On the other hand, the aeronautical mobile service includes all technical means required to support the communications between the aircraft and the ATS services (information, surveillance, and control) based on earth. These communications are typically pilot- controller.
As defined by ICAO Standards documents in Annex 10 Vol II:
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 participate in this service on designated distress and emergency frequencies.
The ultimate goal of this service is to allow communications between pilot and controller. In particular, in one control sector, the controller must be able to communicate with all aircraft inside the sector using only one of these radio channels (each sector has a unique frequency assigned). Therefore, the number and dimension of the sectors condition the location of the communication centres. The frequency assigned to each sector establish a double direction channel: pilot-controller; controller-pilot. That is the fundamental instrument in the functions of information, surveillance, and control of aircraft in flight.
Figure 11.1: Radio communications
The categories of messages handled by the aeronautical mobile service and the order of priority in the establishment of communications and the transmission of messages shall be as follows:
- Distress calls, distress messages, and distress traffic (emergency messages).
- Urgency messages.
- Communications relating to direction finding (to modify the course).
- Flight safety messages (movement and control).
- Meteorological messages (meteorological information).
- Flight regularity messages.
There are two types of aircraft-controller communications:
- Controller-pilot voice communications.
- Controller-pilot data-link communications (CPDLC).
Voice communications These services are provided wireless, using radio channels. In the case of aeronautical communications, it is used the VHF (Very High Frequencies) band and HF (High Frequency) band. The channels in HF are only used for long-distance communications, when it is impossible to establish communication using VHF. VHF radio communications (for civil aviation) operate in the frequency range extending from 118MHz to137MHz.7 HF radio communications utilize practically the whole HF spectrum (3MHz to 30MHz), depending on times of the day, seasonal variations, solar activity, etc.
Figure 11.2: Datalink control and display unit (DCDU) on an Airbus A330. © User:SempreVolando / Wikimedia Commons / CC-BY-3.0.
CPDLC communications: A mean of communication between controller and pilot, using data link for ATC communication. Messages can be transmitted using both VHF bands or satellite bands. The way it works is rather simple: when any of either the pilot or the controller wants to establish contact, a message containing the request/instructions is sent. Figure 11.2 illustrates a pilot interface for sending and receiving CPDLC messages.
The first data link ground-air communication were due to ACARS (Aircraft Communication Addressing and Reporting System) in 1978. This service is provided via Inmarsat satellite. Its main drawback is that is not compatible with the ATN. CPDLC was later generalised under FANS (Boeing’s avionics equipment), which has evolved to FANSB, a system with advanced capabilities, e.g., radar mode S, RNP.
7. Notice that the VHF range is 30MHz to 300MHz.