COMMUNICATION IN AVIATION
(STUDY MATERIAL FOR CPL/ ATPL)
Communication in aviation is mainly achieved by voice modulation of radio waves. The future however, seems to be in data transfer, which can be achieved without using the human voice. Not withstanding that, for the safe movement of air traffic, voice communication is still important.
Choice of Frequency Band for Long-Range Communication
To achieve communication globally which involved large distances, the choice lay in the bands between VLF and HF. The frequency bands above HF being limited to direct wave and thus less range, or ‘line-of-sight’ propagation. Although these higher frequency bands are now been in 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 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. The reasons are:
- Aerials are shorter and less expensive to install.
- Static noise is less than in MF and tolerable.
- By using sky waves by day and night, very long ranges are obtained for relatively less power.
- Higher frequencies suffer less attenuation in the ionosphere.
- Efficiency is further increased by beaming the radiation in the direction of the receiver.
The principle of efficient HF communication relies on choosing a frequency appropriate for a given set of ionospheric conditions that will produce the first return at the required skip distance from the transmitter. If the height of the refracting layer is known, the signal’s path from the transmitter 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 frequency whose critical angle that equates to. That frequency is the maximum usable frequency, which will give communication at the estimated range, given the prevailing ionospheric 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 minimum 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 graphs take into consideration such factors as the station’s position in latitude and longitude, time of the day, density of the ionosphere and any abnormal condition prevailing, and the distance at which the first sky return is required. Nowadays, of course, computers make the calculations, and can automatically select the optimum frequency for communication between the aircraft and any required ground station.
Because of the diurnal variation in the ionospheric density, if transmission 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, as we saw in the previous chapter, the electron density decreases and the signal travels higher in the ionosphere before refraction, and is refracted less. For these reasons, the working frequency is lowered at night. This lowering of the frequency adjusts the skip distance because the lower frequencies are refracted from lower levels and require smaller critical angles. Despite the lower frequency the attenuation is less because the electron density is less. In practice the night time frequencies are approximately half of the day time values.
The HF frequency band allocated to commercial aviation ranges from 2 MHz to 22 MHz, but in practice it is only used up to around 18 MHz. The Flight Information Publication (FLlP) lists each Air Traffic Control Centre (ATCC) or Area Control Centre (ACC) ground station with the frequencies available, which aircraft can use to communicate with them. The transmissions are amplitude modulated and a single sideband (SSB) emission, coded J3E, is used to economise on power and bandwidth or channel space.
In the early days when MF and HF wireless telephony 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 by recessed aerials electronically adjusted and conveniently located to give all-round reception from the ground stations. To give an indication of power required, a mere 100 W transmitter can provide transatlantic voice communication.
Factors Affecting HF Range. The factors affecting HF range are:
- Transmission power.
- Time of day, as it affects the electron density.
- Season of the year also affects the electron density.
- Any disturbances in the ionosphere (solar flares, etc.).
- Geographical location.
- Frequency in use which determines the critical angle and the depth of ionospheric penetration.
High Frequency Data Link (HFDL) is an ACARS communications media used to exchange data such as Aeronautical Operational Control (AOC) messages, Controller Pilot Data Link Communications (CPDLC) messages and Automated Dependent Surveillance (ADS) messages between aircraft end-systems and corresponding ground-based HFDL ground stations. Using the unique propagation characteristics of HF Radio Waves, the ground stations provide data link communications to properly equipped aircraft operating anywhere in the world. The result, pilots can always communicate with someone on the ground. Today, HFDL is an air/ground data link standard with coverage in virtually every corner of the globe, approximately 168,000,000 square miles (440,000,000 km2) where aircraft are never out of touch both in the air and on the ground.
HF Datalink (HFDL) is a facility used in Oceanic Control to send and receive information over normal HF frequencies, using the upper sideband of the selected frequency. The signal is phase modulated to send digital information. Modern equipment converts voice signals into similar digital information (like a digital mobile telephone), and vice versa, to provide digital voice communications.
The advantages claimed for digital HF, whether data or voice, include more rapid initial establishment of the communications link because of the automatic frequency selection. Once established, the link can be maintained continuously without a crew member constantly having to make transmissions, which allows messages to be passed quickly. A major benefit is that voice signal clarity is greatly improved by converting the message into digital form. Due to the digital nature of HFDL, it uses between 1/3 and 1/2 of the bandwidth that voice requires, so data can continue to be decoded when voice is unusable. This was proven during the 2003 Halloween Solar Storm when aircraft were still using HFDL on polar routes when voice was unusable.
There are around 15 HF ground stations (HGS) available today, and, like a canopy within a jungle, the stations provide overlap and redundancy in the unlikely event of a HGS failure. These 15 stations provide nearly complete global coverage, including both poles, and system availability is 100 percent. The design of the system allows for 4 channels per ground station. Currently, 13 stations are only using ½ of the designed capacity. 2 others are using 3 of the 4 designed channels. Stations are actively monitored for traffic load and can determine when additional channels needs to be added by the service providers.
With the advent of satellite communications, HF is losing its importance in oceanic flight. However, routes over and close to the North Pole, which are outside the cover of geostationary satellites, are becoming more common. HF communication, by voice and datalink, are likely to remain vital in such areas. Communications computers can control all the radios in an aircraft, and while receiving signals from all of them, can select automatically the most useful method of sending whatever signal the crew or the aircraft flight management computer wishes to send.
Choice of Frequency Band for Short-Range Communication
There is a requirement to provide communication out to 80 nm range at 5000 ft, and 200 nm at 20000 ft. As these are very short ranges, frequency bands from VLF to HF, with their disadvantages of complexity and static interference, are not necessary. The VHF band provides a practical facility. At frequencies above VHF, aerial requirements become more complicated. The signal strength received by a simple antenna at a given range is proportional to the wavelength. Thus a longer wavelength (lower frequency) will give better reception.
The VHF band is chosen for RTF communication at short ranges, the operating frequencies being kept at the lower end of the band, 117.975 MHz to 137.000 MHz. Within this band, communications channels are available at 8 kHz (actually 8.33 recurring) separation, although older equipments are still available at 25 kHz or even 50 kHz separation. The transmission is amplitude modulated, the type of emission being A3E. A transmitter producing 20 W power would be considered adequate for the intended ranges. VHF is practically free from static, but being vertically polarised the receiver aerials do pick up some background noise. If absolute clarity of reception were required, a frequency modulated UHF signal could provide that, but the equipment would become more complex and expensive.
The highest frequencies in the band, from 136.900 to 136.975 MHz, are reserved for datalink purposes. Originally, VHF frequencies were allocated at 100 kHz spacing. The spacing progressively reduced through 50 kHz to 25 kHz, and finally, at the time of writing, to 8.3 recurring kHz. Older radios were kept in service as the frequency spacing reduced, and most ATS frequencies outside controlled airspace were still allocated at 50 kHz spacing. Frequencies in controlled airspace were allocated at 25 kHz spacing, but mainly at high levels.
The newer 8.33 kHz frequency spacing was introduced in 2000 in the most congested airspace in Europe, above Flight Level (FL) 245. It was not possible to make old radio receivers compatible with the new frequency spacing, because they had been designed with broad bandwidths to accept signals 7.5 kHz removed from the basic frequencies. This was to allow single frequency operation from different transmitters along airways. This meant that totally new radios had to be developed. Unfortunately, the new radios are not totally compatible with the old ones used for ground stations at small aerodromes. The new airborne radios have to be used with both types of ground station, so the simplest method of doing that is to have two separate receivers inside the aircraft radio sets, and a means of switching between the two receivers.
Factors Affecting VHF Range
The formula for calculating the maximum range of a VHF signal is: D = 1.25 √ HT + 1.25 √ HR. Therefore, the factors affecting the range of a VHF transmission are as follows:
- Height of the transmitter.
- Height of the receiver.
- Transmission power both at aircraft and ground station.
- Obstacles at or near the transmission site which will block the signals or scatter with inevitable attenuation.
- Any obstruction in the line-of-sight between the aircraft and the ground station will have a similar effect to that above.
- 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 radios 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 as well as receiver noise. The SELCAL system allows pilots to mute the receiver until ATC transmits a group of two pulses. These pulses are designated ‘RED x’, where x is a letter corresponding to the audio frequency of the pulses transmitted as a modulation on the carrier frequency. Each code is allocated to a specific aircraft listening on the 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 de-select the mute function and use normal communications.
There are restrictions on the use of SELCAL. It can only be used if all the following conditions are fulfilled:
- The ground station is notified as capable of transmitting SELCAL codes.
- The pilot informs the ground station that he intends to use SELCAL, and informs them of his codes.
- The ground station does not raise any objection to the use of SELCAL.
- A preflight functional check must be carried out satisfactorily.
- If the serviceability is suspect, listening watch must be resumed.
Internal Communications (INTERCOM)
Most aircraft communications systems include an intercom facility. This basically consists of an amplifier, which directly amplifies the input from each crew member’s microphone. Intercom signals can therefore be received in every other crew member’s headset, or a loudspeaker, at a similar strength to those amplified from external radio waves. Because there are many external signal inputs coming into most cockpits, it is usual to combine the intercom system with all the other inputs in an audio control console. In this console, all the received signals from the radios and navigation aids may be selected for listening independently as required. The volume controls on each individual control unit determine the actual volume of each signal in the pilot’s headset.
Satellite Communications (SATCOM)
SATCOM or Satcom may refer to:
- Short for Satellite Communications and used frequently in the context of VSAT (Very Small Aperture Terminal)
- Communication Satellites or comsats
- Satcom (Satellite), one of the earliest geostationary communications satellites
- Used by some airliners to transmit ACARS messages
- Generic term for mobile telephony via satellite (i.e., aircraft and watercraft)
Although once a novelty, we now have satellite communications in many homes, giving us television pictures and sound. Aviation also uses satellites for communications, mainly via the International Maritime Organisation constellation INMARSAT. These satellites are positioned in ‘geostationary’ orbits very high over the equator, and provide communications by accepting transmissions of digital signals in the 6 GHz band. The signals from the satellites cover the whole of the earth between 80° North and 80° South.
What are the requirements for a satellite to be geostationary?
1. Its revolutionary direction must be same as that of the earth, i.e. from west to east.
2. The time period of satellite’s revolution must be same to the time period of the rotation of earth along its polar axis, which is equal to 24 hours.
3. The equatorial plane of earth must be coplanar with the orbital plane of the satellites revolution.
The name given to the orbit of the geostationary satellites is synchronous orbit. Due to this geostationary satellites are also called as geo-synchronous satellites. Geo-synchronous orbit is at a height of nearly 36000 km from the surface of earth.
These orbits are capable of giving a successful communication link between two stations present on the earth. These satellites can handle communication up to large distances. But it is impossible for a single geostationary satellite to cover the whole earth and provide a communication link. Due to curvature of earth the stations will be out of sight after covering some distance. If we want to cover the whole earth then we have to put three satellites onto the geosynchronous orbit. These satellites can cover the earth if all are inclined at an angle of 120o to each other.
These signals are virtually unaffected by meteorological conditions or static. However, special aerials are required for transmission and reception on these frequencies. The satellites do not reflect the signals but instead they receive them and re-transmit them on different frequencies, thereby reducing the attenuation of the signal. Those re-transmitted to ground stations are sent in the 4 GHz band and those to aircraft in the 1.5 GHz band.
Ground stations are positioned in a network so that they service each 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 receivers operate 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 messages are digitised by the equipment using specific algorithms.
Search and Rescue Satellites
A further use of the satellite constellation is for search and rescue. All the INMARSAT satellites listen 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 uses 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.
The International Cospas-Sarsat Programme is a satellite-based search and rescue (SAR) distress alert detection and information distribution system, established by Canada , France, the US, and the former Soviet Union in 1979. It is best known as the system that detects and locates emergency beacons activated by aircraft, ships and backcountry hikers in distress. Over the years many countries have joined the project, either as providers of ground segments or as user states. As of 2011, 26 country or regional governments (Algeria, Argentina, Australia, Brazil, Chile, People’s Republic of China, Greece, Hong Kong, India, Indonesia, Italy, Japan, Republic of Korea, New Zealand, Nigeria, Norway, Pakistan, Peru, Saudi Arabia, Singapore, South Africa, Spain, Thailand, Turkey, United Arab Emirates, United Kingdom, Vietnam) and one organization (Chunghwa Telecom) are providers of ground segments, while 11 countries are user states (Cyprus, Denmark, Finland, Germany, Madagascar, Netherlands, Poland, Serbia, Sweden, Switzerland, Tunisia). The Secretariat of the International Cospas-Sarsat Programme is based in Montréal, Québec, Canada, and is headed by Steven Lett of the United States.
The system consists of a ground segment and a space segment:
- Distress Radio Beacons to be activated in a life-threatening emergency
- SAR signal repeaters (SARR) and SAR signal processors (SARP) aboard satellites
- Satellite downlink receiving and signal processing stations called LUTs (local user terminals)
- Mission Control centres that distribute to Rescue Coordination Centres distress alert data (particularly beacon location data) generated by the LUTs
- Rescue Coordination Centres that facilitate coordination of the SAR agency and personnel response to a distress situation.
The space segment of the Cospas-Sarsat system currently consists of SARR instruments aboard seven geosynchronous satellites called GEOSARs, and SARR and SARP instruments aboard five low-earth polar orbit satellites called LEOSARs.
One of these satellites can receive signals transmitted at 121.5 MHz, for example from a survivor’s Personal Locator Beacon (PLB). The satellite retransmits 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 (explained in the chapter on Doppler radar). That Doppler shift will only be the equivalent of the satellite’s velocity, if the transmitter is directly below the satellite’s path. Any difference means the transmitter is to one side of the path. The maximum difference 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, so a search area can be calculated.
Signals on 121.5 MHz can only be re-transmitted to LUTs which are in line-of-sight from the satellite. Signals from transmitters 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 transmission to a LUT, even though none is in line-of-sight when the original message is received. For this reason, 406 MHz emergency position indicating radio beacons (EPIRBs) are preferred for ocean voyages and flights.
Starting on 1 February 2009, the Cospas-Sarsat System stopped processing signals from the older 121.5 MHz and 243 MHz beacons. Now only signals from 406 MHz beacons are processed. The switch to 406 MHz is expected to result in a substantial reduction in wasted use of SAR resources on false alerts while simultaneously increasing the responsiveness of the system for real distress cases.
Despite the above, many modern radio beacons continue to transmit a homing signal on 121.5 MHz or 243 MHz (in addition to 406 MHz). While no longer detected by satellite, this signal may be used to aid direction finding efforts by local search and rescue teams, after first receiving a distress signal on 406 MHz and navigating to within a sufficiently close range.
The Aircraft Communications Addressing and Reporting System (ACARS)
ACARS as a term refers to the complete air and ground system, consisting of equipment on board, equipment on the ground, and a service provider.
The Aircraft Communications Addressing and Reporting System, known as ACARS, is the air-to-ground data communications infrastructure hundreds of airlines around the world use to communicate with air traffic control, national aviation authorities and their own operations centers. Airlines use ACARS VHF and HF ground stations as well as satellite communications to send and receive billions of Air Traffic Control (ATC) and Airline Operational Control (AOC) messages every year.
Depending on where the aircraft is and its equipage (Equipment for a particular purpose), ACARS messages are routed through a global network of thousands of ground stations or satellite constellations that cover the earth. When the aircraft is over land a network of VHF stations, including VDLM2 that is 10x the speed of traditional VHF, route and deliver ACARS messages. Over the ocean a message can be delivered via HFDL ground stations, Inmarsat satellite communications or Iridium satellite communications. If the aircraft is over either of the poles it can use HFDL or Iridium.
AOC messages include take-off and landing confirmation, weather information, gate information, and engine reports. ATC messages include navigation information, aircraft positional reporting, departure clearances, oceanic clearances, runway conditions, and weather data. Currently AOC messages take up 80% of ACARS network traffic versus 20% for ATC, but the FAA NextGen program will shift more and more voice communications to data in the near future.
Equipage varies depending on the age and chief use of the aircraft. Typically domestic aircraft have only traditional ACARS (VHF) capabilities and one long range option. Nearly all aircraft produced before 2000, domestic or long-haul, have only traditional ACARS. Aircraft produced today however are equipped to support traditional VHF and VDLM2. Most all are equipped for either Inmarsat or Iridium, but not both.
The Aircraft Communications Addressing and Reporting System (ACARS) is another system designed to reduce pilot workload in airliners. Much of the communication on airliner radios used to be on company frequencies, passing information about aircraft system serviceability, crew and passenger requirements, fuel state and requirements, and many other routine messages. As aircraft became larger and more complicated, these messages increased, usually requiring transmission during periods of high cockpit workload such as the descent into the destination.
With the advent of Flight Management Systems (FMS) , most information which might need transmission already exists in digital form on the aircraft’s computers. The ACARS can send that information from the FMS computer to computers on the ground. The crew can prepare their messages using the keyboard and scratchpad 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 messages to the FMS for display on the scratchpad of the CDU. Information from other computers on 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 (fax) machine. A data message can be delayed automatically until the frequency is vacant. It is compressed, so uses less time than a voice message. The ACARS equipment acknowledges messages automatically, and many aircraft 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 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 and 136.975 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 datalink services by a particular transmitter.
(A) FIS Book on Avionics