AIS (Automatic Identification System) is a VHF based automated system for identification, collision avoidance, and fleet tracking, Most commercial vessels are equipped with "Class A" equipment which transmits identity, positon, course and speed on two dedicated VHF channels. Other information such as destination and ETA are also sent, but require to be input by the crew. Transmit power is 12.5W and the transmissions are repeated every few seconds if the ship is travelling fast or executing a turn; once a minute if anchored.
Participating vessels can display the location of all ships in the vicinity, and the data can be integrated with radar and electronic displays to warn of potential collisions, and to enable unambiguous communications. Coastguard and harbour control stations can use the system to help them organise efficient traffic control and promote better safety at sea.
Smaller vessels not required to carry Class A AIS can participate at lower cost by fitting a Class B transceiver. This has the same receiving capability, but transmits less often and at lower power (2W). It also has a simplified interface and does not require data to be input for each journey.
By fitting an AIS receiver, vessels can opt to just "listen in" to these transmissions. This allows much easier identification of big ships which might be on a collision course, and is a big advantage if used in combination with other methods such as radar and Mk.1 eyeball. In particular you can detect vessels out of sight around a bend or on the other side of a headland. However with just a receiver you do not appear on anyone else's AIS display, so they cannot use AIS to contact you or avoid you.