AIRCRAFT COMMUNICATION SYSTEM

● THE RADIO HISTORY

Maxwell first suggested the existence of
electromagnetic waves in 1864
Later, Heinrich Rudolf Hertz used an
arrangement of rudimentary resonators to
demonstrate the existence of electromagnetic
waves.
In 1894, Marconi demonstrated the
commercial potential of the phenomenon that
Maxwell predicted and Hertz actually used in
his apparatus. It was also Marconi that made
radio a reality by pioneering the development RADIO WAVE
of telegraphy without wires. - Radio waves are directional and propagate
Marconi was able to demonstrate very out into space at 186,000 miles per second.
effectively that information could be exchanged The distance they travel depends on the
between distant locations without the need for frequency and the amplification of the signal
a ‘landline’. AC sent to the antenna. The electric field
The military applications of radio were first component and the electromagnetic field
exploited during the First World War (1914 to component are oriented at 90° to each other,
1918) and, during that period, radio was first and at 90° to the direction that the wave is
used in aircraft. traveling.
● RADIO WAVES
A radio wave is invisible to the human eye. It is
electromagnetic in nature and part of the
electronic spectrum of wave activity that
includes gamma rays, x-rays, ultraviolet rays,
infrared waves, and visible light rays, as well
all radio waves.
The relationship between frequency and
wavelength is inversely proportional. A high TYPES OF RADIO WAVES
frequency wave has a short wave length and a - Ground Waves or Surface Waves
low frequency wave has a long wave length. o Have VLF, LF, and MF waves
ELECTROMAGNETIC WAVES SPECTRUM o Ranges from 3kHz to 3mHz
o Have long wavelengths and utilize long
antennas
o Particularly useful for long distance
transmissions.
o They follow the curvature of the earth as
they travel from the broadcast antenna to
RADIO FREQUENCY SPECTRUM the receiving antenna.
- The frequency of an AC signal is the number o Automatic direction finders (ADF) and
of times the AC cycles every second. The LORAN navigational aids also use these
number of times it does this every second is frequencies.
known as the radio wave signal frequency or - Sky Waves
radio frequency. As the current flows through o Higher than HF transmission
the antenna, corresponding electromagnetic o Includes VHF (30-300Mhz), UHF
and electric fields build, collapse, build in the (300MHz-3GHz) and SHF (3GHz-30GHz)
opposite direction, and collapse again. radio waves
o transmission is often along the line-of-sight
of the antenna
o but DO NOT bounce off of the Ionosphere
o most aviation communication and - Wavelength (λ)
navigational aids operate with space waves o Distance along the direction of propagation
between points on the wave.
o λ = vT = v/f

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● MODULATION
The production and broadcast of radio waves
does not convey any significant information.
AVIATION RADIO FREQUENCY The basic radio wave discussed above is
- VHF communication radios are the primary known as a carrier wave. To transmit and
communication radios used in aviation. They receive useful information, this wave is altered
operate in the frequency range from 118.0 or modulated by an information signal.
MHz to 136.975MHz The information signal contains the unique
- VHF radios are used for communications voice or data information desired to be
between aircraft and air traffic control (ATC), conveyed. The modulated carrier wave then
as well as air-to-air communication between carries the information from the transmitting
aircraft radio to the receiving radio via their respective
- When using VHF, each party transmits and antennas.
receives on the same channel. Only one Two common methods of modulating carrier
party can transmit at any one time. waves are amplitude modulation and
WAVE PROPAGATION frequency modulation.
- Any wave in which the vibration direction is AMPLITUDE MODULATION (AM)
perpendicular to the direction of propagation - A radio wave can be altered to carry useful
is called a transverse wave. information by modulating the amplitude of
- Typical transverse waves are the wave. A DC signal, for example from a
electromagnetic waves; like light waves and microphone, is amplified and then
radio waves. superimposed over the AC carrier wave
- In contrast, Longitudinal or Compressional signal. As the varying DC information signal
Wave’s vibration direction is parallel to the is amplified, the amplifier output current
direction of propagation; like sound waves. varies proportionally.
WAVE TERMINOLOGIES - The oscillator that creates the carrier wave
- Period (t) does so with this varying current. The
o Time it takes for the wave to go through one oscillator frequency output is consistent
complete cycle. It is the number of seconds because it is built into the oscillator circuit.
per cycle. But the amplitude of the oscillator output
- Frequency (f) varies in relation to the fluctuating current
o Number of cycles per second in Hertz input.
o f = 1/T - When the modulated carrier wave strikes the
- Crests receiving antenna, voltage is generated that
o Top points of the wave. is the same as that which was applied to the
- Troughs transmitter antenna. However, the signal is
o Bottom points of the wave. weaker. It is amplified so that it can be
- Amplitude demodulated.
o Maximum disturbance undergone during a - Demodulation is the process of removing the
cycle. original information signal from the carrier
 wave. Electronic circuits containing resulting in the transmission of useful
capacitors, inductors, diodes, filters, etc., information through the atmosphere or
remove all but the desired information signal space.
identical to the original input signal. Then, - Transmitters
the information signal is typically amplified o Consists of a precise oscillating circuit or
again to drive speakers or other output oscillator that creates an AC carrier wave
devices. frequency. This is combined with
amplification circuits or amplifiers. The
distance a carrier wave travels is directly
related to the amplification of the signal sent
to the antenna.
o A transmitter prepares and sends signals to
an antenna that, in the process described
above, radiates the waves out into the
atmosphere. A transmitter with multiple
channel (frequency) capability contains
tuning circuitry that enables the user to
select the frequency upon which to
broadcast. This adjusts the oscillator output
to the precise frequency desired. It is the
oscillator frequency that is being tuned
FREQUENCY MODULATION (FM)
- It is widely considered superior to AM for
carrying and deciphering information on
radio waves. A carrier wave modulated by
FM retains its constant amplitude. However,
the information signal alters the frequency of
the carrier wave in proportion to the strength
of the signal. Thus, the signal is represented
as slight variations to the normally consistent
- Receivers
timing of the oscillations of the carrier wave.
o Antennas are simply conductors of lengths
- FM has a steady current flow and requires
proportional to the wavelength of the
less power to produce since modulating an
oscillated frequency put out by the
oscillator producing a carrier wave takes less
transmitter. An antenna captures the
power than modulating the amplitude of a
desired carrier wave as well as many other
signal using an amplifier.
radio waves that are present in the
atmosphere. A receiver is needed to isolate
the desired carrier wave with its information.
o The receiver also has circuitry to separate
the information signal from the carrier wave.
It prepares it for output to a device, such as
speakers or a display screen. The output is
the information signal originally introduced
into the transmitter.

TRANSIMITTERS, RECEIVERS,
TRANSCEIVERS
- Radio transmitters and receivers are
- Transceivers
electronic devices that manipulate electricity
 o A transceiver is a communication radio that the upper atmosphere. Due to variations in
transmits and receives. The same height and intensities of the ionised regions,
frequency is used for both. When different frequencies must be used at
transmitting, the receiver does not function. different times of day and night and for
The push to talk (PTT) switch blocks the different paths. There is also some seasonal
receiving circuitry and allows the transmitter variation (particularly between winter and
circuitry to be active. summer). Propagation may also be
o In a transceiver, some of the circuitry is disturbed and enhanced during periods of
shared by the transmitting and receiving intense solar activity. The upshot of this is
functions of the device. So is the antenna. that HF propagation has considerable
This saves space and the number of vagaries and is far less predictable than
components used. Transceivers are half propagation at VHF
duplex systems where communication can - Selective Calling (SELCAL)
occur in both directions but only one party o This system involves the transmission of a
can speak while the other must listen. VHF short burst of audio tones. Each transmitted
aircraft communication radios are usually code comprises two consecutive tone
transceivers. pulses, with each pulse containing two
Modulation / Demodulation simultaneously transmitted tones. The
- https://www.youtube.com/watch?v=beFoCZ7 pulses are of 1-second duration separated
oMyY by an interval of about 0.2 seconds. To
BASIC COMMUNICATION SYSTEM ensure proper operation of the SELCAL
- VHF Communication decoder, the frequency of the transmitted
o Very high frequency (VHF) radio has long tones must be held to an accuracy of better
been the primary means of communication than 0.15%
between aircraft and the ground. The - Flight Interphone System
system operates in the frequency range o The flight interphone system provides the
extending from 118 MHz to 137 MHz and essential connecting link between the
supports both voice and data aircraft’s communication systems,
communication (the latter becoming navigation receivers and flight-deck crew
increasingly important). members. The flight interphone system also
o In the VHF range (30 MHz to 300 MHz) extends communication to ground
radio waves usually propagate as direct personnel at external stations (e.g. the nose
line-of-sight (LOS) waves. Sky wave gear interphone station).
propagation still occurs at the bottom end of o It also provides the means by which
the VHF range (up to about 50 MHz members of the flight crew can
depending upon solar activity) but at the communicate with the cabin crew and also
frequencies used for aircraft make passenger address announcements.
communication, reflection from the The flight interphone system comprises a
ionosphere is exceptionally rare. number of sub-systems including amplifiers
o The maximum LOS distance between an and mixing circuits in the audio accessory
aircraft and a ground station, in nautical unit, audio selector panels, cockpit
miles (nm), is given by the relationship: speakers, microphone/headphone jacks and
PTT switches.
Where “h” is in feet (ft) and “d”
is in Nautical Miles (nm)
- HF Communication
o High frequency (HF) radio provides aircraft
with an effective means of communication
over long distance oceanic and trans-polar
routes. In the HF range (3 MHz to 30 MHz)
radio waves propagate over long distances
due to reflection from the ionised layers in
 AIRCRAFT NAVIGATION SYSTEM tuning capabilities can be used for ADF. This
includes those from AM radio stations. Audible
● In the early years of aviation, a compass, a
identifier tones are loaded on the NDB carrier
map, and dead reckoning were the only
waves.
navigational tools. These were marginally
The most modern ADF systems have two loop
reassuring if weather prevented the pilot from
antennas mounted at 90° to each other. The
seeing the terrain below.
received signal induces voltage that is sent to
● For aviation to reach fruition as a safe, reliable,
two stators in a resolver or goniometer.
consistent means of transportation, some sort
● Radio Magnetic Indicator (RMI)
of navigation system needed to be developed.
To save space in the instrument panel and to
● VHF Omni-directional Range (VOR)
consolidate related information into one easy
Navigation
to use location, the radio magnetic indicator
One of the oldest and most useful navigational
(RMI) has been developed. It is widely used.
aids. The system was constructed after WWII
The RMI combines indications from a magnetic
and is still in use today. It consists of
compass, VOR, and ADF into one instrument
thousands of land-based transmitter stations,
or VORs, that communicate with radio
receiving equipment on board aircraft.
Ground VOR transmitter units are also located
at airports where they are known as TVOR
(terminal VOR). The U.S. Military has a
navigational system known as TACAN that
operates similarly to the VOR system. ● Instrument Landing System (ILS)
Sometimes VOR and TACAN transmitters An ILS is used to land an aircraft when visibility
share a location. These sites are known as is poor. This radio navigation system guides
VORTACs. the aircraft down a slope to the touch down
VOR uses VHF radio waves (108–117.95 area on the runway. Multiple radio
MHz) with 50 kHz separation between each transmissions are used that enable an exact
channel. This keeps atmospheric interference approach to landing with an ILS.
to a minimum but limits the VOR to line-of sight A localizer is one of the radio transmissions. It
usage. To receive VOR VHF radio waves, is used to provide horizontal guidance to the
generally a V-shaped, horizontally polarized, center line of the runway. A separate
bi-pole antenna is used. glideslope broadcast provides vertical
The signals produced by a VOR transmitter guidance of the aircraft down the proper slope
propagate 360° from the unit and are used by to the touch down point.
aircraft to navigate to and from the station with Compass locator transmissions for outer and
the help of an onboard VOR receiver and middle approach marker beacons aid the pilot
display instruments. in intercepting the approach navigational aid
● Automatic Direction Finder (ADF) system. Marker beacons provide distance
Operates off of a ground signal transmitted from-the-runway information. Together, all of
from a Non-Directional Beacon (NDB). Early these radio signals make an ILS a very
radio direction finders (RDF) used the same accurate and reliable means for landing
principle. aircraft.
A vertically polarized antenna was used to ILS Components:
transmit LF frequency radio waves in the 190 - Localizer
kHz to 535 kHz range. A receiver on the o The localizer broadcast is a VHF broadcast
aircraft was tuned to the transmission in the lower range of the VOR frequencies
frequency of the NDB. Using a loop antenna, (108 MHz–111.95 MHz) on odd frequencies
the direction to (or from) the antenna could be only. Two modulated signals are produced
determined by monitoring the strength of the from a horizontally polarized antenna
signal received. complex beyond the far end of the approach
Any ground antenna transmitting LF or MF runway.
radio waves in range of the aircraft receiver’s
 o They create an expanding field that is 21 ⁄2° ILS identifier. Once located, the pilot
wide (about 1,500 feet) 5 miles from the maneuvers the aircraft to fly down the
runway. The field tapers to runway width glidepath to the runway
near the landing threshold. The left side of - Marker Beacons
the approach area is filled with a VHF o Marker beacons are the final radio
carrier wave modulated with a 90 Hz signal. transmitters used in the ILS. They transmit
The right side of the approach contains a signals that indicate the position of the
150 MHz modulated signal. aircraft along the glidepath to the runway.
- Glideslope As mentioned, an outer marker beacon
o The vertical guidance required for an aircraft transmitter is located 4–7 miles from the
to descend for a landing is provided by the threshold.
glideslope of the ILS. Radio signals funnel o The transmission is very narrow and
the aircraft down to the touchdown point on directed straight up. A marker beacon
the runway at an angle of approximately 3°. receiver receives the signal and uses it to
o The transmitting glideslope antenna is light a blue light on the instrument panel.
located off to the side of the approach This, plus the oral tone in combination with
runway approximately 1,000 feet from the the localizer and the glideslope indicator,
threshold. It transmits in a wedgelike pattern positively locates the aircraft on an
with the field narrowing as it approaches the approach.
runway.
o The frequency is paired to the localizer
frequency of the ILS. When the VOR/ILS
receiver is tuned for the approach, the
glideslope receiver is automatically tuned.
Like the localizer, the glideslope transmits
two signals, one modulated at 90 Hz and
the other modulated at 150 Hz. The
aircraft’s glideslope receiver deciphers the
signals similar to the method of the localizer
receiver. It drives a vertical course deviation
indicator known as the glideslope indicator.
The glideslope indicator operates identically
to the localizer CDI only 90° to it. The
VOR/ILS localizer CDI and the glideslope
are displayed together on whichever kind of
instrumentation is in the aircraft.
- Compass Locators
o It is imperative that a pilot be able to
intercept the ILS to enable its use. A
compass locator is a transmitter designed
for this purpose. There is typically one
● Distance Measuring Equipment (DME)
located at the outer marker beacon 4–7
Many VOR stations are co-located with the
miles from the runway threshold. Another
military version of the VOR station, which is
may be located at the middle marker
known as TACAN. When this occurs, the
beacon about 3,500 feet from the threshold.
navigation station is known as a VORTAC
o The outer marker compass locator is a 25
station. Civilian aircraft make use of one of the
watt NDB with a range of about 15 miles. It
TACAN features not originally installed at
transmits omnidirectional LF radio waves
civilian VOR stations called DME
(190 Hz to 535 Hz) keyed with the first two
A DME system calculates the distance from
letters of the ILS identifier.If a middle marker
the aircraft to the DME unit at the VORTAC
compass locator is in place, it is similar but
ground station and displays it on the flight
is identified with the last two letters of the
deck. It can also display calculated aircraft
 speed and elapsed time for arrival when the is imminent, the TCAS II computer issues a
aircraft is traveling to the station. Resolution Advisory (RA). This is an aural
● Area Navigation (RNAV) command to the pilot to take a specific evasive
Area navigation (RNAV) is a general term used action (i.e., DESCEND). The computer is
to describe the navigation from point A to point programmed such that the pilot in the
B without direct over flight of navigational aids, encroaching aircraft receives an RA for
such as VOR stations or ADF nondirectional evasive action in the opposite direction (if it is
beacons. It includes VORTAC and VOR/DME TCAS II equipped).
based systems, as well as systems of RNAV
based around LORAN, GPS, INS, and the
FMS of transport category aircraft.
All RNAV systems make use of waypoints. A
waypoint is a designated geographical location
or point used for route definition or
progress-reporting purposes. It can be defined
or described by using latitude/longitude grid
coordinates or, in the case of VOR based
RNAV, described as a point on a VOR radial
followed by that point’s distance from the VOR
station (i.e., 200/25 means a point 25 nautical
miles from the VOR station on the 200° radial)
● Radar Beacon Transponder ● Radio Altimeter
A radar beacon transponder, or simply, a A radio altimeter, or radar altimeter, is used to
transponder, provides positive identification measure the distance from the aircraft to the
and location of an aircraft on the radar screens terrain directly beneath it. It is used primarily
of ATC. For each aircraft equipped with an during instrument approach and low level or
altitude encoder, the transponder also provides night flight below 2500 feet. The radio altimeter
the pressure altitude of the aircraft to be supplies the primary altitude information for
displayed adjacent to the on-screen blip that landing decision height.
represents the aircraft. It incorporates an adjustable altitude bug that
Radar capabilities at airports vary. Generally, creates a visual or aural warning to the pilot
two types of radar are used by air traffic control when the aircraft reaches that altitude.
(ATC). The primary radar transmits directional Typically, the pilot will abort a landing if the
UHF or SHF radio waves sequentially in all decision height is reached and the runway is
directions. not visible
● Traffic Collision Avoidance System (TCAS) A radar altimeter is more accurate and
Traffic collision avoidance systems (TCAS) are responsive than an air pressure altimeter for
transponder based air-to-air traffic monitoring AGL information at low altitudes. The
and alerting systems. There are two classes of transceiver is usually located remotely from the
TCAS. TCAS I was developed to indicator. Multifunctional and glass cockpit
accommodate the general aviation community displays typically integrate decision height
and regional airlines. This system identifies awareness from the radar altimeter as a digital
traffic in a 35–40 mile range of the aircraft and number displayed on the screen with a bug,
issues Traffic Advisories (TA) to assist pilots in light, or color change used to indicate when
visual acquisition of intruder aircraft. TCAS I is that altitude is reached. Large aircraft may
mandated on aircraft with 10 to 30 seats. incorporate radio altimeter information into a
TCAS II is a more sophisticated system. It is ground proximity warning system (GPWS)
required internationally in aircraft with more which aurally alerts the crew of potentially
than 30 seats or weighing more than 15,000 dangerous proximity to the terrain below the
kg. TCAS II provides the information of TCAS aircraft.
I, but also analyzes the projected flightpath of ● Weather Radar
approaching aircraft. If a collision or near miss
 There are three common types of weather ● Long Range Aid to Navigation System
aids used in an aircraft flight deck that are (LORAN)
often referred to as weather radar: Long range aid to navigation system (LORAN)
1. Actual on-board radar for detecting and is a type of RNAV that is no longer available in
displaying weather activity the United States. It was developed during
2. Lightning detectors World War II, and the most recent edition,
3. Satellite or other source weather radar LORAN-C, has been very useful and accurate
information that is uploaded to the aircraft to aviators as well as maritime sailors. LORAN
from an outside source uses radio wave pulses from a series of towers
and an on-board receiver/computer to
positively locate an aircraft amid the tower
network.
There are twelve LORAN transmitter tower
“chains” constructed across North America.
● Emergency Locator Transmitter (ELT) Each chain has a master transmitter tower and
An emergency locator transmitter (ELT) is an a handful of secondary towers. All broadcasts
independent battery powered transmitter from the transmitters are at the same
activated by the excessive G-forces frequency, 100 KHz. Therefore, a LORAN
experienced during a crash. It transmits a receiver does not need to be tuned.
digital signal every 50 seconds on a frequency
of 406.025 MHz at 5 watts for at least 24
hours. The signal is received anywhere in the
world by satellites in the COSPAS-SARSAT
satellite system
Two types of satellites, low earth orbiting
(LEOSATs) and geostationary satellites
(GEOSATs) are used with different,
● Global Positioning System (GPS)
complimentary capability. The signal is partially
Global positioning system navigation (GPS) is
processed and stored in the satellites and then
the fastest growing type of navigation in
relayed to ground stations known as local user
aviation. It is accomplished through the use of
terminals (LUTs). Further deciphering of a
NAVSTAR satellites set and maintained in orbit
signal takes place at the LUTs, and appropriate
around the earth by the U.S. Government.
search and rescue operations are notified
Continuous coded transmissions from the
through mission control centers (MCCs) set up
satellites facilitate locating the position of an
for this purpose
aircraft equipped with a GPS receiver with
extreme accuracy
There are three segments of GPS: the space
segment, the control segment, and the user
segment. Aircraft technicians are only involved
● COSPAS-SARSAT SYSTEM with user segment equipment such as GPS
receivers, displays, and antennas.
● Wide Area Augmentation (WAAS)
To increase the accuracy of GPS for aircraft
navigation, the wide area augmentation
system (WAAS) was developed. It consists of
approximately 25 precisely surveyed ground
stations that receive GPS signals and
ultimately transmit correction information to the
aircraft.
● Inertial Navigation System (INS)
An inertial navigation system (INS) is used on
some large aircraft for long range navigation.
This may also be identified as an inertial
reference system (IRS), although the IRS
designation is generally reserved for more
modern systems. An INS/IRS is a
self-contained system that does not require
input radio signals from a ground navigation
facility or transmitter
The system derives attitude, velocity, and
direction information from measurement of the
aircraft’s accelerations given a known starting
point. The location of the aircraft is
continuously updated through calculations
based on the forces experienced by INS
accelerometers. A minimum of two
accelerometers is used, one referenced to
north, and one referenced to east.
An inertial navigation system (INS) is used on
some large aircraft for long range navigation.
This may also be identified as an inertial
reference system (IRS), although the IRS
designation is generally reserved for more
modern systems. An INS/IRS is a
self-contained system that does not require
input radio signals from a ground navigation
facility or transmitter
• The system derives attitude, velocity, and
direction information from measurement of the
aircraft’s accelerations given a known starting
point. The location of the aircraft is
continuously updated through calculations
based on the forces experienced by INS
accelerometers. A minimum of two
accelerometers is used, one referenced to
north, and one referenced to east.