilitate automatic landings using the
Instrument Landing System (ILS).
 Ground Proximity Warning System (GPWS) to provide height, and rate change of
height information.
The outputs from the Radio Altimeter can directly, or via a data bus, feed to the Electronic
Flight Instrument System (EFIS) and he Flight Management Computer (FMC)

Importantly, the height measured by the Radio Altimeter is absolute, so flight over undulating
terrain results in systematic variations in the indications of the height of the aeroplane on the
display.

13.2 THE RADIO ALTIMETER SYSTEM

A Radio altimeter determines the time taken for a radio wave to travel from the aeroplane in
the ground directly beneath the aeroplane and back again. The system consists of a
transmitter/receiver, a modulator, an integral timing or past frequency counter, a transmitter
aerial, a receiver aerial and a display as shown below.

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 Fig 3.1

Some Radio altimeter systems alternatively use a mechanical circular display, as shown
below, where the height displays linearly up to 500 ft and logarithmically from 500 – 2500ft,
making the lower range of altitudes easier to read more accurately.

Fig. 13.2

In this type of display, the maximum height (2500ft) is obvious, but it is not so apparent when
using a moving vertical scale presentation, as shown on the previous page.
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 Notably, all radio altimeter displays have a method of setting the decision height, which is
normally set at 100ft, and a flashing DH indicator light is given when reaching this point. The
required height is set using a decision height (DH) setting knob, and a bug or index marker
indicates the set height. The setting control knob on some systems also normally doubles up
as a press-to-test (PTT) facility, which when engaged, drives the display to a predetermined
value, which is typically 100ft.

With reference to the upper display, an „OFF‟ or „FAIL‟ flag is visible if:
 There is a power failure
 The returning signals is too weak
 Local reflections are received from the airframe itself

A mask also covers the height pointer if:

 The equipment is switched off
 There is a fault in the transmitted signal
 The altitude exceeds 2500 ft.

The Decision Height (DH) light flashes continuously if the aeroplane goes below the set
height and remains so until the aeroplane climbs, or until setting the DH at a lower value. At
approximately 50ft above the set decision height, an audible alert sound with increasing
loudness until reaching the actual decision height.

13.3 PRINCIPLE OF OPERATION OF A RADIO ALTIMETER

A Radio altimeter measures the time taken for a radio wave to travel from the aeroplane to the
surface directly beneath and back again, and provided that the path followed by the wave is
vertical, the total elapsed time is a function of the aero plane‟s height. During this time, the
transmitted frequency changes, and the equipment measures the difference between the
transmitted and received signals. The frequency change is a measure of the time taken for the
radio wave to travel to and from the surface and thus, the greater the frequency change the
greater the height. To achieve this, the Radio Altimeter system makes use of primary radar
principles and transmits a Frequency Modulated continuous Wave (FMCW), at a frequency of
4250 MHz to 4350 MHz, which is in the Super High Frequency (SHF), or Centimetric
wavelength band.

A complete modulation cycle or frequency sweep is illustrated on the next page. In this
system, the total sweep of the modulated or carrier frequency is automatically varied by ± 50
MHz approximately 300 times per second, from an initial datum of 4300 MHz.

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 Fig. 13.3

Throughout the cycle, there are two very short periods when the modulation changes from
positive to negative and vice-versa. The frequency difference, which occurs when the
transmitter alters the direction of its frequency sweep, is overcome by relating the aeroplane
height directly to the average beat frequency (i.e. the difference between the transmitted and
received frequency, observed over a short sampling period). The frequency changeover
points essentially should be ignored in the height calculation, so that the difference in
frequency is directly proportional to the aeroplane‟s height

At low altitudes, the reflected radio wave returns almost instantaneously, which gives an
erroneous height, so a wider sweep is necessary to provide a measurable frequency difference.
In order to overcome this ambiguity, the sweep rate is lowered (i.e. the time for a complete
frequency sweep is made longer, so that all normal heights within the normal operating
range of the radio altimeter are covered).

13.4 PERFORMANCE AND ACCURACY OF A RADIO ALTIMETER

The accuracy of the radio altimeter is normally:
 0-500ft: ±3ft or 3% of the height, whichever is the greater
 Above 500ft: 5% of the height

When the aeroplane is on the ground, the Radio Altimeter may show a small negative value,
since the equipment is normally calibrated to indicate zero when the main wheels first contact
the runway surface on landing. The effect is particularly noticeable on aeroplanes with multi-
wheel undercarriage assemblies, which are inclined at an upward angle when deployed in
flight.

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13.5 ERRORS ASSOCATED WITH A RADIO ALTIMETER
A Radio Altimeter may be susceptible to the following errors:

1. LEAKAGE ERROR

This may occur if the Transmitter (Tx) and Receiver (Rx) antennae on the underside
of the aeroplane are fitted too close together (i.e. the spilling through of the side-lobes
directly into the Rx antenna). Placing the antennae far enough apart to avoid any
interference also provides adequate screening.

2. MUSHING ERROR
This may occur if the antennae are placed too far apart. As the aeroplane comes close
to the ground, the Tx antenna, reflection point, and Rx antenna form a triangle, so that
the actual distance traveled by the wave can become greater than twice the vertical
height between the surface and the aeroplane, thus giving a false height indication, as
illustrated below.

Fig.13.4

13.6 THE ADVANTAGES OF A RADIO ALTIMETER

Radio altimeters have the following advantages:
 They indicate the actual (absolute) height of an aeroplane
 They provide an easy crosscheck with the barometric altimeter for terrain clearance
 They provide an aural warning signal prior to reaching the preset DH, and a visual
warning when reaching the DH.

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14.0 AREA NAVIGATION SYSTEMS(RNAV)

14.1 INTRODUCTION
In the early days of expansion of commercial air transport a system of air routes was
developed to provide a safe means of control and separation of aircraft.
These routes, which became known as airways, were defined by radio navigation facilities
sited at strategic distances apart and at significant navigational points such as airway
intersections, turning points and FIR boundaries. By following these airways, aircraft have
been provided with a system of navigational checkpoints and, by being enclosed (by ATC)
in a clear box of airspace, separation from other known aircraft in the vicinity.
For many years the airway system has provided an adequate means of routing aircraft, in spite
of the fact that navigation from departure aerodrome to destination not normally being a
direct.
In recent years however, a number of factors have led to a review of this situation. These
include such elements as:
 Increasing congestion on the airways system. This is resulting in flow control and subsequent,
frequently extensive, delays to flights.

 The development of improved and enhanced navigation and communication systems that
permit an aeroplane‟s position to be determined accurately and transmitted speedily to the
responsible ATC unit .

 The development of enhanced ATC systems that make it possible to provide aeroplanes with
safe separation from other air traffic without the need to confine them to narrow corridors
of airspace
 An urgent need to conserve costs which demand that the shortest route from departure to
destination should be followed.

To respond to these problems and to enhanced capabilities, a system known as area Navigation
(RNAV) is being introduced. This is a system of navigation which is not dependant upon routing
between points which are coincident with the position of a radio facility but is capable of
providing navigational guidance along other non airway routes marked by waypoint as illustrated
in Fig.14.1. A waypoint is a predetermined geographic position. This is defined in terms of
latitude and longitude but, where appropriate, may also be defined as a radial and range of a
VOR/DME beacons. This is known as rho/theta (ρ / Ө) system or by ranges from two DME
stations, known as rho/rho (ρ/ρ). The rho/theta system is shown on fig. 14.2.

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 Figs 14.1 Rho-theta (ρ-Ө) navigation

Fig. 14.2 Rho/theta system

Subject to an aeroplane being properly equipped, area navigation will be available as follows:
 FIXED PUBLIHSED RNAV ROUTES – These can be nominated in a flight plan only if
the aeroplane is fitted with an approved RNAV capacity
 CONTIGENCY RNAV ROUTES Published routes useable by suitably equipped
aeroplanes during specified times
 RANDOM RNAV ROUTES - Unpublished routes. These may be flight planned within
designated areas

14.2 AREA NAVIGATION CONCEPTS

JAR OPS 1 requires that an aeroplane‟s navigation equipment should include “an Area
Navigation System when area navigation is required for the route being flown”.

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 There are three types of area navigation viz;
 Systems where the information is derived from airborne navigation equipment which
is a self-contained system. That is a navigation system that is independent of any
external information source. A typical self-contained navigational system will use the
outputs from an Inertial Reference System (IRS). Inertial navigation is completely
independent of any external visual or electronic reference. It simply updates the
position of the aircraft by sensing its accelerations and integrating those, with respect
to time, to establish distance and direction of movement from the start position. An
integral navigation computer carries out all related navigation calculations.

 Externally referenced systems in which information from an external source (or

sources) is required in order to provide navigational guidance.

 Hybrid systems that use information from a selection of self-contained and externally
referenced navigation systems.

Many commercial operators have been quick to realize the benefits of such a system and are
not only specifying a suitable system for new Aeroplanes but are, at considerable cost,
actively refit their older fleets with similar equipment.

Most equipment installations on commercial Aeroplanes will form an integral part of a

comprehensive avionics package and will be capable of providing area navigation even when
out of range of ground based navigation facilities. Such systems will normally be of the
hybrid types.

Many modern general aviation Aeroplanes are fitted with a basic RNAV system (so called
BRNAV) as standard. These are generally based on the rho/theta or rho/rho system using
inputs from VOR/DME. However, the Global Navigation Satellite System (GNSS), which is
based on satellite navigation, will be increasingly utilized as the prime source for the required
navigation information.

Within the areas of coverage, LORAN C may be used as a source of information to BRNAV.

14.3 Basic RNAV

VOR/DME based area navigation is a navigation and guidance system which, as its basic
signal inputs to compute track and distance to a waypoint uses VOR bearing and DME slant
range. In some more sophisticated systems barometric altitude input may also be provided.
Fig. 15.3 illustrates such a system. The simple system, most commonly installed in general
aviation aircraft, usually consists of a computer in which each waypoint is defined as a radial
and range from a VOR/DME. Such waypoints are often called phantom or ghost stations.

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 Fig. 14.3 Typical sophisticated area navigational system

The computer‟s memory is able to store a limited number of successive waypoints, normally a
maximum of nine, so that the pilot can enter the planned route before departure.

A more sophisticated system will utilize a navigational data base stored either within the
navigation computer or in an external storage unit. The navigational database contains all the
necessary information regarding routes between
airports, VOR/DME stations and waypoints. It is obviously important that this database is
kept up to date. It will be updated every 28 days.
The Control Display Unit (CDU) is used to enter information into the computer and to
display navigation information. In a basic system the navigation computer resolves the
navigation problem. This receives a radial from the VOR receiver, DME distance from the
DME interrogator and altitude from the encoding altimeter. The altitude information is used
to calculate the difference in altitude between the DME antenna and the aeroplane, for further
calculation of the horizontal distance DME aeroplane.
These parameters are used to establish the aeroplane‟s current position. This is
compared to the position of the next waypoint and an error signal is generated which is used
to provide steering signals to a Course Deviation Indicator (CDI), Horizontal Situation
Indicator (HIS) or other suitable display. A “distance to go” is also derived.

Fig. 14.4 VOR bearing and DME slant ranging

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 In some aircraft installations, the computer may also send track correction signals (lateral
steering commands) to the autopilot roll channel. Since the use of Area Navigation System
permits waypoints to be accurately defined, determined and flown, the need to follow the
„facility determined‟ structure of the airways is removed and this permits direct routing and
more effective use of the available airspace. The aircraft equipment will consist of the normal
VOR/DME receivers, a navigation (course line) computer and a simple interface display such
as illustrated in fig 14.5.

Fig. 14.5 Basic RNAV (VOR/DME) receiver

14.4 Use of basic RNAV

When operating the course line computer, the pilot selects a VOR/DME station that is within
the line of sight range of the desired waypoint. The radial and distance from the station to the
desired waypoint is then manually entered. This can be repeated for a number of waypoints (if
the equipment permits).

Once the waypoint information is stored, the pilot can select the sector (waypoint „from‟ and
„to‟) and the course deviation indicator will act as if a VOR radial has been selected. You
should note that it is possible to select sectors that do not connect successive waypoints. This
allows waypoints to be by passed.

Start navigating in the same manner as you would when tracking a VOR station. Distance to
go will appear in the normal display.

With area navigation, the amount of CDI needle deflection does not vary with distance to the
waypoint as it would when tracking inbound to a VOR. It always represents a distance off
track for a given deflection.

On a 5 dot CDI, one dot deflection equals one mile of deviation, regardless of the distance to
the waypoint.

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14.5 RNAV Accuracy
 BRNAV – Track-keeping accuracy?
5 mm for 95% of flight time
 PRNAV – Track-keeping accuracy?
1 NM for 95% of flight time

14.6 RNAV Limitations

The VOR/DME based BRNAV suffers the dependent limitations of VOR/DME equipment. If
this signal is lost and the Nav warning flags appear on the CDI or HSI you must use an
alternative method of navigation.

You may fly the area navigation route, using guidance signals to the waypoint, only as long as
the aircraft is within the operational range of the appropriate VOR/DME station.

The practical range limit is around 200 NM from the associated VOR/DME at the highest
altitudes normally used for civil aviation, but remembers that the range limit for a light
aeroplane will normally be considerably lower because of operational altitude.

The accuracy of the position information and any derived steering signals is affected by the
same sources of error as the VOR/DME in use.

Some systems use DME/DME navigation, with frequency scanning DME interrogators. Such
systems provide a more accurate navigation when being within range of 2 or more DMEs.

If a navigation system is relying on a single source only, any errors from that source will
naturally affect the accuracy of position fixes. However, in a hybrid system, in which the data
from a number of sources is electronically compared and the best information is used, tends to
provide a higher and more consistent degree of accuracy.

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15.0 Traffic Collision Avoidance System

15.1 Introduction
TCAS is a system used for detecting and tracking aircraft in the vicinity of your own aircraft,
(see fig 15.1). By interrogating their transponders it analyses the replies to determine range,
bearing and, if reporting altitude, the relative altitude of the intruder. Should the TCAS
processor determine that a possible collision hazard exists, it issues visual and audio
advisories to the crew for appropriate vertical avoidance maneuvers.

TCAS is unable to detect any intruding aircraft without an operating transponder.

Fig. 15.1 TCAS detection area

There are two types of cockpit displays for TCAS, the Resolution Advisory (RA) display and
the Traffic Advisory (TA) display. The RA display is incorporated into the vertical speed
indicator (VSI). By illuminating red and green areas around the dial it displays the required
rate, or limitation of climb or descent, to avoid a possible collision.

The TA display shows the intruding aircraft‟s relative position and altitude with trend arrow
to indicate if it is climbing or descending at greater than
500ft per minute. This TA display may be provided on the weather radar indicator, on a
dedicated TCAS display or a EFIS display.

The display identifies the relative threat of each intruder by using various symbols and
colours.

Complementing the displays, TCAS provides appropriate synthesized voice announcements.

ATC procedures and the “see and avoid concept” will continue to be the primary means of

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 ensuring aircraft separation. However, if communication is lost with ATC TCAS adds
significant backup for collision avoidance.

TACS information can also be displayed on EFIS or a combined weather radar and traffic
screen.

15.2 The TCAS System

 Is compatible with and independent of the ATC System
 Determines if a threat exists
 Provides display and audio announcement to the crew
 Position information displayed on CRT and/or EFIS
 Vertical guidance displayed on VSI
 Synthesized voice.
 Calculates appropriate vertical evasive manoeuvre
 Co-ordinates manoeuvre of two or more TCAS equipped aircraft via Mode S
transponder communication between aircraft.

15.3 System Architecture

The TCAS system is based on the Mode S SSR and comprises a control panel, a combined
range and VSI display, a processor and a voice message system. An additional Mode S aerial
is required, as well as additional aerials about the fuselage for amplitude comparison direction
finding.

15.4 Control Panel

The control panel is a conventional SSR electronic display/push button panel but with
additional switches for TCAS. Fig 15.2 shows the additional controls.

Fig. 15.2 Transponder panel

The TA and TA/RA positions allow the pilot to dictate whether collision information will be
displayed as only TA (say in a busy environment during an approach) or as two categories –
TA and RA by selecting TA/RA. When TA is selected, a warning flag will be displayed is the
TCAS VSI. Selection of XPDR (transponder only) means that the TCAS capability is
inhibited, but does not prevent other suitably equipped aircraft from interrogating the Mode S

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 information available from your aircraft, thus making it operate as a normal SSR, XPDR is
also used for parallel approaches.

The F/L button provides the facility for changing the height display on the TCAS VSI from
relative altitude to absolute altitude.

An A/N/B knob selects the relative height band in which “intruders” will be displayed. The
collision avoidance function and display are not affected by this switch, (see fig 15.3).

Fig. 15.3 TCAS vertical limits

15.5 TCAS Processor

The TCAS processor will use target altitude from either the target aircraft‟s Mode C or Mode
S. If no altitude is available, the processor will assume that the target is co-altitude, which is
more pessimistic, If both aircraft have Mode S, they “will have a chat” and the best avoidance
option will be computed. At low altitudes, when in high traffic density, the processor lowers
the system sensitivity (rather like the DME beacon), which is a trade-off between safety and
unnecessary alerts. TCAS calculates threat level on range and relative altitude (including
ROC/ROD), but not relative bearing.

15.6 TCAS OPERATION

The TCAS system monitors the airspace surrounding your aircraft by interrogating the
transponder of intruding aircraft. The interrogation reply enables TCAS to compute the
following information about the intruder:
 Range between your aircraft and the intruder
 Relative bearing to the intruder
 Altitude and vertical speed of the intruder, if reporting altitude;
 Closing rate between the intruder and your aircraft.

Using this data TCAS predicts the time to and the separation at, the intruder‟s closest point of
approach (CPA). Should TCAS predict that certain safe boundaries may be violated, it will
issue a Traffic Advisory (TA) to alert the crew that closing traffic is in the vicinity.

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 If the intruder continues to close, TCAS will issue a Resolution Advisory (RA) to obtain or
maintain safe vertical separation between your aircraft and the intruder. TCAS bases the
alarms on a five second crew reaction time to begin the separation manoeuvre. Increase or
reversal of a RA requires a reaction in two and one half seconds.

15.7 Traffic Display Symbols

TCAS will display four different traffic symbols on the Traffic Advisory displays. The
symbols change shape and colour to represent increasing levels of urgency. The traffic
symbols may also have an associated altitude tag which shows relative altitude in hundreds of
ft, indicating whether the intruder is climbing, flying level or descending A + (plus sign) and
number above the symbol means the intruder is above your altitude. A – (Minus sign) and
number beneath indicates it is below your altitude. A trend arrow appears when the intruder‟s
vertical rate is 500 ft per minute or greater.

If the intruder is Non-Altitude Reporting (NAR) the traffic symbol appears without an altitude
number or trend arrow. The type of symbol selected by TCAS is based on the intruder
location and closing rate. If TCAS direction finding techniques fail to locate the azimuth of
another aircraft, a NO BEARING message appears on the screen.

15.8 Non-Threat Traffic

An open white or cyan diamond indicates that an intruder‟s relative altitude is greater than
plus or minus 1200 ft vertically or its distance is beyond 6 NM range. It is not yet considered
a threat. This one is 1700 ft below your own altitude, climbing at 500 ft per minute or greater,
(see fig 15.4).

Fig. 5.4 Non-threat traffic

15.9 Proximity Intruder Traffic or Proximity Traffic

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 A filled white or cyan diamond indicates that the intruder aircrafts is within plus or minus
1200 ft vertically and within 6 NM range, but is still not considered a threat. This intruder is
now 100 ft below your aircraft and climbing, (see fig 15.5).

Fig. 15.5 Proximity traffic

15.10 Traffic Advisory (TA)

A symbol change to a filled yellow circle indicates that the intruding aircraft is considered to
be potentially hazardous. Depending on your altitude TCAS will display a TA when the time
to CPA is between 20 and 48 seconds.

Here the intruder is 900 ft below your aircraft, climbing at 500 ft per minute or greater. A
voice announcement is heard in the cockpit, advising, “TRAFFIC, TRAFFIC”. Under normal
conditions a TA will precede an RA by 10 to 15 seconds.

The crew should attempt to gain visual contact with the intruder and be prepared to
manoeuvre should an RA be sounded 10 to 15 sec later. The crew should take no evasive
actions based solely on a TA, (see fig 15.6).

Fig 15.6 Traffic advisory

15.11 Resolution Advisory (RA)

A solid red square indicates that the intruding aircraft is projected to be a collision threat.
TCAS calculates that the intruder has reached the point where a resolution advisory is
necessary. The time to closest approach with the intruder is now between 15 and 35 seconds

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 depending on your altitude, the symbol appears together with an appropriate audio warning
and a vertical manoeuve indication on the RA/VSI.

This aircraft is now 600 ft below your altitude and still climbing. A synthesized voice
announces vertical manoeuvre command, such as,

“Climb, Climb, Climb” or

“Climb, Climb”

The pilot should smoothly but firmly initiate any required vertical manoueuvre within 5
seconds from the time the RA is posted, using the vertical speed indicator (1500 ft/min). An
intruder must be reporting altitude in order to generate an RA. Therefore, the RA symbol will
always have an altitude tag,
(see fig 15.7).

Fig. 15.7 Regulation advisory

15.12 Off Scale Traffic

The presence of TA and RA aircraft that are beyond the selected display range is indicated by
one half of the traffic symbol at the edge of the screen. The position of the half-symbol
represents the bearing of the intruder.

15.13 The RA/VSI Instrument

TCAS guidance is incorporated into the vertical speed indicator. Two rows of coloured lights,
one green and one red arc, are located around the vertical speed scale. TCAS uses the lights to
indicate whether to climb, descend or remain level. The lights are OFF unless an active
resolution advisory is in progress.

Resolution Advisories are grouped as Corrective Advisories or Preventive Advisories.

Corrective advisories require a positive action by the crew accompanied by a green arc on
the RA/VSI showing “Fly To” guidance.

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 Preventive advisories require that no action be taken to alter the flight path of the aircraft.

When TCAS issues an RA, certain segments in the row of red lights will be turned on.
Segments in the row of green lights will be on when the pilot is required to actively
manoeuvre the aircraft to satisfy the resolution advisory. For safe separation from the intruder,
the pilot should manoeuvre the aircraft within the vertical speeds represented by the green
lights. Vertical speeds within the red area must be avoided.

A RA may be presented on the VSI requiring avoidance of two or more threat aircraft
simultaneously. For example a “do not descend” indication may be visible at the same time a
“limit climb rate” indication appears because of threat aircraft above and below your own
aircraft, (see fig 15.8).

Fig. 15.8 VSI with TCAS

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15.14 The TA/VSI Instrument
The TA/VSI combines the plan position of intruding aircraft and TCAS guidance on the
vertical speed instrument. A pointer and circular vertical speed scale indicate aircraft vertical
rate. Resolution advisories are shown as red and green bands outside of the scale. The centre
of the display presents intruding traffic (see fig 15.9).

Fig. 15.9 TCAS indications

15.15 Audio Announcements
Synthesized voice announcements are issued by TCAS over the aircraft audio system. Table
15.1 and table 15.6 lists all of the resolution advisories, audio messages and advisories in the
TCAS vocabulary.

Condition Advisory Message

Traffic advisory “TRAFFIC, TRAFFIC”
RA cleared “CLEAR OF CONFLICT”
Self test passed TCAS SYSTEM TEST OK”
Self test failed “TCAS SYSTEM TEST FAILED”
Table 15.1 audio message
“TRAFFIC” is spoken once if a second TA appears
Resolution advisories and synthesis voice announcements (see table 15.2)
RA Category Corrective Preventive
Climb “CLIMB, CLIMB, CLIMB” “MONITOR VERIDICAL SPEED
“MONITOR VERIDICAL SPEED
Descent DESCEND, DESCEND, “MONITOR VERIDICAL SPEED
DESCEND “MONITOR VERIDICAL SPEED
Crossover climb “CLIMB CROSSING CLIMB “MONITOR VERIDICAL SPEED
CLIMB, CROSSING CLIMB” “MONITOR VERIDICAL SPEED
Crossover descent “DESCEND, CROSSING “MONITOR VERIDICAL SPEED
DESCEND - DESCEND “MONITOR VERIDICAL SPEED
CROSSING DESCEND
Vertical speed restricted “REDUCE CLIMB “MONITOR VERIDICAL SPEED
(climbing) REDUCE CLIMB” “MONITOR VERIDICAL SPEED
Table 15.2 – Resolution advisories and synthesized voice announcement

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 “MONITOR VERTICAL SPEED” is spoken once if downgraded from a previous corrective
advisory.
The following resolution advisories are changes from those previously issued and require two
or more half seconds response time.
List of resolution advisories, (see table 5.3).
Change from climb to “DESCEND, DESCEND NOW – DESCEND, (N/A)
descend DESCEND NOW”
Change from descend “CLIMB, CLIMB NOW- CLIMB, CLIMB NOW. (N/A)
to climb
Increase climb rate “INCREASE CLIMB-INCREASE CLIMB (N/A)
Increase descend rate INCREASE DESCENT-INCREASE DESCENT” (N/A)

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15.16 Warnings and Limitations
The capability of TCAS is dependent upon the type of transponder in the intruding aircraft:
 The intruding aircraft must be equipped with a properly operating transponder for
normal TCAS operation; TCAS is unable to detect any aircraft without an oprating
transponder.
 If the intruder is Non-Altitude reporting (NAR), TCAS will display only the range and
bearing. It can issue a Traffic Advisory based on distance and direction of flight but
will not generate a Resolution Advisory.
TCAS assumes Non-Altitude Reporting (NAR) traffic is at the same altitude as your
own aircraft.

TCAS does not display NAR traffic above 14 500 ft.

A TCAS resolution Advisory is based on the expectation that the crew will comply
within 5 seconds. Any modification to the initial RA, including an increase or reversal
to an RA, requires two and one half seconds reaction time.

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16 GLOBAL NAVIGATION SATELLITE SYSTEM
INTRODUCTION
This is the latest in long line of navigation systems, the developments of which have been
greeted as the ultimate solution to all our navigation problems. The capabilities of this
system in coverage, redundancy and possible accuracy certain exceed anything we have seen
before and its long term potential is excellent.
Satellite navigation is really the product of a military requirement for a highly accurate,
reliable means of providing navigational information to military platforms – 24 hours a day,
every day of the year. Systems were developed by the Russians (GLONASS) and by the
Americans (Global positioning system – GPS). Since both systems work on the same
principle, we will study the more widely used GPS system before highlighting the significant
differences between it and the GLONASS system.
A new European GNSS called Galileo is on the design table.
The Chinese are developing theirs – called Beidou
GPS is capable of providing users with information in respect of:
 Position, in three dimensions
 Velocity determination
 Time
1.6.2 Basic Principle Of Operation
The basic principle used is similar to radar in that we measure Time of Arrival (TOA) of radio
signals to determine range from the source transmitter. Let us use your imagination (it might
help you if you draw a diagram to help you think of the scene). Imagine you have some
friends to assist you in this scenario.

You all synchronise your watches exactly to the second. One friend, let us call him Not Ra
Dam, is detailed to go up to the top of a bell tower and, at a precise time – say 10:00:00, ring
the bell. You and your other friends are arranged around the tower listening for the bell and
monitoring your watch. At the instant you hear the bell, note the time on your watch. It may
show 10:00:10 – that is 10 seconds since the bell was struck. If you know that the speed of
sound is 340m / second, you can derive your range as being 340 x 10m = 3400m from the
bell. If your other friends, one of whom is in a ballon above the tower also measures 10:00.10
then they are at a range 3400m from the bell. This is called Time of Arrival (TOA) ranging.
Of course the range so calculated depends on a number of things, such as:
 Did Not Ra Dam strike the bell at exactly 10:00:00?
 Was the speed of sound 340m per second or did it vary for each observer?
 Was your watch and those of your friends still exactly synchronized or
did you experience a „clock error‟?.

In addition to all these sources of possible errors we still only have one range. Each of us knows we
are somewhere on a circle of position at a range of 3400m. At least, each of us on the surface knows
that; but what about our friend in the balloon? He does not know his height and, in fact, he could be
anywhere on a spherical surface with a radius of 3400m from the bell.

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For an observer on the ground a non- ambiguous fix could be obtained by measuring propagation
time delays from three bell towers. For an airborne observer, however, a minimum of four bell towers
would be needed in order to provide an exact location in three dimensions. If your watch did have an
error, the range from each tower would have the same distance error. This would present you with
three possible positions P1, P2 and P3, (see fig. 16.1). By biasing each of these ranges equally you
could deduce the accurate position Pa and, at the same time, determine the magnitude of the clock
error (known as clock bias).

Fig. 16.1 Ambiguity due to clock error

In GPS, the bell towers are replaced by satellites and the sound of the bell by radio signals which
propagate at the speed of light.

The GPS system utilizes a constellation of 24 satellites in 6 precisely controlled orbital planes
inclined at 550 with the Equator plane, 4 satellites in each plane, about 20,200km up in space. A
sketch of this satellites network is shown on fig. 16.5. Each satellite is continuously transmitting
parameters enabling a receiver to calculate its exact position together with a time signal accurate to
within 3 nanoseconds. By locking on to theses signals, any GPS receiver can, with great precision,
triangulate its precise latitude / longitude coordinates.

By measuring the travel time of the signals transmitted by these satellites multiplied by the speed of
light, a GPS receiver on earth can determine its distance from each satellite being tracked. By locking
onto the signals from four satellites, a GPS receiver can, in additional to time, calculate your current
latitude, longitude and altitude (3D).
For a 2D position on the earth surface 3 satellites are sufficient.

FIXING POSITION
Each satellite can be regarded as having an individual radius of coverage, providing a spherical range
similar to DME. As with DME, to obtain a fix, a minimum of two satellites are needed, but to

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resolve ambiguity, three radii, or spheres, are required for a two dimensional fix on the earth‟s
surface, (see fig. 16.2)

Fig. 16.2 Three satellite fixing

However, DME gives slant range and therefore incurs an error depending on an aircraft‟s height.
With satellites navigation, in order to obtain the most accurate, or three – dimensional fix, a fourth
satellite is used. This fourth range sphere then fixes the aircraft‟s position in space, (see fig16.3).

Fig. 16.3 Four satellite fixing

THE GPS SYSTEM

The GPS system consists of three segments, namely:

 The space segment
 The control segment
 The user segment

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1. THE SPACE SEGMENT
This is made up of a group of satellites known as a „constellation‟, which provides the
navigation signals. At this date, 24 satellites from the GPS constellation. These are distributed
between six orbital paths, four to each path, the orbits being synchronous and inclined at 55o
to the equator.

Each orbit path is separated from the next (at the Equator) by 60o of space longitude. The
orbits, (see fig.10.5), are at an altitude of 20 200km and each satellite completes an orbit in 11
hours 58 minutes.

Fig. 16.4 The Three GPS segments

The satellites are positioned on their orbit in such a way that an observer anywhere on the
earth at any given time can detect signals from a minimum of five satellites. In the jargon of
GPS, this is stated as having five satellites “in view” or “visible”. Satellites have a designed
life span of seven years and there are programmes of continuous replacement.

The operational satellites are all in the block II series; the Block II R version being the latest.
These will be followed by the Block II F version, which has a planned designed life of 11.5
years. Satellites on – board equipment includes aerials capable of providing a wide coverage,
operating transmitters, receivers and atomic clocks using, in the later models, a combination
of cesium and rubidium atomic frequency standards. This provides a clock stability of 3 x 10 –
13
or, to put it another way, about 0.003 seconds every 1000 years. All on board equipment
has redundancy capability, which is another way of say that every unit has at least one back
up system.

The satellites all transmit on two navigation frequencies known as L1 (operating at

1575.42MHz) and L2 (operating at 1227.60 MHz), (see fig. 16.6). The Block II F versions of
the satellites will also have a capability for the inclusion of a new frequency (L5), which is
intended to permit civil users to perform automatic corrections for time delays caused by
ionospheric effects on the signals. The actual frequency for L5 is yet to be finalized but it

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 must be greater than 200MHz from L1 in order to optimize the ionospheric corrections. It
must also retain sufficient separation from the other systems in the same band, such as DME.

These signals use pseudo Random Noise (PRN) to carry message. PRN consists of binary
pulses of 1s and 0s which appear to be totally random but are actually generated in a totally
predictable manner by the on- board clock. This is achieved by using Binary phase shift
keying (BPSK) of the in which the phase of the carrier wave is reversed when the PRN code
changes from 0 to 1 and vice versa.

Since the PRN sequence is generated by the on board clock, the start time of each sequence is
precisely known and can be carried with the sequence.
The PRN sequences also make – up two codes known as:
 Coarse / Acquisition code – C/A
 Precision code - P
L1 carrier both C/A and P
L2 carriers only P, (see fig. 16.6).

Fig. 16.5 The GPS satellite constellation

Since the P (precision) code is only available to authorized users (primarily military) we will
not spend too much time on it.

Superimposed on both C/A and P codes is a 50 Hz navigation message.

(NAV – msg). This message contains five discrete sub – frames, (see fig. 16.7) containing:
 Clock data for the satellites being tracked
 Ephemeris for the satellite being tracked
 Message – data on obtaining UTC and, for C/A users, ionospheric delay corrections
 Almanac data – information about all the satellites in the constellation

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 Fig. 16.6 GPS Satellite transmission signals

Fig.16.7 Frame build-up of the NAV-MSG (50Hz modulation)

2. THE CONTROL SEGMENT

This provides the control and support system for GPS. It is made up of a master
control station (MCS) and 5 monitoring stations (MS). The function of the master
station is to track, monitor and manage the satellite constellation. It also provides an
updating service for the satellites‟ „NAV – msg‟.

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 The monitor stations are sited on precisely surveyed locations. They consist of very accurate
receivers which „listen‟ to the GPS satellites in view, measure ranging data and obtain the
NAV – msg from each. This data is transmitted to the master control station where it is
processed.

From this, the MCS can establish the satellite‟s exact orbit and location (known as the
satellites ephemeris) along with the actual and predicted clock parameters. This enables the
MCS to transmit to each satellite updated ephemeris and clock data to be included in the NAV
– msg.

3. THE USER SEGMENT

Essentially this is you, the user. In order to make use of the GPS satellite signals, you require
a receiver, which includes a signal processor. Since, as you will recall, the use of precision (P)
code is restricted to those who have a clearance from the US Department of Defense, receiver
which are intended for civil use are designed to make the best use of the C/A code.

For civil transport aeroplanes, the most favoured unit is the multi – channels receiver. In thus
type, a minimum of 4 RF channels are employed, permitting the simultaneous tracking of four
satellites and so resolving the problems of the single channel receivers.
Advantage can be gained by using more than four channels in an “all in view” system. If, for
example, five channels are used, it is possible to track all five satellites “in view” so that, if a
satellite is temporarily obscured from the aerial, there are sill four satellites “in view” and
providing the full position, velocity and time data.

The receiving antenna is an omni – directional, low profile type, located on the top of the
aircraft fuselage, probably near to the center of gravity, to minimize masking of satellite
signal whist banking and reflections from the aircraft structure.

16.4 GPS OPERATING PRINCIPLES

Since we have looked at the fundamental principle and the components, we are now in a
position to consider the principles of operation of the system. As we have noted, current
satellites signals are transmitted on two frequencies. These are identified as L1
(1575.42MHz) and L2 (1227.60MHz). Each RF signal is modulated by “Binary phase shift
keying” (BPSK).

The modulation is used to provide pseudo Random Noise (PRN) sequences that carry
messages and make up two codes, (see fig.16.8).

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 Fig. 16.8 GPS Satellite signals
These two codes are known as:

 Coarse/ Acquisition code (C/A) on which a standard position service (SPS) is

provided. This is available to all users. The C/A code has 1023 bits, and modulates the
L1 carrier at a chip rate of 1.923 MHz

 Precision code (p) on which a precision position service (PPS) is provided. The
availability of the “P” code is limited to users authorized by the US Department of
Defense. These are generally military users and we have no need to give any further
consideration to the operation of this code. The P code repeats 267 days with a chip
rate of 10.23 MHz, but only one week is used at a time.

L1 carrier both C/A and P codes. L2 carries P code only. Since this is not available to
unauthorized users, GPS receivers for use in civil applications will not be equipped to receiver
this channel. Each transmitted PRN sequence is known and is predictable relative to the start
time of that code sequence. Since these sequences are „known and predictable‟, it is possible
to replicate them.

At the GPS user unit, illustrated in fig.16.9, the satellites signal arrives at the aerial and is fed
to the RF amplifier and then to the phase modulation receiver. This determines the PRN code,
identifies the satellites and collects the NAV message. Information on the „time basis‟ used by
the satellite is transmitted as a component of the transmitted NAV message. The receiver
detects this and uses this time base to replicate the code sequence.

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 Fig. 16.9 GPS user unit

16.4 THE GPS RECEIVERS

The use of GPS is spreading rapidly and its potential applications appear to expand on a daily
basis. For the moment, let us just consider the common current equipment. We will first look
at three different GPS receiver / processors.

Fig.16.10 shows a typical of handheld GPS receiver low cost, but effective units available for
use in a light aeroplane. It consists of a self contained receiver / processor along with a
navigational database with a capacity of 250 waypoints and up to 20 pre –stored flight plans.
The receiver, which can use its own integral aerial or a remote aerial, can track up to 12
satellites simultaneously.

Fig. 16.10 Garmin handheld GPS

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 The control/ Display is on the fact of the instrument. The controls allow the pilot to interact
with the computer, selecting or entering waypoints, routes etc.
The display can show position or distance and track to next waypoint, track and ground speed.
Also displayed is the actual time, the ETA and cross track error. The set may be operated for a
period of a few hours on internal batteries, or be connected to an extended power source.

A schematic diagram of a typical GPS avionics receiver is shown at fig. 16.11. The receiver
is fed from an aerial, generally mounted on the top of the fuselage.

Fig. 16.11 Simplified GPS receiver block diagram

The aerial is designed to provide uniform sensitivity for all signals from satellites above a
specified angle of elevation. It is also shielded from low elevation signals which are prone to
multi- path signals i.e signals arriving from a low elevation satellites on more than one
propagation path. These multi – paths may occur from reflections and refraction from the
earth and its environment, and aircraft environment, if received, would cause significant
errors. The antenna will also shield from random or illegal signals radiated from the surface of
the Earth.

16.5 SYSTEM LIMITATIONS

There are two levels of positioning service:
 PPS – precision positioning service is not available to civil users so need not concern
us
 SPS – standard positioning service is carried on the C/A code, is available to all and is
the one that concerns us.

1. NUMBER OF USERS
Since the pseudo ranges are determined from signals, which are broadcast without specific
address, the number of users that can be served at any instant is virtually limitless.

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 2. COVERAGE
Of the 24 satellites in orbit, only 21 need to be fully operational; the remaining three can be
considered as „spare units‟ that can be rapidly re – deployed to replace any failed units. As has
been noted, the satellites are arranged so that there are six orbital paths each with four satellites.
The positions of these satellites are so adjusted that for any observe at any point on the earth
therefore will always b a minimum of four satellites above the horizon and there “visible” to the
“user segment” aerial. To be “visible” they must be 50 above the horizon for the PPS, as shown on
fig.16.12.

Current minimum operational standard calls for a minimum of five satellites to be

above a mask angle of 7.50

Fig. 16.12 Only satellites above 5o are visible

3. RELIABILITY /INTERGRITY
Inherently, the system is reliable
Constant monitoring of the satellite, by its own internal systems and by the
five monitor stations and the master control station of the ground segment,
ensures that signal performance degradation is detected at an early stage.
However, the satellites health message, which is a component of the satellite
NAV message may only be changed after a cycle of 25 frames transmission.
That means there is a possible period of 12.5 minutes plus, before
your receiver/processor is told that the signals it is processing are wrong at source.

In addition to that, the system operators (the Department of Defense) have installed a circuit
which is known as SELECTIVE AVAILABILITY (SA). When activated, the circuit inserts
controlled errors into the satellite signal. This renders the system inaccurate for unauthorized
(non-military) navigators. The DOD reserves the right to activate SA as and when they see a
need to do so without warning.

Both of these situations make the idea of using GPS as a „sole guidance‟ during a CAT 3
landing seems a bit unnerving.

This is obviously not a situation that civil users can live and two steps are available to resolve
the integrity problem.

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 (i) RECEIVER AUTONOMOUS INTEGRITY MONITORING (RAIM)
As the name suggests, this is integrity monitoring at the user segment. The receiver /
processor can self evaluate by using five visible satellites and, from them, determine
four separate positions. If one of the satellites exhibits anomalous pseudo range errors,
this can be easily detected and voted out of the calculations. For this kind of RAIM
activity there must be six satellites visible including a “spare” to replace any faulty
satellites.

An alternative method that may be employed is to link the GPS receiver / processor
into an integrated on board navigation system, which takes inputs from other
navigational sources. This is known as receiver augmentation. As an example, the
system could compare the GPS position to that derived from an IRS. If the navigation
computer detects a sudden marked deviation of the GPS position, satellite failure, poor
coverage, or deliberate interference may be suspected.

(ii) GPS INTEGRITY BROADCAST (GIB).

The second method is the use of a system known as the GPS integrity Broadcast
(GIB). This consists of a ground – based satellite monitoring system in which the
monitors, sited at precise locations, measure the satellite ranges on a continuous basis.
From the measured values, the range errors are computed and broadcast, via satellite,
to all users.
In some user segments, the equipment is sufficiently sophisticated to use a
combination of RAIM AND GIB in order to enhance the level of system integrity.

4. COVERAGE PROBLEM
A number of occasions have been reported when the GPS coverage was not as expected.
These arise from transient „holes‟ in the coverage and have earned the star TREK style
name of “WORM HOLES”. In some area „worm holes‟ appear on a regular basis. Some
have lasted for only a few minutes while others have lasted for days. In general, they
cause a weakening of the GPS signal and a degradation of accuracy but they have been
known to cause a total loss of the GPS service.
The cause of the holes is a blocking of the GPS receiver‟s capabilities to detect the signals.
This can result from any of the following:
a) Other transmission on the same frequency
b) Multiples (harmonics) of other transmissions (e.g. from television broadcast)
c) Intermodulation which causes a similar effect to (b)
d) Suppressing or blocking interference where the GPS receiver is swamped by strong
RF signals and is de – sensitized.
e) Deliberate jamming.

16.6 ACCURACY
1. ACCURACY FOR CIVIL USE
The rated accuracy of the SPS is, on 95% of occasions that the GPS position will lie
within a radius (of the true position) of:
 (less than /equal to)13 m horizontally
 (less than /equal to)22 m vertically
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 Time transfer accuracy:
(less than /equal to) 40 m nanoseconds
However these standards do not take into account signal delays due to ionospheric,
tropospheric, receiver and multipath conditions, or interference problems.

2. DILUTION OF PRECISION
Like all position determining systems that use lines of position, the geometry of
the intersecting lines greatly affects the potential accuracy of any resultant fix. If
the satellites in use, as viewed by the GPS receiver aerial, are close together, (see
fig. 16.13), the surface of position will have a poor angle of intersection and, as a
consequence, there will be a significant loss of accuracy. This is referred to, in
GPS jargon, as a “High Dilution of precision.”

Fig. 16.13 At least four satellites connected

DILUTION OF PRECISION (DOP) is frequently referred to by different acronyms as

follows:
 GDOP – Geometric DOP as just described
 VDOP – DOP in the vertical (altitude)
 HDOP – DOP in the horizontal
 PDOP – DOP of position, which is a combination of VDOP and HDOP
 TDOP – DOP in time
 The best configuration would be 3 satellites spaced 1200 apart, near the horizon,
with one overhead, see fig. 16.13

3. ERROR PREDICTATIONS
Up to a point, DOP error sources can be predicted for a specific receiver at a specific
time in a specific place. Compensations can therefore be made in the processor.
However, not all error sources are totally predictable (e.g. ionospheric refraction,
solar wind), so there is always the possibility or near probability of uncompensated
errors existing.

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17.0. DOPPLER NAVIGATION SYSTEM
17.1. Introduction
Otherwise known as Doppler Radar System, the Doppler Navigation system uses the Doppler
principle to measure an aircraft‟s ground speed and drift. The Doppler radar functions by continuous
measurement of Doppler shift and converting the measured values to GS and drift angle. In early
systems the aircraft‟s departure point was loaded into a navigation computer, which when converted
the aircraft‟s heading and Doppler GS/drift inputs into a continuous display of aircraft position; this
then was then displayed as latitude and longitude, and/or as distance to go along track and position,
left or right of track, in nautical miles.
A Doppler navigation system:
i. is completely self-contained and requires no ground-based Navaids
ii. is useable worldwide
iii. is most accurate overland
iv. is less accurate during flights over the sea because the surface winds,
tides and currents
v. Sometimes fails to measure a GS and drift during flights over smooth,
glassy sea

The latest improved Doppler Navigation Systems combine the inherent accuracy of Doppler GS and
drift measurement with information from Decca, IRUs, Loran C, GPS and VOR/DME, in various
combinations. These navigational inputs also help to eradicate the errors of the original Doppler
Navigation Systems, caused by inaccurate heading reference and degradation, or loss of Doppler GS
and drift when flying over large expanses of water.
The Doppler principle is also utilized in other navigation systems, such as VOR and VDF, and some
Radar equipment.

17.2. The Doppler Principle

Whenever there is a relative motion between a transmitter and receiver, a frequency shift or change
occurs which is proportional to their relative motion. This change in frequency, fd is known as the
Doppler shift, Doppler effect or Doppler frequency.
In an airborne Doppler the transmitter and receiver are screened from each other, but share the same
aerial. An array of beams are transmitted towards the earth surface at an angle of depression of
between 60° and 70° and the receiver measures the reflected frequency shift, which is caused by the
aircraft‟s speed along track, GS and speed across track drift.

17.3. Illustration of the Doppler Principle

To explain the Doppler principle, a separate ground-based transmitter, T and receiver, R are
considered.

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 Fig. 17.1. DNS System

CASE I: Both Transmitter and Receiver are Stationary.

Fig. 17.2. Both T and R Stationary

The stationary transmitter broadcasts at a carrier frequency, f. The stationary receiver receives f
waveforms per second at the speed of light, c, in m/s. Thus, the received frequency = C/λ, which is
the transmitted frequency. Hence, no frequency shift occurs.

CASE II: The transmitter is stationary, while the receiver is moving towards the transmitter at a
speed, V m/s.

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 Fig. 17. 3. R moves towards T

The speed of the transmission remains constant, but each transmitted wavelength arrives at the
moving R at a shorter time interval. This is perceived as a wavelength reduction.
We know that, C = f X λ. Then as λ appears to decrease, f must increase.
This apparent increase in frequency is due solely to the relative motion between T and R. The
difference in transmitted frequency, fT and received frequency, fR is known as the Doppler shift, fd.
Therefore, , fd = fR - fT.
The frequency shift, fd is also given as: fd = V/ λ, where, V is the speed of the moving aircraft.
But λ = C/ f. This implies that fd becomes, fd = (V fT)/ C.
Therefore, the relationship frequencies and speeds can be expressed as follows:
fd/ fT = V/ C.
In other words, the ratio of the frequencies equals the ratios of speeds.

CASE III: The receiver is moving away from the transmitter. Here the wavelengths take longer time
to reach the transmitter. Hence, this is perceived as a wavelength elongation. This means a decrease
in frequency, giving us a negative frequency shift.

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17.4. Summary

Fig. 4. Summary Table

17.5. Airborne Doppler

A typical DNS antenna consists of separate transmitting and receiving arrays, designed to produce
one of the common aerial beam configurations. This technique of using opposing beams is called a
JANUS array. Each aerial of a particular array transmit at an angle of depression Θ (60° ≤ Θ ≤ 70°).

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 Fig.17. 5. Depression Angle
This is a compromise:
i. If Θ is too close to 90° the Doppler shift approaches zero.
ii. If Θ is too small the transmissions would strike the surface at a shallow angle, causing the
signals to reflect away from the aircraft, thereby resulting in weak un-measurable Doppler
shift returns at the aircraft‟s receiver.

Using the four Janus array, zero drift and an aircraft traveling forwards: The received frequency from
the two front beams is shifted upwards and that from the two rear beams is shifted downwards,
equally in proportion the aircraft‟s GS.
If the aircraft is drifting then there will be a difference in the frequencies received from the port and
starboard beams. This information is then electronically converted in modern fixed aerial equipment
to a continuous indication of drift.
The higher the Doppler frequency, the more sensitive and efficient it becomes at assessing the
frequency shifts to be converted to GS and drift, and the narrower the beam widths (1° to 5°) for a
given aerial dimension. An excessive increase in the transmitted frequency causes absorption and
reflection from precipitation. The frequencies allocated to DNS are 8800 MHz or 13300 MHz.
The Janus arrays also reduce errors caused by minor variations in the transmitted frequency, pitch,
roll and vertical speed changes and unlocking during flight over an uneven surface. When a Doppler
system unlocks, it reverts to “memory” and ceases to compute GS and drift.

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17.6. Janus Array System

Fig. 6. The JANUS Arrays

A modern 4-Janus system transmits at a frequency of 13.325 GHz. The depression angle to the centre
of each beam is 67°. The depth and width of each is 5.6° and 11° respectively. The accuracy for this
system is 0.3% of GS and drift on 95% of the occasions.

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17.7. The Control Display Unit (CDU) of DNS

Fig. 17.7. DNS CDU

i. STBY function is selected when the aircraft is close to structures and people. This
safeguards the equipment, prevents damaging the health of people in the radiation path
and allows the equipment to be energized for immediate use when the aircraft is clear.

ii. The SEA indicator illuminates when the aircraft is flying over the sea or large expanse of
water. As stated previously, the reflected returns from water are less than those from land
due to “spillage” of reflected energy from the front of the forward beams and the rear of
the rearward beams.

iii. This results in a smaller measured fd spectrum from the four beams, evidenced by a
reduction in the actual GS readout. Circuitry within the computer will compensate for this
GS reduction and increase the readout for the assessed GS loss.

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