ADS (Automatic Dependent Surveillance) is a surveillance technique in which aircraft automatically provide, via a data link, data derived from on- board navigation and position- fixing systems, including aircraft identification, four-dimensional position and additional data as appropriate. �ADS data is displayed to the controller on a screen that resembles a radar screen.
CPDLC (Controller Pilot Data Link Communications) is a means of communication between controller and pilot, using data link for ATC communications). CPDLC is a two -way data-link system by which controllers can transmit messages to the pilot without the use of voice communications. The message is displayed on a flight deck visual display. CPDLC is an essential element of Automatic Dependent Surveillance (ADS)
Several different forms of ADS are currently in use or under development, including:
Automatic Dependent Surveillance-Broadcast (ADS-B) is a function on an aircraft or surface vehicle that broadcasts position, altitude, vector and other information for use by other aircraft, vehicles and by ground facilities. It has become the main application of the ADS principle.
Automatic Dependent Surveillance-Contract (ADS-C) functions similarly to ADS-B but the data is transmitted based on a contract between a ground system and an aircraft: Demand contract, periodic contract, event contract and emergency contract. This application is most likely to find application to sparsely trafficked transcontinental or transoceanic crossings.