Eddie Sez:

ADS, CPDLC, ATC COM, ACARS, AOC, Whaaah? Here's how we keep this straight:

  • ADS-B Out — You can think of Automatic Dependent Surveillance - Broadcast Out (ADS-B Out) as a higher tech replacement for your transponder. ADS-B OUT sends your GPS position to air traffic control and other aircraft equipped with ADS-B In. It is much more accurate than a radar blip.
  • ADS-B In — You can think of Automatic Dependent Surveillance - Broadcast In (ADS-B In) as a more accurate version of your TCAS. While TCAS aircraft positions shown in your cockpit are approximate, ADS-B In positions are exact.
  • ADS-C — You can think of Automatic Dependent Surveillance - Contract (ADS-C) as a replacement for Air Traffic Control 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.
  • CPDLC — You can think of Controller Pilot Data Link Communications (CPDLC) as a replacement for your HF when oceanic and VHF over some domestic areas. It is far superior to your HF and has distinct advantages over VHF.
  • ATC COM, ACARS, AOC — Air Traffic Control Communications (ATC COM) is Honeywell-speak for CPDLC. Aircraft Communication Addressing and Reporting System (ACARS) used to be Airline Communication Addressing and Reporting System and is simply communications through a data service provider and includes your PDC, digital ATIS, and most of your oceanic clearances. Aeronautical Operational Control (AOC) is Gulfstream-speak for ACARS.

ADS-C procedures are closely aligned with those for CPDLC:

What follows comes from the references shown below. Where I think it helpful, I've added my own comments in blue.

Functional Description


[ICAO GOLD, pg. 104] Automatic dependent surveillance — contract (ADS-C). 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.

[AC 91-70A, ¶5-4]

  1. Automatic Dependent Surveillance-Contract (ADS-C). ADS-C is an Air Traffic Service (ATS) application established by contract in which aircraft automatically transmit, via data link, data derived from onboard navigation systems. As a minimum, the data includes a 3-D position, the corresponding time of the position data, and a Figure of Merit that characterizes the accuracy of the position data. You may provide additional data as appropriate.

  2. ADS Data. It uses the various systems aboard the aircraft to provide aircraft position, velocity, intent, and meteorological data. The aircraft can transmit this data to the ATS provider system for estimating and predicting aircraft position.

  3. ADS-C Reports. The ATS provider applies a contract request to an aircraft. ADS-C reports are issued by the aircraft per the contract request. The contract identifies the types of information and the conditions that the aircraft transmit.

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.


Figure: Examples of applying RCP and RSP specifications, from ICAO GOLD, Tables 2-3 and 2-4.

[ICAO GOLD, ¶] The Safety and Performance Standard for Air Traffic Data Link Services in Oceanic and Remote Airspace (Oceanic SPR Standard, RTCA DO-306/EUROCAE ED-122), provides operational, safety and performance criteria for data link services that are applicable in airspace, where procedural separation is being applied, for normal ATC communication and surveillance to support separation assurance, route conformance monitoring, re-routes, and weather deviation management. These criteria include specifications for required communication performance (RCP) and required surveillance performance (RSP), taking into consideration the following data link applications:

  • Data link initiation capability (DLIC);

  • CPDLC for ATC communication;
    • RCP 240 operations; and

    • RCP 400 operations;
  • ADS-C for surveillance - automatic position reporting;
    • RSP 180 operations; and

    • RSP 400 operations;

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.

Contract Types

[AC 91-70A, ¶5-4.d.] A periodic contract, event contract, and demand contract all define three types of reporting. The aircraft may also initiate emergency reporting, which is a special case of periodic reporting. In response to a periodic contract, the aircraft assembles and transmits a message containing the fields at the interval specified in the contract request. Event contracts define certain events (such as an altitude change), which causes the aircraft to send a report, independent of any periodic contract in effect. Send one demand contract each time the ATS provider system commands it. The contract request may specify several different data groups. These include the basic position report, which contains 3-D position and time, and additional on-request groups. These groups include aircraft and wind velocity, vertical speed, and limited waypoint information.

Periodic Contracts


  • A periodic contract allows an ATSU to specify:
    • The time interval at which the aircraft system sends an ADS-C report; and

    • The optional ADS-C groups that are to be included in the periodic report. Each optional group may have a unique modulus which defines how often the optional group is included with the periodic report (e.g. a modulus of five indicates that the optional group would be included with every fifth periodic report sent).
  • The range and resolution of the time interval parameter in the periodic contract allows for an interval to be specified between 1 second and 4,096 seconds (approximately 68 minutes). However, RTCA DO-258A/EUROCAE ED-100A limits the minimum interval to 64 seconds.

  • An ATSU can establish only one periodic contract with an aircraft at any one time. A number of ATSUs can each establish their own periodic contract and specify their own conditions for the report with the same aircraft at the same time.
  • The ATSU determines what information it wants and how often it wants it. From the flight crew standpoint, you will see differing numbers of items on the periodic contract. The example photo shows the contract from YQXE2YA wants FLIGHT ID and METEORO information every 1088 seconds.

Event Contracts

[ICAO GOLD, ¶] An event contract allows an ATSU to request an ADS-C report whenever a specific event occurs. An ATSU can establish only one event contract with an aircraft at any one time. However, the event contract can contain multiple event types. These types of optional events include:

  • Waypoint change event (WCE);

  • Level range deviation event (LRDE);

  • Lateral deviation event (LDE); and

  • Vertical rate change event (VRE).

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.

Demand Contracts

[ICAO GOLD, ¶] A demand contract allows an ATSU 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.

Log On

Photo: G450 MCDU, ADS Armed, from Eddie's aircraft.

[ICAO GOLD, ¶] The logon is the first step in the data link process. A logon, initiated either by the flight crew or by another ATSU, is performed prior to the ATSU establishing a CPDLC and/or ADS-C connection. The purpose of the logon is to:

  • Provide the ATSU with the data link application “context” of the aircraft, namely:
    • The ATS data link applications supported by the aircraft system (e.g. CPDLC, ADS-C) and the associated version numbers of these applications; and

    • The unique identification of the aircraft;
  • Provide the ATSU with the relevant aircraft information required to allow the ATSU to correlate the logon information with the aircraft’s corresponding flight plan.

  • On receipt of a logon request, the ATSU correlates the logon information with the relevant information in the flight plan held by the ATSU. This ensures that messages are sent to the correct aircraft and that automation associated with ADS-C reports or CPDLC messages updates the correct flight plan.

  • When making this correlation, the ground system ensures that the aircraft identification in the logon request matches that in Item 7 of the associated flight plan and at least one of the aircraft registration or aircraft address provided match the corresponding descriptors (following the REG and/or CODE indicators, respectively) in Item 18 of the flight plan;

The log on procedure will vary with aircraft. In the case of a Gulfstream 450/550, for example, the ADS-C log on takes places as a consequence of the CPDLC log on. The CPDLC FIR region code is entered into the ATC LOGON STATUS page of the MCDU after ensuring all other items are correct. The information is sent and once accepted the MCDU scratch pad will show "ADS ESTABLISHED" and the ADS ARMED entry will change to ADS ACTIVE. For more about how to do this, see: Communications / CPDLC: Logon.

Position Reporting

Position Reporting in an ADS-C Environment

[ICAO Gold, ¶5.6.3]

Position Reporting in a non-ADS-C Environment

[ICAO Gold, ¶5.6.2]

The Importance of FMS Waypoint Sequencing

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 Gold, figure 5-2.

[ICAO Gold, ¶5.6.1]

  • The flight crew should ensure that waypoints are sequenced correctly. If an aircraft passes abeam a waypoint by more than the aircraft FMS waypoint sequencing parameter, the flight crew should sequence the waypoints in the FMS, as appropriate.
  • 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.

  • When using CPDLC or FMC WPR to provide position information, the flight crew should use latitudes and longitudes encoded as waypoint names in the ICAO format.
  • The point here is do not use ARINC 424 format. The ATSU doesn't understand "N3030" but wants to see the full latitude and longitude.

Transfer of ADS-C Services Between ATSUs

[ICAO GOLD, ¶5.2.3]

Exiting CPDLC and ADS-C Service Areas

[ICAO GOLD, ¶5.2.5]

ADS-C Reports

[ICAO GOLD, ¶] 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 Gold, figure 2-38.

Figure: ADS-C Flight Identification Group, from ICAO Gold, figure 2-39.

Figure: ADS-C Earth Reference Group, from ICAO Gold, figure 2-40.

Figure: ADS-C Air Reference Group, from ICAO Gold, figure 2-41.

Figure: ADS-C Airframe Identification Group, from ICAO Gold, figure 2-42.

Figure: ADS-C Meteorological Group, from ICAO Gold, figure 2-43.

Figure: ADS-C Predicted Route Group, from ICAO Gold, figure 2-44.

Figure: ADS-C Fixed Projected Intent Group, from ICAO Gold, figure 2-45.

Figure: ADS-C Intermediate Projected Intent Group, from ICAO Gold, figure 2-46.

Book Notes

Portions of this page can be found in the book International Flight Operations, Part V, Chapter 3.


Advisory Circular 91-70A, Oceanic and International Operations, 8/12/10, 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

ICAO Doc 4444 - Air Traffic Management, Fourteenth Edition, Procedures for Air Navigation Services, International Civil Aviation Organization, 2001 *

ICAO Doc 4444 - Air Traffic Management, Fifteenth Edition, Procedures for Air Navigation Services, International Civil Aviation Organization, 2007 *

* Not all of Doc 4444 seems to have been reproduced in the 15th edition, so you might need to look at the 15th edition and then then 14th edition for some sections.

ICAO Global Operational Data Link Document (GOLD), International Civil Aviation Organization, Second Edition, 26 April 2013