Automatic Identification System (Automatic Information System)
The Automatic Identification System (AIS) is a radio transponder on board a ship. It broadcasts the ship's details on VHF so that nearby ships and shore stations can see who is around them. AIS was introduced for collision avoidance, and today the same signals feed live ship maps such as the map on this site.
The name "automatic information system" also appears, for example in a text by the International Maritime Organization (IMO). IMO rules require AIS on all ships of 300 gross tonnage and upwards on international voyages, cargo ships of 500 gross tonnage and upwards on other voyages, and all passenger ships regardless of size. The requirement has applied to all ships since the end of 2004.
What AIS transmits
Static data is entered once and rarely changes: the ship's name, call sign, IMO number, MMSI number, type, length and beam.
Voyage data is entered by the crew before each voyage: destination, estimated time of arrival and draught.
Dynamic data comes from the ship's sensors, mainly GPS: position (latitude and longitude), course over ground, speed over ground and rate of turn. It is updated every few seconds to a few minutes, depending on the ship's speed.
How AIS reaches the map
Ships transmit AIS only over VHF radio, not satellite or mobile networks, so sending and receiving it over the radio costs nothing beyond the equipment. Shore receivers pick up ships within roughly 40 nautical miles, and further when the antennas stand high. Satellites receive AIS in the open ocean, where there are no shore stations.
Why some ships are missing
A ship may be out of range of any receiver, its transponder may be switched off, or it may send wrong data. Military ships often keep AIS off. When a ship is out of range, the map shows its last known position.
Browse the regional maps to open the map already zoomed to a sea or strait.