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Communication System


Communication in aviation is currently mainly achieved by voice modulation of radio waves. The future seems to lie in data transfer, which can be achieved without using the human voice. Nonetheless, voice communication is still important for the safe movement of air traffic, and will remain so in many parts of the world for sometime.


Long Range Communication - Choice of Frequency

To achieve communication on the basis of global distances, the choice traditionally lay in the band between VHF and HF the frequency band above HF being limited to direct wave, or 'line-of-sight' propagation. Although these higher frequency bands can now be used in association with satellite technology, many parts of the world still require this traditional means of communication.

Starting at the lowest end, we could obtain very long ranges in the VLF and LF bands and settle for them without further ado, but there are some inherent disadvantages in the employment of these bands. Just the two requirements,of aerial size and power alone, are sufficiently forbidding to spur researchers to investigate alternative possibilities.

These possibilities are MF and HF. Of these two, HF is considered to be far superior. 

  • Aerials are shorter and less expensive to install.

  • Static noise is less than in MF and tolerable.

  • By using sky waves day and night , very long range are obtained for relatively less power.

  • Higher frequency suffer less attenuation in the ionosphere. 

  • efficiency is further increased by radiating in the direction of receiver .

HF communication

The principle of effective HF communication relies on choosing frequency appropriate for a given set of atmospheric condition that will produce the return and the required distance from the transmitter. If the height of the reflecting layer is known, the signals path from the transmitted to the receiver via the ionosphere can be plotted and, from this the angle of incidence the signal makes at the ionosphere can be calculated. An operator can use the angle of incidence to find the frequencies whose critical angle that equates to. That frequency is the maximum and usable frequency for a given communication the estimated range, given the prevailing atmospheric conditions.

If we use a frequency higher than this maximum usable frequency, the signal will return beyond the receiver. At the maximum usable frequency itself, any ionospheric disturbance may increase the skip distance and cause the signal to be lost, so a slightly lower frequency is used. As we lower the frequency, attenuation increases and we need more transmitter power to produce an acceptable signal, until we are unable to produce enough power. When this limit is reached, we have reached the maximum usable frequency (LUHF or Lowest Usable High Frequency).

In practice, graphs and nomograms are made available to the radio stations from which these values are directly extracted . The graph takes into consideration such factors such as the station's position in latitude and longitude, time of the day, density of the ionosphere and any abnormal conditions prevailing, and the distance at which sky returns is required. Now a days computers make this calculation, and can automatically select the optimum frequency for communication between the aircraft and any required ground station.

Because of the diurnal variation in the ionosphere density, if transmitter is continued at night on a daytime frequency, a longer skip distance will result, leaving the receiver in the 'dead space'. This is because at night the electron density decreases; signal travels higher in the ionosphere before refraction, and is refracted less. For these reasons the working frequency is lower at night. his lowering of the frequency adjusts the skip distance because the lower frequency are refracted more, and at lower levels. Attenuation is also less, despite the lower frequency, because the electron density is less.


The HF frequency band allocated to commercial aviation range from 2 MHz to 22 MHz, but in practice only upto

18 MHz is used. The Aeronautical Information Publication (AIP) of each country list the Air Traffic Control Centre (ATCC) or Area Control Centre (ACC) ground station with the frequencies available which the aircraft can use to communicate with them.

The transmission are Amplitude Modulated (AM) and a Signal Sideband (SSB) emission, coded J3E, is used to economise on power and bandwidth or channel space.

In the early days when MF and HF w/t was in the forefront, aircraft were equipped with a trailing aerial. It consisted of a coil of wire which was wound out and held downwards by a weight. Normally it disappeared at the first sight of a thunderstorm, either by the pilot for safety or in the turbulence. In another system, a permanently fixed wire was used, stretching along the length of the fuselage. These aerials have now been replaced with recessed aerials electronically adjusted and conveniently located to give all-round reception to ground stations. To give an indication of power required, a mere 100 W transmitter can provide transatlantic voice communication

Factor affecting HF Range

  • Transmission Power

  • Time of the day: this affects the electron density

  • Any disturbance in the Ionosphere (solar flare,etc)

  • Geographical Location

  • Frequency in use; this determines the critical angle and the depth of ionospheric penetration.

HF Data Link

HF Data Link (HFDL) is a facility used in Oceanic Control to send and receiver information over normal HF frequencies, using the upper sideband of the selected frequencies. The signal is phase modulated to send digital information. Modern equipment converts voice signals into similar digital information, and vice versa, to provide digital voice communication. 


The advantage claimed for digital HF, weather data or voice, include more rapid initial establishment of the communication link because of the automatic frequency selection. Once established, the link can be maintained continuously without the crew member constantly having to make transmissions, which allows message to pass quickly. A major benefit is that voice signal clarity is generally improved by converting the message into digital form.

With the advent of satellite communication HF is loosing its importance  in the oceanic flights. However, route over and close to the North Pole, which are outside the cover of geostationary satellites, are becoming more common. HF communication, by voice and data link, are likely to remain vital in such areas. Communications computer can be control all the radio in the aircraft, and while receiving signal from all of them, can select automatically the most useful method of sending whatever the crew or the Flight Management System computer wishes to send.

Short Range Communication - Choice of Frequency

As there are requirements to provide short range communication out to 80 Nm range at 5000 ft and 200 Nm at 20,000 ft, frequency band from VLH to HF, with their disadvantages of of complexity and static interference are not required. The VHF band provides a practical facility.  


VHF Communication

The VHF band is chosen for RTR communication at short ranges the operating frequencies being kept at the lower end of the band, 118 MHz to 136 MHz. Within this band communications channel are available at 8 kHz (8.33 kHz recurring) separation, although older equipments are still available at 25 kHz or even 50 kHz separation. 

The transmission is Amplitude Modulated (AM), type of emission being A3E. A transmitter producing 20 W power would be considered adequate for the intended range.

VHF is practically free from static, but being vertically polarised the receiver aerials do pick up some background noise. If absolute clarity of the reception were required, a frequency modulated UHF signal could provide that, but the equipment would become more complex and expensive.

Frequency Allocation

The highest frequency in the band, from 136.900 to 136.975 MHz are reserved for data link purposes. Originally, VHF frequencies were allocated at 100 kHz spacing. The spacing progressively reduced through 50 kHz, to 25 kHz, and finally to 8.33 kHz.

Factor affecting VHF range

  • Transmitter power both at aircraft and ground station

  • Height of the Transmitter

  • Height of the Receiver

  • Obstacles at or near the transmission site will block the signal or scatter them with inevitable attenuation

  • Any obstruction in the line-of-sight between aircraft and the ground station will have similar effects

  • In certain circumstances the aircraft may receive both direct and reflected waves which may cause fading or even short-term loss of communication.

Selective Calling System (SELCAL)

Pilots on long-haul flights used to have to listen to the radio all the time, waiting for their own callsign to alert them to a message for them. This was tiring, especially on HF frequencies with a lot of static noise as well as receiver noise. The SELCAL system allows the pilot to mute the receiver until ATC transmits a group of two pulses. These pulses are designated 'REDx', where x is a letter corresponding to the audio frequency of the pulse transmitted as a modulation on the carrier frequency. Each code is allocated to a specific aircraft listening frequency. When the relevant code is received, it activates an alarm in the cockpit, either a light or a bell or both, telling the crew to deselect the the mute function and use normal communication.

Audio Integration System (AIS)

The function of the Audio Integrated system is to provide an interface between the pilot's mic. & tel. and the selected receiver & transmitter. It provides provision for selection of radio system audio outputs and inputs and crew intercommunications. 

Audio Integrated System comprises of following services:- 

  • Flight Interphone: It allows flight deck crew to communicate with each other or with ground stations. 

  • Cabin Interphone: It allows flight deck and cabin crew to communicate. 

  • Service Interphone: It allows ground staff to communicate with each other and also with the flight crew. 

  • Passenger Address (PA): It allows announcements to be made by the crew to the passengers. 

  • Inflight Passenger Entertainment System (IFES): It allows the showing of movies and the piping of music. 

  • Ground Crew Call System: allows flight and ground crew to attract each other's attention. 

  • Cockpit Voice Recorder: It meets regulatory requirements for the recording of flight crew audio for subsequent accident investigation if necessary. 

Control Panel

There are two control panel associated with HF, VFH and audio integration system

  • Frequency Select Panel

  • Radio Tuning Panel

Frequency Select Panel

This panel is use to select the frequency on which transmission is to be done. It also allows us to select the standby frequency, which can be transferred to the active frequency by the use to Transfer switch. The frequency can be selected by using a rotating knob or number keys, depending on the type of system used.

The VHF or HF system can also be selected using the panel

Frequency Select Panel.jpg


Radio Tuning Panel

The radio tuning panel is use for mic and speaker selection of all the communication system fitted in the aircraft. Only one mic can be selected on at a time but all the speakers can be selected on together. The speaker switches are potentiometer, which also control the intensity of the sound.

Radio Tuning Panel.jpg


Satellite Communication (SATCOM)

Aviation uses satellites constellation INMARSAT. These Satellites are positioned in 'geostationary' orbits very high over the equator, and provide communications by accepting transmission of digital signals in the 6 GHz band. The signal from the satellite covers the whole world of the earth between 80 deg North and 80 deg South.

These signals are virtually unaffected by meteorological conditions or static. However, special aerials are required for transmission and receptions on the frequencies. The satellite do not reflect the signals; they receive then and re-transmit them on different frequencies, so reducing the attenuation of the signal. Those re-transmitted to ground station are sent in the 4 GHz band, those to aircraft in the 1.5 GHz band.

Ground Station are positioned in a network so that they service each other of the four satellite regions or 'segments' and link into the conventional public and private telephone networks. This means that a pilot using the system is effectively using an ordinary telephone, as do his passengers from their seats.

The aircraft satcom receiver operates on frequencies between 1544 and 1555 (ideally up to 1559) MHz. The aircraft SATCOM transmitters use frequencies between 1626.5 and 1660.5 MHz in ideal conditions, but generally between 1645.5 and 1656.5 MHz. Voice message are digitised by equipment using specific algorithms, which are laid down in ICAO Annex 10.

Search and Rescue Satellites

A further use of the satellite constellations is for search and rescue. All the INMARSAT satellite list constantly for signals on the international emergency frequencies, and can alert SAR centres to emergency beacons carried by survivors. The earlier but still functioning international COSPAS-SARSAT system is dedicated to the provision of search and rescue facilities, and use a different system of four polar orbiting satellites to cover all the globe. Cospas is the Russian name, Sarsat the US name for this joint venture.

One of these satellites can receive signals transmitted at 121.5 MHz, for example from a survivor's Personal Locator Beacon (PLB). The satellite re-transmits the signal to a ground station called a Local User Terminal or LUT, where the exact  frequency received is measured and compared with the datum 121.5 MHz. The difference is the Doppler shift. That Doppler shift will only be the equivalent of the satellite's path. Any difference means that the transmitter is to onside of the path. The maximum change comes as the satellite passes abeam the transmitter, and the variation of Doppler shift gives an indication of the lateral distance from the satellite's path. Any difference means the satellite passes abeam the transmitter, and the variation of Doppler shift gives an indication the lateral distance from the satellite's path, so search area can be calculated.


Signal on 121.5 MHz can only be re-transmitted to LUT's which are in line-of-sight from the satellite. Signals from transmitter using 406.025 MHz, the international UHF search and rescue frequency, are sent as digital data streams, which include an individual identification signal. The data streams can be stored in the satellite for future transmissions to a LUT, even through none is in line-of-sight when the original message is received, For this reason, 406 MHz emergency position indicating radio beacon (EPIRB's) are preferred for ocean voyages and flights.


Aircraft Communication Addressing and Reporting System (ACARS)

The Aircraft Communication Addressing and Reporting System (ACARS) is another system designed to reduce pilot workload in airlines. Much of the communication on airlines radios used to be con company frequencies, passing information about aircrafts systems serviceability, crew and passenger requirements, fuel state and requirements, and many more other routine messages. As aircraft became larger and more complicated these messages increased, usually requiring transmission during periods of high cockpit workload such as descent into the destination.

With the advent of Flight Management System (FMS), most information which might need transmission already exists in digital form on the aircraft's computers. The ACARS can send that information from FMS computer on the ground. The crew can prepare their messages using the keyboard and scratchpads on the Control and Display Unit (CDU) if required, but many transmissions are automatic, requiring no extra workload on the flight crew. The ground computer can also send message to the FMS for display on the scratchpad of the CDU. Information from other computer no the aircraft can also be sent, allowing ground engineers to monitor the aircraft systems while it is in flight, and arrange maintenance.

The ACARS can be compared to a facsimile machine. The data message can be delayed automatically until the frequency is vacant.  It is compressed, so uses less time than voice message. The ACARS equipment acknowledges messages automatically and many aircrafts have a printer to produce hard copy of the messages.


The ACARS uses a normal aircraft VHF radio set to send its signals, Pulse Modulating (PM) the carrier to send digital signals. Usually such a set is dedicated to ACARS, but sometimes its use may be shared between the ACARS and normal communications by use of a VOICE/DATA switch. Frequencies 136.900 to 136.975 are reserved for datalink communications, but any frequency between 118.000 to 136.975 itself may be used at a frequency separation of 25 kHz. The frequency of 136.975 itself is reserved as a worldwide common signalling channel to announce the availability of VHF data link services by a particular transmitter.

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