What is AIS?

The transponder transmits automatically over VHF radio, every few seconds while the ship is under way. Each message carries the vessel’s identity, its position from satellite navigation, its course and its speed. The crew add the destination and an expected arrival time by hand.

The system was built so that ships could see each other and avoid collisions. The SOLAS convention requires it on ships of 300 gross tonnage and upwards on international voyages and on every passenger ship, which covers every container ship in ocean service. Other vessels and coastal stations receive the signal directly. VHF reaches only a limited distance, so far from land the messages are collected by satellite.

You meet it as a map: a shipping line’s tracker that draws a ship on its route is plotting AIS, and so are the free ship-tracking websites. To use one, you need the vessel’s name. Take it from the “loaded” event on the line’s tracking page, which names the ship and the voyage, and look that ship up.

This site shows neither ships nor containers on a map. The container number checker verifies a number, names the owner of its prefix and points you to the line’s own tracker, where the vessel is named.

AIS knows nothing about cargo. If the container was discharged at a hub for transshipment, or was never loaded because it missed the cut-off, the ship sails on and the map keeps showing it. People have watched a vessel arrive and sent a truck for a box that was still two ports behind. Confirm with the line’s event list that the container is aboard before you follow the ship.

Two smaller traps follow. The destination and arrival time in an AIS message are typed in by the crew and can be out of date; the line’s own estimate is the one to plan with. And a ship that vanishes from a free map in mid-ocean has usually just sailed beyond the reach of coastal receivers. On land there is no dot at all, only terminal events. A position for the box itself exists only on containers fitted with their own tracking devices.