I had some difficulty understanding it as well. It seemed that the information was scattered around in various documents and only partial answers or difficult to understand information. So I downloaded stuff and took the best quotes from several articles, simplified it and made my own notes as written below. Therefore, there could be inaccuracies or out of date information. Maybe ATC Chopper can proof read it. Enjoy........
ADS-C is an automatic reporting system that is part of the FANS 1/A equipment in an aircraft. The primary objective of the ADS-C application is to provide automated aircraft position and intent data for ATC. It may also be useful in air traffic flow and airspace management. ADS-C replaces pilot position reports by voice and/or CPDLC. This allows controllers to obtain position data and other information from appropriately equipped aircraft in a timely manner in accordance with the controllers requirements allowing the aircraft to be tracked in airspace where radar surveillance service is not available.
The ADS-C application supports the following services: 4-Dimensional Trajectory Data Link, Information Exchange and Reporting, and Position Reporting. The ATC unit is capable of requesting the aircraft system to provide the ADS-C reports to the ATSU system in three ways:
a) on demand;
b) on a periodic basis;
c) when triggered by certain events.
The ATC unit specifies under what conditions ADS-C reports will be initiated, and what data will be contained in the reports. This requested information can vary from unit to unit.
ADS-C relies on specific contracts (or agreements) being established between the ground system and the aircraft's avionics. Basically, this means that the ground system provides the aircraft with a list of reports it is required to send and specifies when they are required to be sent.
The aircraft automatically complies by providing the information that its installation is capable of providing which can vary from aircraft to aircraft, and depending on its capability, may or may not include all the ATC requested information. ADS-C is a Dependent Surveillance because the ground system is dependent on reports from the aircraft - rather than primary radar which gets aircraft position information independent of what the aircraft is doing. The initial purpose of ADS-C has been to provide surveillance for aircraft operating in non-surveillance(non-radar) airspace.
The ADS reports are converted by data link equipped ground stations into an ADS track and presented on the controller's air situation display to provide enhanced situational awareness and the potential for reduced separation standards. ADS-C data can be used by the ground system automation to identify violations and potential violations of separation minima.
The indication given to the ADS-C controller varies for different equipment vendors, but generally an actual report is shown and then, based on information received in the report (ETA, speed...), extrapolation symbols are displayed until the next report is received.
Types of Contracts
The use of ADS-C with a minimum reporting rate (not greater than every 14 mins) allows longitudinal and lateral separation standards in oceanic areas to be reduced to 30NM/30NM. There are a number of contract types:
Periodic Contract: A report sent every "x" minutes. This can vary among various ground stations to suit their needs. The Periodic Contract specifies the reporting rate at which the avionics is required to assemble and downlink the requested information to the ground system. Once a periodic contract is established, it remains in place until it is cancelled or replaced by another periodic contract.
Event Contract: This can be one of four events which are waypoint crossing, Lateral Deviation Event(actual position exceeds the contracted lateral distance from the aircraft's expected position on the active FMS flight plan), Altitude Range Change Event(exceeding contracted value), and Vertical Rate Change Event(exceeding contract limits).
Demand Contract: a one-off request made by a controller for an ADS report containing only the Basic ADS Group. The Demand Contract is commonly known as a "one shot" report and is useful for updating ADS data and position information.
Each report from the aircraft includes the Basic ADS Group, which contains the current location and the altitude/level. The ground system receives the reports, and based on the vendor, will interpolate between the received data points. Current ATC designs are using more frequent update requests and fewer interpolations.
The Waypoint Change Event is triggered by a change made to the Next, or the Next-plus-one waypoint. This change normally occurs due to normal waypoint sequencing by the FMS. The Next or Next-plus-one waypoint can be either an ATS waypoint or a pilot inserted waypoint.
A situation where a Demand Contract report is useful is when an ADS aircraft is climbing or descending. The ADS level displayed for the flight data record does not update dynamically. That is, unlike a Mode C level readout, the ADS level displays the last level information reported and does not change until a new report is received.
Periodic Reports can also contain various groups of optional data generated by the avionics. Some ground systems will ask for automatic inclusion of this other data as part of the contract. Depending on its options, the aircraft may or may not be able to comply. This can include items such as wind/temperature(MET Group), ETA, altitude and Lat/Long for next position, next position +1(Predicted Route Group), true track, ground speed, vertical rate(Earth Ref Group), and Intent Group for route conformance.
The majority of information is exchanged system-to-system and is not specifically seen by either the controller or the pilot. In fact, the pilot can only turn the ADS-C application on or off, and select the ADS-C emergency mode on or off. And, in many ground systems, the controller can only change the periodic reporting rate for a particular aircraft to cater for situations, such as traffic density, where a higher or lower reporting rate is required or to manually request a Demand Contract in order to update the ATC display.
Other than emergency contracts, ADS-C reporting is controlled by the ground station in all situations Only the flight crew can declare and cancel ADS emergency reporting. Only the aircraft is capable of initiating, changing, or canceling the emergency/urgency mode. The provision of the emergency/urgency indication does not affect the operation of any ADS-C contract(s).
ADS contracts are established by the ground station following a logon from an aircraft. Where possible an agreement should be established between an aircraft and the ground system prior to the entry into airspace where ADS-C is in use. An agreement may also remain in effect for a period of time after an aircraft has exited this airspace. Termination of an ADS-C agreement may should be achieved automatically by the ground system.
Although, ADS and CPDLC are separate applications, they both use the same logon from the aircraft for their own purposes. ADS and CPDLC differ in the number of possible connections between one aircraft and various ground stations. For CPDLC, only one unit can communicate with a particular aircraft at any one time, and there can be only one other (inactive) CPDLC connection. For ADS, an aircraft can have connections with a maximum of four ground stations, plus another connection to the Airline's Operational Control area (AOC).
When an aircraft is on an offset route from the FMS route that exceeds 21 miles, any relevant waypoints on the original route will not be sequenced by the FMS. When a waypoint is not sequenced by the FMS, the intent information will continue to project to the just passed abeam waypoint until manually corrected by the pilot.
ADS-B is a separate system from ADS-C. As the name suggests, it operates in a broadcast mode where the aircraft broadcasts positional information on a regular basis into the ether. Any appropriately equipped station, including other aircraft, can intercept ADS-B reports from one aircraft. The reporting rate for ADS-B is significantly higher than ADS-C, which makes ADS-B a good candidate as a pseudo radar replacement system in traffic situations.
Note: when crossing into a new FIR, a position report is to be manually sent even if the aircraft is ADS-C equipped. This provides confirmation to ATC that the ADS-C equipment is selected to the proper FIR. It should not be assumed that a position report will automatically be sent at the FIR boundary if there is a waypoint at that location. Anchorage Oceanic uses 20 minute intervals for position reports not waypoints.