Airborne radio telephone communication via a satellite is usually abbreviated to the term SATCOM.
Use of satellites for this purpose complements satellite-based navigation capability. Aircraft onboard equipment for SATCOM includes a satellite data unit, a high power amplifier and an antenna with a steerable beam. A typical aircraft SATCOM installation can support data link channels for ’packet data services’ as well as voice channels. SATCOM data link is currently used for only a small proportion of en route ATM communications in contrast to the much more extensive use as an alternative to VHF and HF for non-ATC purposes. The Asia-Pacific Region has been a particular focus for many of the early developments in the use of SATCOM for ATM data link. SATCOM functionality, which primarily depends upon geostationary satellites, is poor in polar regions, where HFDL (HF Data Link) provides equivalent service for some uses.
Aircraft Satellite Communication
Even in non-ATC use, SATCOM voice communications still require standard radio discipline procedures. Any attempt to use a SATCOM link like a normal telephone can easily lead to misunderstandings. Satellite Voice-equipped aircraft can initiate calls using either INMARSAT or IRIDIUM assigned security phone numbers (ICAO short codes), or can direct dial using commercial phone numbers and country codes. Ground Earth Stations can originate calls to SATCOM Voice-equipped aircraft using their unique 8-digit Aeronautical Earth Station (AES), aircraft ID (OCTAL) code, or phone number.
Data Link Communications
Introduction of satellite-based data link services for en route ATM, both for CPDLC and for surveillance, has allowed suitably equipped ANSPs to trial reduced oceanic procedural separation standards such as 50 nm longitudinal and 30nm longitudinal/30nm lateral. However, inconsistent data link performance mainly attributed to a combination of satellite outages, and poor Ground Earth Station (GES) availability and data link capacity issues, have temporarily reduced confidence in some early applications. Satellite providers have since worked together to address these concerns and have identified infrastructure solutions which States are being asked to help implement.
Future Use of SATCOM for Air Traffic Management (ATM)
Most agencies consider that SATCOM capability will eventually form a more significant part of the ATC element of the ATN system, since it is probably the only way to ensure reliable data link coverage of oceanic and remote continental environments at the capacity levels required and may also eventually be required due to voice and data communication capacity constraints in parts of Europe and the USA. Current ATM SATCOM for en route ATM uses satellites and a radio frequency spectrum shared with ACARS and other communication channels rather than the dedicated ones which would be possible if the system was ultimately to be applied more widely.
ICAO Annex 10 currently notes that “ITU Radio Regulations permit systems providing mobile-satellite service to use the same spectrum as Aeronautical Satellite (route) Service without requiring such systems to offer safety services and that this situation has the potential to reduce the available spectrum ATM use. States are advised to consider this issue in radio frequency planning and in the establishment of national or regional spectrum requirements".
Since 2003, ICAO activities related to SATCOM have been coordinated by the Aeronautical Communications Panel (ACP), which has responsibility for developing the necessary SARPs as well as associated guidance material, for both voice and data air-ground aeronautical communications. There has been concern in some quarters that the practical development of data link SATCOM for en route purposes is running ahead of the development through ICAO of corresponding SARPs, with a consequent risk that present action may not necessarily be compatible with the likely longer term needs. European planning for an eventual extension of SATCOM ATM is being coordinated by the NexSAT Steering Group, which functions in liaison with the separate initiatives of the FAA.