I think of Automatic Dependent Surveillance - Contract (ADS-C) as a replacement for Air Traffic Control's radar contact. When you have logged on to an Air Traffic Service Unit with ADS-C, you have agreed to contracts which send information to the ATSU, such as your position, on a regular basis. It is a part of the data link system that keeps you on the same page as Air Traffic Control.
Everything here is from the references shown below, with a few comments in an alternate color.
[ICAO Doc 10037, ¶1.2.5]
Note.— Although the terms are similar, ADS-C and ADS-B are two different applications. In comparison, ADS-B (PSR, SSR or any comparable ground-based system that enables the identification of aircraft) is an ATS surveillance system. An ADS-B-capable aircraft supports ATS surveillance services and broadcasts information at a relatively high rate, and any appropriate receiver on the ground or in another aircraft within range can receive the information.
ADS-C gives you a better Required Surveillance Performance (RSP) and CPDLC gives you better Required Communication Performance (RCP). Combined, they allow you to fly in airspace with tighter separation minima which means you have a greater selection of airspace available to you.
An ADS Contract is an agreement from you, the pilot, to the Air Traffic Service (ATS), to provide information. You can provide information through various types of contracts and you can do this with up to four different ATS providers. The data is extracted automatically from various electronics in your aircraft. In exchange they grant you access to the airspace you are in, or coordinate with nearby airspace. ADS-C will take the place of voice position reports in many regions of the world. You need special equipment and authorization to use ADS-C.
[AC 91-70B, ¶A.2.16.2] Automatic Dependent Surveillance—Contract (ADS-C) (ICAO). A means by which the terms of an ADS-C agreement will be exchanged between the ground system and the aircraft, via a data link, specifying under what conditions ADS-C reports would be initiated and what data would be contained in the reports. The abbreviated term “ADS contract” is commonly used to refer to ADS event contract, ADS demand contract, ADS periodic contract, or an emergency mode.
[ICAO Doc 10037, ¶22.214.171.124]
[ICAO Doc 10037, ¶126.96.36.199]
Note.— ADS-C groups are referred to as data blocks in ICAO Doc 4444.
Note.— The ANSP ensures that separation minima are applied in accordance with appropriate standards. The ground system may prevent the controller from selecting a periodic reporting interval that is longer than the maximum interval specified in the standard for the separation minima being applied.
[ICAO Doc 10037, ¶188.8.131.52.4] A demand contract allows an ATS unit to request a single ADS-C periodic report. A demand contract does not cancel or modify any other ADS contracts that may be in effect with the aircraft.
[ICAO Doc 10037, ¶184.108.40.206.6.]
Note.— In accordance with ICAO Doc 4444, paragraph 220.127.116.11.3.2, in airspace where procedural separation is being applied, as a minimum, WCE, LRDE, and LDE shall be contained in ADS-C agreements.
In the example photo, the ATS will be notified automatically if the aircraft's vertical velocity exceeds 5,056 fpm, lateral deviation exceeds 5 nm, the altitude goes below 40,700 or above 41,300 feet, and at every waypoint change. The waypoint change is determined by the FMS so anything you do to the FMS that affects the next waypoint and the waypoint one after that will be reported. For example, you you were to insert your ETP before the next waypoint, that gets reported.
Photo: G450 MCDU, ADS Armed, from Eddie's aircraft.
In my Gulfstream the ADS-C log on happens automatically as a function of the CPDLC log on. I'm not sure if that is the case with other aircraft but it is with mine. For more about this see: CPDLC / Logon.
[ICAO Doc 10037, ¶18.104.22.168.1] When using CPDLC to provide position information, the flight crew should report unnamed waypoints (latitudes/longitudes) using the ICAO format of nn[N/S]nnn[E/W] or, if both degrees and minutes are required, nnnn[N/S]nnnnn[E/W].
Note.— The flight crew and flight operations officers/dispatchers should not use the ARINC 424 format. ARINC 424 describes a 5-character latitude/longitude format for aircraft navigation databases (e.g. 10N40 describes a lat/long of 10N140W). The ATS unit may reject or be unable to process any downlink message containing waypoint names in the ARINC 424 format.
[ICAO Doc 10037, ¶22.214.171.124]
[ICAO Doc 10037, ¶126.96.36.199]
Note.— In an ADS-C environment, the flight crew should not provide position reports or revised waypoint estimates by CPDLC or voice, unless otherwise instructed or under conditions in certain airspace as stipulated in AIP (or other appropriate publication)
Note.— Some ANSPs require a single CPDLC position report, even when in an ADS-C environment, to provide the controlling ATS unit confirmation that it is the CDA and the only ATS unit able to communicate with the aircraft via CPDLC (refer to Appendix B).
In general you should not provide position reports or revised waypoint estimates by CPDLC or voice unless you see something that tells you to do so. Append B can be misleading. The subject is only brought up in the North Atlantic region where you are explicitly told "For ADS-C flights, the flight crew should not submit position reports via voice to reduce frequency congestion, unless requested by aeronautical radio operator." Makes sense, but don't let that talk you into thinking now you need to do so in other regions because the same phrase is missing.
Note.— If the flight crew inserts non-ATC waypoints (e.g. mid-points) into the aircraft active flight plan and activates the change, the aircraft system may trigger an ADS-C waypoint change event report at the non-ATC waypoint, or include information about the non-ATC waypoint in the predicted route group, as well as the intermediate and fixed projected intent groups. As a result, the ADS-C report will include information about the non-ATC waypoint, which is not expected by the ATC ground system.
Note.— If the flight crew activates a non-ATC cleared route into the aircraft system, the ADS-C reports will include information that will indicate the aircraft is flying a route that is deviating from the cleared route.
You don't have to worry about flying a Strategic Lateral Offset because your FMS will sequence the waypoints even if your are two miles away from the intended course. But if you are further than the tolerance allowed by your FMS, it may not sequence. If that happens, the position report will not be made.
Figure: Waypoint sequencing anomaly, from ICAO Doc 10037, figure 4-6.
[ICAO Doc 10037, ¶188.8.131.52]
As shown in [the figure], when an aircraft passes abeam a waypoint in excess of the defined sequencing parameter (refer to , paragraph F.7) for specific aircraft types), the FMS will not sequence the active waypoint. If the flight crew does not sequence the waypoint, incorrect information will be contained in ADS-C reports, CPDLC position reports and FMC waypoint position reports – the next waypoint in these reports will actually be the waypoint that the aircraft has already passed.
[ICAO Doc 10037, ¶4.2.3.]
Note.— The flight crew should not need to reinitiate a logon.
Note.— If a flight is transferred to a new ATS unit with an open CPDLC message, the message status will change to ABORTED. If the flight crew has not yet received a response from the controller, the downlink request will also display the ABORTED status. Refer also to Appendix C, paragraph C.8.
[ICAO Doc 10037, ¶4.2.5]
[ICAO Doc 10037, ¶184.108.40.206.1] The aircraft system sends specific aircraft data in different groups of an ADS-C report. Each group contains different types of data. An ADS-C event report contains only some of the groups, which are fixed. The ADS-C periodic report can contain any of the ADS-C groups, which the ATSU specifies in the contract request.
Figure: ADS-C Basic Group, from ICAO Doc 10037, figure 1-39.
Figure: ADS-C Flight Identification Group, from ICAO Doc 10037, figure 1-40.
Figure: ADS-C Earth Reference Group, from ICAO Doc 10037, figure 1-41.
Figure: ADS-C Air Reference Group, from ICAO Doc 10037, figure 1-42.
Figure: ADS-C Airframe Identification Group, from ICAO Doc 10037, figure 1-43.
Figure: ADS-C Meteorological Group, from ICAO Doc 10037, figure 1-44.
Figure: ADS-C Predicted Route Group, from ICAO Doc 10037, figure 1-45.
Figure: ADS-C Fixed Projected Intent Group, from ICAO Doc 10037, figure 1-46.
Figure: ADS-C Intermediate Projected Intent Group, from ICAO Doc 10037, figure 1-47.
Portions of this page can be found in the book International Flight Operations, Part V, Chapter 3.
ICAO Document 10037 - Global Operational Data Link (GOLD) Manual, First Edition, 2017
Advisory Circular 91-70B, Oceanic and International Operations, 10/4/16, U.S. Department of Transportation
Asia/Pacific Information Package, FAA Flight Technologies and Procedures Division (AFS-400), June 15, 2012
FANS-1/A Operations Manual, FAA Aeronautical Communications Aviation Safety (AVS), Version 6.0, 25 September 2008
Guidance Material for ATS Data Link Services in NAT Airspace, The North Atlantic FANS Implementation Group (NAT FIG), The North Atlantic Systems Planning Group (NAT SPG), Version 19.1, 14 September 2009
Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.
Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013
Gulfstream Operating Manual Supplement for G350, G450, G500, and G550 Airplanes, Supplement Number GAC-OMS-4, Future Air Navigation System (FANS 1/A), Automatic Dependent Surveillance (ADS-C), Controller Pilot Data Link Communications (CPDLC), Revision 1, July 1, 2012
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