How ship tracking works
Ship tracking looks like magic: open a map and see tens of thousands of vessels move in real time, but the system behind it is surprisingly simple. This page explains where the data comes from, how accurate it is, and how to read the details you see when you click a ship.
AIS: the system behind every ship tracker
Since 2004, international rules require AIS transponders on all passenger ships and on cargo ships above 300 gross tonnage on international voyages. The transponder broadcasts the ship’s identity and GPS position every few seconds while under way. Shore stations receive those broadcasts; a typical station reaches roughly 15–20 nautical miles, more if the antenna is mounted high. Everything beyond coastal range is covered by satellite AIS, with longer update intervals.
How accurate and how live is the map?
Along coasts and shipping lanes, positions are usually seconds to a couple of minutes old. In mid-ocean, a position can be older, because the ship is between satellite passes. So if a vessel seems frozen far from shore, it usually hasn’t stopped: the map is simply waiting for the next signal.
Reading the details: course, draught, ETA
Course is the direction the ship is moving over the ground, in degrees. Speed is given in knots (one knot is 1.852 km/h). Draught is how deep the hull sits in the water: a loaded tanker sits much deeper than an empty one. Destination and ETA are entered by the crew, so they are usually right but not guaranteed.
What people use ship tracking for
Shipping professionals plan port calls and monitor fleets. Travellers follow the cruise ship of family members. Spotters check which vessels will pass their stretch of coast. And if you’re expecting cargo, following the carrying vessel gives a realistic idea of when it will arrive.