AIS (Automatic Identification System)

AIS is a transponder system that broadcasts a vessel's identity, position, course and speed over VHF radio. Mandatory on commercial ships over 300 gross tons on international voyages, it is the backbone of modern vessel tracking: coastal stations and satellites collect AIS signals to build the live maritime traffic picture used by ports, carriers and tracking platforms.

AIS data powers most vessel-tracking services, including congestion analytics and estimated times of arrival. Its limits matter too: transponders can be switched off, and signal coverage at open sea depends on satellite constellations.