AIS vessel tracking

AIS vessel tracking uses the Automatic Identification System, a VHF radio transponder that most commercial and many fishing vessels are required to carry, which broadcasts position, speed, heading and vessel identity every few seconds to a few minutes. Shore-based receivers and, increasingly, satellites collect these broadcasts into a continuous global feed of ship movement.

Analysts reconstruct vessel tracks from the raw message stream, interpolating gaps and filtering noise, then apply spatial analysis such as speed and course changes to infer behavior like fishing activity, transshipment at sea, or loitering near a restricted zone, often within a geographic information system. This differs from satellite imagery analysis of vessels, which detects ships optically or by radar regardless of whether they are broadcasting, and is the main way analysts catch vessels that turn off their AIS transponder to hide activity, known as "going dark."

AIS data underpins fisheries enforcement, environmental monitoring of protected marine areas, sanctions compliance, and supply-chain due diligence, and combining it with geofencing around marine protected areas flags unauthorized entry automatically, a common technique in maritime geospatial intelligence work. The main pitfalls are that AIS is self-reported and can be spoofed or switched off, coverage from shore-based receivers thins far from coastlines, and small or non-compliant vessels may not carry a transponder at all.