Also known as: ADS-C, ADS-Contract
ADS-C (Automatic Dependent Surveillance–Contract) is an addressed, point-to-point aviation surveillance service in which an aircraft sends position and intent reports to an air-traffic control centre under a negotiated contract, carried over ACARS via satellite, HF, or VHF datalink.1 Unlike the omnidirectional broadcast of ADS-B, ADS-C is a private conversation between one aircraft and one ATC unit, which makes it the workhorse for oceanic and remote airspace beyond radar and ADS-B ground coverage.
Overview
ADS-C is one half of the FANS (Future Air Navigation System) toolset; its companion is CPDLC controller–pilot text messaging. A ground ATC system establishes a contract with the aircraft’s avionics specifying what to report and when. The aircraft then autonomously sends reports drawn from its own navigation system (GNSS/INS) — hence “dependent” — without further prompting, until the contract is modified or cancelled.
Technical characteristics
| Property | Value |
|---|---|
| Model | Addressed request/contract, not broadcast |
| Contract types | Periodic, event, and on-demand |
| Report content | Position, altitude, time, ground vector, intent/waypoints |
| Bearer | ACARS over Inmarsat/Iridium satcom, HFDL, or VDL |
| Position source | Aircraft FMS (GNSS/INS) |
| Companion | CPDLC (clearances and text) |
| Standards | ICAO FANS-1/A, ARINC 745/622 |
Three contract types coexist: periodic (report every N minutes), event (report when crossing a waypoint, changing level, or deviating), and on-demand (a single report on request). Because reports are addressed and acknowledged over ACARS, ADS-C tolerates the long latency and low throughput of oceanic satcom and HF links far better than the once-per-second broadcast of ADS-B.
History
ADS-C emerged from the FANS-1/A programme in the 1990s, led by Boeing and Airbus with ICAO, to bring datalink surveillance to oceanic regions that radar could never cover. It let controllers reduce the large procedural separation minima previously required over the oceans, increasing capacity on busy tracks such as the North Atlantic.
Deployment
ADS-C is standard equipment on long-haul transport aircraft and is used by oceanic and remote-area control centres worldwide, typically over Inmarsat or Iridium satcom, with HFDL as an HF fallback in polar regions. It complements rather than replaces ADS-B: ADS-B for surveillance where ground receivers exist, ADS-C for the airspace where they do not.
Decoding it with GopherTrunk
Not decoded. ADS-C is an addressed application riding inside ACARS over satellite and HF bearers, not a broadcast VHF/UHF signal in GopherTrunk’s land-mobile or 1090 MHz ADS-B scope. Enthusiasts can observe ACARS-borne ADS-C with satcom or HFDL SDR setups and dedicated decoders, but reconstructing the contract application is outside GopherTrunk’s remit.
Sources
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Automatic Dependent Surveillance–Contract — Wikipedia, for the ADS-C contract model, periodic/event/on-demand reports, ACARS/satcom/HF bearers, and its FANS oceanic-surveillance role alongside CPDLC. ↩