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• AIS is a system used by ships and vessels for identification at sea.

it helps 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 (Vessel Traffic Services) stations. The IMO SOLAS requires AIS to be fitted aboard all ships of gross tonnage > 300 tons for International voyages. The ship data are displayed on an AIS data capable chart plotter with a ship symbol or 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(Global Positioning System) 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’s name and VHF call sign ,is entered when installing the equipment and is transmitted less frequently.

• In order to ensure that the VHF transmissions of different AIS transponders do not occur at the same time they are time multiplexed.

 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 a conventional marine radar.

• AIS transceivers send the following data every 2 to 10 seconds depending on speed while underway, and every 3 minutes while at anchor.
• This data includes

MMSI number of vessel-unique identification
Navigation status –“at anchor”-“under way using engine”

Rate of turn

Speed over ground

Position accuracy

Longitude and latitude

Course over ground

True heading

Time stamp

• Not all this data will be displayed as this depends on software in chart plotter.-MMSI, speed and course.
  Every 6 minutes IMO number, radio call sign, name of vessel , type of vessel , dimensions, draught, destination, ETA.
  
• The access of this information to the leisure market using suitable receivers linked to a chart plotter offers an additional safety and navigation service especially when transiting areas used by commercial shipping such as shipping lanes or Port facilities. As always this should be supplemental to good lookout practise and correct crossing routines.