Flight tracking equipage gaps – ADS-B or C?

Philip ClinchPhilip Clinch argues that the ITU approval of rules to expand up to satellite altitude the protection of the 1090 MHz spectrum used by aircraft transponders for ADS-Broadcast has reminded everybody of the ICAO process to require airlines to track their aircraft over oceans by obtaining position reports at least every 15 minutes.

The ITU rules will protect satellite access to the Automatic Dependent Surveillance Broadcasts included in transmissions aircraft radar transponder send every few seconds to radar stations to keep them interrogating the aircraft. The ADS-B function in the transponder takes from the aircraft navigation systems the position data calculated using GPS signals and inserts that data in its broadcasts, enabling better accuracy than the radar station estimate of position based on the characteristics of the radio signal it receives.

The draft ICAO requirements for tracking during normal flight, have not required airlines to obtain position updates at the once a minute frequency provided by ADS-Broadcasts because the transponder output cannot be picked over the oceans out of range of ground stations. The draft requirement to get position updates at least every 15 minutes allows compliance using a different version of ADS – Automatic Dependent Surveillance Contract – an automated version of the historical pilot position reporting via HF voice radio to Oceanic Air Traffic Control.

Most long range aircraft delivered since 2000 have been equipped with the ADS-Contract function included in “Future Air Navigation Systems” (FANS) avionics which use ACARS communications via satellite, HF or VHF links. The ADS-C capability has sufficiently improved position reporting accuracy and reliability compared to HF voice reports to enable aviation regulators and ANSPs to safely increase capacity in oceanic regions to meet growing demand for flights. The ANSPs covering the North Atlantic are requiring FANS equipage to access the most efficient routes and altitudes.

The ICAO group defining tracking requirements wanted to ensure quick progress by defining requirements that airlines could comply with quickly using existing avionics. The draft 15 minute requirements do work for the majority of flights in oceanic airspace that are carried out by aircraft with FANS avionics connected to a satellite communications system. But full fleet compliance cannot be achieved quickly because many short haul aircraft that fly over short areas of sea or desert have no satellite communications systems to transport their position reports.

While ADS-C has improved on voice reports, the position reporting it provides via a communications system will become unnecessary when the aircraft transponder output becomes accessible via the satellite receivers protected by the new ITU rules. When ADS-B output becomes accessible to Oceanic ATC system, it will enable a move from the “procedural” ATC required in airspace with no surveillance coverage to a more complex continental ATC system that can handle far more flights. Global ATC access to surveillance transponder output will eventually enable seamless integration of the systems handling all airspace and remove the distinction between oceanic and continental airspace.

Equipping aircraft with new systems takes a long time so airlines will not realistically be able to get satellite communications systems on all their short haul aircraft that sometimes leave surveillance coverage before the end 2017 start date of the Aireon satellite network of 1090 MHz receivers picking up ADS-Broadcasts. Maybe instead of just relying on aircraft to get satellite communications systems to send position data, ICAO and its member states should also be defining oceanic ATC benefits requiring the ADS-B capability in aircraft transponders. Many ANSPs already have deadlines for ADS-B equipage in their continental airspace so expanding the requirement to oceanic airspace would not be extraordinary.

ICAO might not immediately abandon the plan to require airlines to obtain position reports every 15 minutes but the quickest way for airlines to comply could be to upgrade any radar transponders still missing ADS-Broadcast capability so that when the satellite ADS-B receivers start working their position updates will come 15 times more often than needed. The airlines will still need tracking systems to detect problems with their aircraft but they will have better position data to carry out the detection. Aircraft will still need communications systems for Controller Pilot Data Link so eventually the fleet will have multiple ways of providing position data.

Philip Clinch heads SITA’s corporate strategy.


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