ICAO Annex 3 — Meteorological Service for International Air Navigation
CHAPTER 1 - DEFINITIONS

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.

Note.— The abbreviated term “ADS contract” is commonly used to refer to ADS event contract, ADS demand contract, ADS periodic contract or an emergency mode.

The ADS (Automatic Dependent Surveillance) is a FANS application that consists in automatically sending aircraft surveillance data to the ATC. The set of data to downlink is defined automatically, without any crew action.
The different types of ADS contracts can be:

• Periodic – at periodic time intervals.
• On demand – when asked for by the ATC.
• On event – whenever a specified event occurs.

Emergency messages can also be sent through the ADS application. When a MAYDAY datalink message is sent to the ATC, the ADS contract is switched to a high periodic reporting rate.