RADAR AND NAVIGATIONAL

AIDS

What is RADAR?
The word radar is an abbreviation for RAdio
Detection And Ranging
Radar is an electromagnetic systems used for
detection and location of objects such as
aircraft, ship, vehicles, people, natural
environment etc.
Radar systems use modulated waveforms and
directive antennas to transmit
electromagnetic energy into a specific volume
in space to search for targets.
Objects (targets) within a search volume will
reflect portions of this energy (radar returns
or echoes) back to the radar.

Radar Principles
Transmitter generates and transmits electromagnetic wave (sine or
pulse). One antenna can be used for both transmission and
reception.
The distance to the target (Range)- the time taken for the radar
signal to travel to the target and back.
Angular position, of the target - direction of arrival of the reflected
wave front.
Doppler effect moving objects

Range- Distance from you and the

target!
The range of the object is found by the time the
pulse takes to travel to and from the detected
object

Maximum unambiguous range

We send a train of pulses. Not a single pulse. A pulse is sent
and reflected back as echo
We have to wait to get for the echo before we send the next
one
If not the echo for the first pulse we sent will become echo
for the second one. This is called second time around pulse
Due to this the target may look near (as you get the echo
immediately for the send pulse: this echo was for the first
pulse!!!!)
So the pulse repetition frequency (PRF) is important and it
determines the maximum unambiguous range

Run = cTp/2 = c/ 2fp

TO INCREASE THE RANGE

Transmitted power is increased

Gain of the antenna should be high
Large antenna for reception
The receiver should be sensitive to weak signals
(should pick them)
Range equation is an important derivation to
calculate the range.

Radar Equation
The radar equation relates the range of a radar to
the characteristics of the transmitter, receiver,
antenna, target, and environment.
It is useful for determining the maximum distance
from the radar to the target
it can serve both as a tool for understanding
radar operation and as a basis for radar design.
However it cannot give the precise value.
Why??

Statistical nature of noise and signal

Fluctuation & uncertainty of the target
Propagation effect of the wave
Losses

Are affecting the calculation

We will consider each one separately.

RANGE PERFORMANCE

In practice the simple range equation does not

predict range performance accurately. The
actual range may be only half of that predicted
.
Noise is in the system and it is random. So we
have to have probability. If affects the
detection. So we have to get SNR

Threshold detection
false alarm: if the threshold is set as low then the noise may
be detected as target
Missed detection: If the threshold is set high then real target
will be missed.
However we use matched filter which will increase the SNR.
So we conclude that SNR is an important factor and more
analysis is needed.

Receiver Noise and the Signalto-Noise Ratio

Most receivers are super heterodyne.

The noise is mostly internal. It is thermal or
Johnson noise. Its value is 1.38 X 10-23 J/deg.
Noise figure is to be derived and defined below

APPLICATIONS
Navigational aid on ground and sea
Radar altimeters (height measurement)
Radar blind lander (aircraft landing during poor
visibility)
Airborne radar for satellite surveillance
Planetary observations
Police radars ( Highway safety)
Remote sensing (weather monitoring)
Air traffic control (ATC) and Aircraft safety
Ship safety
Non-contact method of speed and distance in industry

MAIN TYPES OF RADAR

There are two main types of radar:
1)Primary Radar
Continuous wave Radar
Pulse Radar
2)Secondary Radar

Pulse Radar

 The transmitter may be an oscillator, such as a

magnetron
radar for the detection of aircraft at ranges of 100 or 200
nmi might employ a peak power of the order of a
megawatt, an average power of several kilowatts
The duplexer might consist of two gas-discharge devices,
one known as a TR (transmit-receive) and the other an
ATR
(anti-transmit-receive).
The mixer and local oscillator (LO) convert the RF signal
to an intermediate frequency (IF).
Matched filter- to improve the signal to noise
ratio.

CW Radar
The radar transmitter may be operated
continuously rather than pulsed if the strong
transmitted signal can be separated from the
weak echo
A feasible technique for separating the
received signal from the transmitted signal
when there is relative motion between radar and
target is based on recognizing the change in the
echo-signal frequency caused by the doppler
effect

CW RADAR

 If the target is in motion with a relative velocity vr

to the Radar, then the received signal will be
shifted by an amount of fd.
The purpose of Doppler amplifier is to eliminate the
echos from stationary targets and to amplify the
Doppler echo signal to a level where it can be used
to operate an indicating device
The low frequency cut-off must be high enough to
reject the dc component caused by stationary
targets. The upper cut-off frequency is selected to
pass the higest Doppler frequency expected
Intermediate receiver-flicker noise-

Isolation between transmitter

and receiver:
In principle, a single antenna may be employed since the
necessary isolation between the transmitted and the received
signals is achieved via separation in frequency as a result of
the doppler effect
A moderate amount of leakage entering the receiver
along with the echo signal supplies the reference
necessary for the detection of the doppler frequency shift
(1) the maximum amount of power the receiver input circuitry
can withstand before it is physically damaged or its sensitivity
reduced
(burnout)
(2) the amount of transmitter noise due to hum,
microphonics, stray pick-up, and instability which enters the
receiver from the transmitter

Disadvantages of CW Doppler Radar

1.When a single antenna is used for both transmission
and reception, it is difficult to protect the receiver
against the transmitter because in constant to pulse
Radar, both are ON all the time.
2. These are able to detect only moving targets, as
stationary targets will not cause a Doppler shift and the
reflected signals will be filtered out.
3. CW Radars are not able to measure range, where
range is normally measured by timing the delay between
a pulse being sent and received but as CW Radars are
always broadcasting; there is no delay to measure.

FM CW Radar
CW radars have the disadvantage that they cannot
measure distance, because it lacks the timing mark
necessary to allow the system to time accurately the
transmit and receive cycle and convert the measured
round-trip-time into range. In order to correct for this
problem, phase or frequency shifting methods can be used
In the frequency shifting method, a signal that constantly
changes in frequency around a fixed reference is used to
detect stationary objects and to measure the rage. In such
Frequency-Modulated Continuous Wave radars
(FMCW), the frequency is generally changed in a
linear fashion, so that there is an up-and-down or a
sawtooth-like alternation in frequency

TRACKING RADAR
Measures the coordinates and provides data to
determine target path
Tracking can be performed in range, angle and
doppler
Classified into two types
Continuous tracking radar
Track-While-Scan radar
Acquisition radar designates targets to the
tracking radar

Error Signal generation

The antenna beam in the continuous tracking radar is
positioned in angle by a servomechanism actuated by
an error signal. The various methods for generating
the error signal may be classified as
1. Sequential lobing,
2. Conical scan, and
3.Simultaneous lobing or
mononpulse.

The tracking radar must first find its target before it

can track. Some radars operate in a search, or
acquisition, mode in order to find the target before
switching to a tracking mode.

 Obviously, when the radar is used in its tracking

mode, it has no knowledge of other potential
targets.
Also, if the antenna pattern is a narrow pencil
beam and if the search volume is large, a
relatively long time might be required to find the
target.
it called an acquisition radar.

SEQUENTIAL LODING
The antenna pattern commonly employed with
tracking radars is the symmetrical pencil beam in
which the, elevation and azimuth beam widths
are approximately equal.
However, a simple pencil-beam antenna is not
suitable for tracking radars unless means are
provided for determining the magnitude and
direction of the target's angular position with
respect to some
reference direction, usually the axis of the
antenna.
The difference between the target position and
the reference direction is the angular error.
The tracking radar attempts to position the

SEQUENTIAL LOBING
Two lobes are required
to track in each axis,
each lobe must be
sequentially switched
four pulses are required
The radar measures
the returned signal
levels
The voltages in the two
switched position
should be equal

CONICAL SCAN
A logical extension of the simultaneous lobing
technique described in the previous section is to
rotate continuously an offset antenna beam
rather than discontinuously step the beam
between four discrete positions. This is known as
conical scan.
The angle between the axis of rotation and the
axis of antenna beam is called squint angle.

CONICAL SCAN

The antenna is
continuously rotated at an
offset angle.
Redirection of beam
Rotating feed
Nutating feed

CONICAL SCAN

DISADVANTAGES
Sequential lobing
1) Angle accuracy can be no better than the size of the
antenna beamwidth.
2) Variation in echo strength on a pulse-by-pulse basis
changes the signal level thereby reducing tracking
accuracy
3) The antenna gain is less than the peak gain in beam axis
direction, reducing maximum range that can be
measured

Conical scan
4) The antenna scan rate is limited by the scanning
mechanism (mechanical or electronic)
5) Sensitive to target modulation
6) Mechanical vibration and wear and tear due to rotating
feed

SIMULTANEOUS LOBING
With a single pulse angular coordinates can be obtained
Maximum unambiguous range is limited only by PRF
Monopulse is free of mechanical vibrations, Errors due to
amplitude fluctuation of target echoes are greatly
reduced
In this technique the RF signals received from two offset
antenna beams are combined so that both the sum and
the difference signals are obtained simultaneously.
The sum and difference signals are multiplied in a phasesensitive detector to obtain both the magnitude and the
direction of the error signal. All the information necessary
to determine the angular error is obtained on the basis of
a single pulse.

THE USES OF ILS

Instrument Landing System :
To guide the pilot during the approach and
landing.
Very helpful when visibility is limited
To provide an aircraft with a precision final
approach.
To provide an aircraft guidance to the
runway both in the horizontal and vertical
planes.

Localize
r

Localizer is the horizontal antenna array located at the

opposite end of the runway.
Localizer operates in VHF band between 108 to 111.975
MHz

ILS COMPONENTS

ILS consists of Ground Installations and

Airborne Equipments There are 3
equipments for Ground Installations, which
are:
1. Ground Localizer (LLZ) Antenna To
provide horizontal navigation
2. Ground Glide path (GP) Antenna To
provide vertical navigation
3. Marker Beacons To enable the pilot
cross check the aircrafts height.
There are 2 equipments for Airborne
Equipments, which are:
1. LLZ and GP antennas located on the

Illustration

How does LOC work ?

Localizer transmit two signals which overlap
at the centre.
It operates in the VHF band: 108MHz to
117MHz
The left side has a 90 Hz modulation and
the right has a 150 Hz modulation.
The overlap area provides the on-track
signal.
For example, if an aircraft approaching the
runway centre line from the right, it will
receive more of the 150 Hz modulation
than 90Hz modulation.

How does the GS work ?!

GS operates in UHF band: 329 to 335 MHz
Glide path antenna produces two signals in
the vertical plane.
The upper has a 90 Hz modulation and the
bottom has a 150 Hz modulation.
For example, if an aircraft approaching the
runway too high, it will receive more of the 90
Hz modulation than 150Hz modulation.
Difference in Depth of Modulation will align
the aircraft with the 3o glide path.

3. What do Marker Beacons do?

They aid in indicating the distance of the aircraft from the runway.
1. Outer Marker (OM)
The outer marker is normally located 7.2 to 10 km (4.5to 6 mi)
from the runway threshold. The cockpit indicator is abluelamp
that flashes in unison with the received audio code. The purpose
of this beacon is to provide height, distance, and equipment
functioning checks to aircraft on intermediate and final approach.
On the aircraft, the signal is received by a 75 MHz marker
receiver. The pilot hears a tone from the loudspeaker or
headphones and a blue indicative bulb lights up.
2. Middle Marker(MM)
The middle marker should be located so as to indicate, in low
visibility conditions, themissed approachpoint, and the point
that visual contact with the runway is imminent, ideally at a
distance of approximately 3,500ft (1,100m) from the threshold.
The cockpit indicator is anamberlamp that flashes in unison
with the received audio code.

Airplane Approaching to the left of runway

center line.

Observe the yellow NAV

vertical pointer line tracking
the runway center line and
moving towards right.

Microwave Landing System

MLS is an all-weather, precision landing
system originally intended to replace or
supplement ILS installations
MLS has a number of operational
advantagesi. wide selection of channels to avoid
interference with other nearby airports,
ii. excellent performance in all weather,
iii. wide vertical and horizontal capture
angles.