What Is AIS Tracking?

The system that made live ship maps possible

The Automatic Identification System (AIS) was designed in the 1990s as a collision-avoidance tool: ships continuously broadcast who they are, where they are and where they're heading, so nearby vessels and coastal authorities can see them electronically. Under the SOLAS convention, AIS is mandatory for all vessels over 300 gross tons on international voyages, all cargo ships over 500 gross tons, and all passenger ships.

What a ship actually broadcasts

AIS transmits two message families. Dynamic data — GPS position, speed over ground, course, heading and rate of turn — is sent every 2 to 10 seconds while under way. Static and voyage data — name, MMSI, IMO number, call sign, dimensions, ship type, draught, destination and ETA — goes out every six minutes. Together they form the vessel profile you see when you click a marker on any AIS live map.

Terrestrial vs satellite AIS

VHF radio is line-of-sight, so shore receivers cover roughly 40–60 nautical miles. Dense receiver networks around chokepoints like the Strait of Hormuz deliver updates within seconds. Beyond coastal range, satellite constellations pick up the same broadcasts, with latency measured in minutes to hours — good enough for ocean passages, less so for tactical monitoring.

How accurate is AIS?

Position data comes straight from a ship's GPS, so accuracy is typically within a few metres. The weaknesses are human and deliberate: destination fields are typed manually (and often stale), transponders can be switched off, and "spoofing" — broadcasting false positions — has been documented in sanctioned trades and conflict zones. Analysts treat sudden AIS gaps in the Gulf as signals in their own right.

See AIS in action

Open the Strait of Hormuz live tracker and every marker you see is an AIS broadcast decoded in near real time. For hands-on searching by name, IMO or MMSI, try the vessel finder.