Automatic identification system

The automatic identification system (AIS) is an automatic tracking system that uses transponders on ships and is used by vessel traffic services (VTS). Originally designed as a terrestrial communication system (ship to ship; ship to shore), satellites are now also used to passively capture the signal traffic. With a few exceptions, AIS traffic is in the form of continual status announcements of a sender to all interested parties in its proximity. AIS information supplements marine radar, which continues to be the primary method of collision avoidance for water transport, since AIS requires faith in the accuracy of announcements sent by strangers while radar does not. Although technically and operationally distinct, the ADS-B system is analogous to AIS and performs a similar function for aircraft.

Information provided by AIS equipment, such as unique identification, position, course, and speed, can be displayed on a screen or an electronic chart display and information system (ECDIS). AIS is intended to assist a vessel's watchstanding officers and allow maritime authorities to track and monitor vessel movements. AIS integrates a standardized VHF transceiver with a positioning system such as a Global Positioning System receiver, with other electronic navigation sensors, such as a gyrocompass or rate of turn indicator.

The International Maritime Organization's International Convention for the Safety of Life at Sea requires AIS to be fitted aboard international voyaging ships with 300 or more gross tonnage (GT), and all passenger ships regardless of size. For a variety of reasons, ships can turn off their AIS transceivers. As of 2021, there were more than 1,644,000 ships equipped with AIS.

AIS offers essentially no security and relies on the cooperation of the participants and on the enforcement of its rules by law and on plausibility evaluation.