Aeronautical Telecommunication Services

Every radio call a pilot makes rests on an international framework that the pilot rarely sees. The frequencies are allocated worldwide by the International Telecommunication Union (ITU); the equipment, procedures and networks are standardised by ICAO in Annex 10 to the Chicago Convention; and States license the aircraft radio stations and the people who operate them. Aeronautical telecommunication services is the collective name for what runs inside that framework: the radio links between aircraft and the ground, the networks between ground units, the radio navigation aids and the broadcasts.

The EASA Communications syllabus tests the definitions almost word for word: aeronautical station, aircraft station, air-ground communication, AFTN. The framework also explains why the same procedures work in any country, and where the next generation of communication, navigation and surveillance systems is taking them.

ICAO Annex 10 and the ITU

The International Telecommunication Union (ITU), the United Nations agency for telecommunications, divides the radio spectrum among services in its Radio Regulations, so that the bands used by aviation are protected from other users worldwide. The ITU also allocates each State a series of radio call sign prefixes, and an aircraft's nationality mark is chosen from the series allocated to its State of Registry: the G of the United Kingdom and the N of the United States come from there.

Within those bands, ICAO Annex 10, Aeronautical Telecommunications, sets the Standards and Recommended Practices (SARPs) for aviation's own systems. It is published in five volumes.

As with every Annex, a State that does not apply a provision notifies the difference, and its AIP lists the differences in GEN 1.7, recommended practices being marked with an asterisk. France, for example, notifies under Volume V that 121.5 MHz is provided at some international aerodromes only where it is considered necessary. In the EU, rule CNS.TR.100 refers to Volume II in its sixth edition of 2001, up to Amendment 89, so France lists later additions, such as the CPDLC definitions introduced by Amendment 90, as differences.

Annex 10 has neighbours that matter to communication. Annex 11 sets up the air traffic services that use the radio, Annex 3 the meteorological information that is broadcast, and Annex 15 the aeronautical information carried on the ground networks. Even Annex 18, on the safe transport of dangerous goods by air, has a radio element: a commander facing an in-flight emergency informs the appropriate air traffic services unit, as soon as practicable, of any dangerous goods on board, so that the aerodrome's rescue and fire-fighting services know what they will meet (see dangerous goods).

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The aeronautical telecommunication services

Annex 10 divides aeronautical telecommunications into four services.

The radio navigation aids are covered in radio navigation aids and position fixing, and the broadcasts in flight information and broadcast services.

The aeronautical mobile service

The aeronautical mobile service is a mobile service between aeronautical stations and aircraft stations, or between aircraft stations, in which survival craft stations may participate; emergency position-indicating radio beacon stations may also take part on the designated distress and emergency frequencies. In practice it is every voice and data exchange between aircraft and the ground, and between aircraft. Its VHF voice channels run from 118.000 to 136.975 MHz, amplitude-modulated, with HF single sideband and satellite links for long range (see VHF, HF and satellite communications).

Messages in the aeronautical mobile service have a fixed order of priority: distress calls and traffic; urgency messages; communications relating to direction finding; flight safety messages; meteorological messages; and flight regularity messages. A request for fuel and steps on arrival is flight regularity traffic and gives way to everything else (see radiotelephony transmitting technique).

Aeronautical and aircraft stations

A station, in this vocabulary, is any equipment used to transmit or receive aeronautical information, on the ground, in an aircraft or on a network.

• An aeronautical station is a land station in the aeronautical mobile service. In some cases it may be on board a ship or on a platform at sea. The radio of every ATC unit is one, as is that of a flight information unit, an airline operations frequency or a flight service station.
• An aircraft station is a mobile station in the aeronautical mobile service, other than a survival craft station, located on board an aircraft.

Aeronautical stations are called by the location name and a suffix giving the type of unit or service, such as TOWER, CONTROL, INFORMATION or RADIO (see radiotelephony call signs). RADIO denotes a station that handles air-ground messages without controlling anyone. The aeradio stations of oceanic airspace are the classic case: in the North Atlantic, HF messages from aircraft, such as position reports and requests, are received by aeradio stations such as Shanwick Radio and Gander Radio and relayed to the oceanic control centres. A satellite telephone call about a communication problem there normally goes to the aeradio station rather than the control centre, unless urgency dictates otherwise.

Air-ground communication

Air-ground communication is two-way communication between aircraft and stations or locations on the surface of the earth. Several related terms complete the vocabulary:

• air-to-ground communication, one-way, from aircraft to stations or locations on the surface;
• ground-to-air communication, one-way, from stations or locations on the surface to aircraft, as in VOLMET;
• air-to-air communication, between aircraft stations, as on 123.45 MHz over the oceans (see communications at non-towered aerodromes);
• a blind transmission, from one station to another when two-way communication cannot be established but the called station is believed able to receive it, the pilot's tool after a receiver failure (see radio communication failure);
• a broadcast, a transmission of information relating to air navigation that is not addressed to a specific station.

The aeronautical fixed service and AFTN

The aeronautical fixed service (AFS) links specified fixed points on the ground: ATS units, aeronautical information and meteorological offices, operators. Its oldest and best-known element is the aeronautical fixed telecommunication network (AFTN), a worldwide system of fixed circuits carrying messages in a standard format: flight plans with their departure, arrival, delay and cancellation messages, NOTAMs, and meteorological reports and forecasts. It began as a teleprinter network and is now mostly digital. When a flight plan is filed, the AFS carries it to every unit along the route (see ATC flight plan).

Every AFTN station has an eight-letter address beginning with the four-letter ICAO location indicator of its site. The French AIP, for instance, lists LFFFYVYX as the AFTN address of the northern regional control centre, which serves the Paris FIR and whose location indicator is LFFF. Alongside the AFTN, ATS units coordinate through direct speech circuits and computer-to-computer data exchange, ATS inter-facility data communication (AIDC).

The ground networks now reach the aircraft too. The aeronautical telecommunication network (ATN), standardised in Annex 10 Volume III, is ICAO's data network for ground-ground and air-ground data; in Europe controller-pilot data link runs over it, using only the VHF data link Mode 2 subnetwork. Commercial providers such as SITA and ARINC operate the networks that carry airline and air traffic data link messages over VHF, HF and satellite (see ACARS and aircraft data link).

The CNS/ATM concept

CNS/ATM, communications, navigation and surveillance / air traffic management, is ICAO's concept for modernising air navigation by adding digital data link, satellite navigation and automatic dependent surveillance to voice, ground beacons and radar, all serving a more integrated air traffic management. It grew from ICAO's work on future air navigation systems, which is why the first avionics packages to implement it were called FANS.

Two families of data link show the concept at work. FANS 1/A uses the ACARS network, over satellite and HF as well as VHF, and combines CPDLC with ADS-C for oceanic and remote airspace; the North Atlantic data link mandate requires it or an equivalent (see oceanic and North Atlantic operations). ATN Baseline 1, called FANS B on Airbus aircraft, provides CPDLC over the ATN in busy continental airspace, above all in Europe; the FAA is not implementing it. Neither replaces voice. The FAA's AIM states that voice remains the primary and controlling means of air-ground communication, and distress and urgency calls are normally made by voice even though the CPDLC message set includes MAYDAY and PAN PAN messages (see controller-pilot data link communications, ADS-B, performance-based navigation).

Radio operator licensing

The radio side of a flight's paperwork has three parts: a licence for the aircraft's radio station, the operator's authority to transmit, and the language.

• Aircraft radio station licence. Article 29 of the Chicago Convention lists the aircraft radio station licence, where radio is carried, among the documents required on international flights, and under EU air operations rules the aircraft radio licence is carried as an original, like the certificates of registration and airworthiness. In the United States the FCC station licence is required only for international flights.
• Radiotelephony operator. States organise this differently. Some include radiotelephony privileges in the pilot licence, while others issue a separate certificate, such as the UK's Flight Radiotelephony Operator's Licence. A US pilot flying internationally needs an FCC restricted radiotelephone operator permit, and most US airlines require one.
• Language proficiency. Under EASA's FCL.055, a pilot required to use the radiotelephone may exercise the licence privileges only with a language proficiency endorsement, in English or in the language used for radio communications on the flight, at Operational Level 4 or above. Level 4 is re-evaluated every 4 years and Level 5, Extended, every 6 years, while Level 6, Expert, needs no further evaluation. The FAA meets the ICAO requirement by marking certificates "English Proficient".

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.