AIS is a system used by ships and vessels for identification at sea. AIS helps to resolve the difficulty of identifying ships when not in sight (e.g. at night, in radar blind arcs or shadows or at distance) by providing ID, position, course, speed and other ship data with all other nearby ships and VTS stations. The IMO SOLAS requires AIS to be fitted aboard all ships of gross tonnage >= 300 for international voyages. The ship datas are displayed on an AIS data capable chartplotter with a ship symbol or even on AIS data capable navigation software.

AIS transponders automatically transmit the position and velocity of the ship at regular intervals via a VHF radio built into the AIS. The position and velocity originate from the ship’s GPS or, if that fails, from an integral GPS receiver.

The AIS also receives heading information from the ship’s compass and transmits this at the same time. Other information, such as the vessel name and VHF call sign, is entered when installing the equipment and is transmitted less frequently. The signals are received by AIS transponders fitted on other ships or on land based systems, such as VTS systems.

In order to ensure that the VHF transmissions of different AIS transponders do not occur at the same time they are time multiplexed. In order to make the most efficient use of the bandwidth available, vessels which are anchored or are moving slowly transmit less frequently than those that are moving faster or are manoeuvring. The update rate of fast manoeuvring vessels is similar to that of conventional marine radar.

AIS transponders automatically transmit the position and velocity of the ship at regular intervals via a VHF radio built into the AIS. The position and velocity originate from the ship’s GPS or, if that fails, from an integral GPS receiver. The AIS also receives heading information from the ship’s compass and transmits this at the same time. Other information, such as the vessel name and VHF call sign, is entered when installing the equipment and is transmitted less frequently. The signals are received by AIS transponders fitted on other ships or on land based systems, such as VTS systems. In order to ensure that the VHF transmissions of different AIS transponders do not occur at the same time they are time multiplexed. In order to make the most efficient use of the bandwidth available, vessels which are anchored or are moving slowly transmit less frequently than those that are moving faster or are manoeuvring. The update rate of fast manoeuvring vessels is similar to that of conventional marine radar.