What is AIS?

AIS = Automatic Identification System
A quick look at the basic technology


The Automatic Identification System (AIS) is a maritime transponder/receiver system defined by the IMO (international maritime organization) AIS operates in the VHF frequency band. AIS main objectives are :

  •  to improve maritime safety
  •  to protect the maritime environment

Practically the AIS:

  • assists collision avoidance (AIS ship-to-ship)
  • enables ports and coastal states to identify ships and to manage and supervise the traffic in their waters (AIS Coast station).


AIS is a system used by ships and vessels for identification at sea. It 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 data is displayed on a AIS data capable chartplotter with an 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 maneuvering. The update rate of fast maneuvering vessels is similar to that of a conventional marine radar.

AIS transceiver sends the following data every 2 to 10 seconds depending on vessels speed while underway, and every 3 minutes while vessel is at anchor. This data includes :

  • MMSI number of vessel – vessel’s unique identification
  • Navigation status – “at anchor”, “under way using engine” or “not under command”
  • Rate of turn – right or left, 0 to 720 degrees per minute
  • Speed over ground – 0.1 knot resolution from 0 to 102 knots
  • Position accuracy
  • Longitude – to 1/10000 minute and Latitude – to 1/10000 minute
  • Course over ground – relative to true north to 0.1 degree
  • True Heading – 0 to 359 degrees from eg. gyro compass
  • Time stamp – UTC time accurate to nearest second when this data was generated

Not all of above data is displayed (Quantity of shown data is software of the Chartplotter).


Typically you can see on the chartplotter following data :

  • MMSI number of vessel – vessel’s unique identification
  • Speed over ground – 0.1 knot resolution from 0 to 102 knots
  • Course over ground – relative to true north to 0.1 degree

In addition, the following data is broadcast every 6 minutes:

  • MMSI number – vessel’s unique identification
  • IMO number
  • Radio call sign – international radio call sign assigned to vessel
  • Name – Name of vessel, max 20 characters
  • Type of ship/cargo
  • Dimensions of ship – to nearest meter
  • Location of positioning system’s (eg. GPS) antenna onboard the vessel
  • Draught of ship – 0.1 meter to 25.5 meters
  • Destination – max 20 characters
  • ETA (estimated time of arrival) at destination – UTC month/date hour:minute


The AIS is mandatory since 2002 for ships constructed on or after July 2002. It is already fully effective for passenger ships and tankers constructed before July 2002 engaged on international voyages. End of July 2004, it will be mandatory to all cargo ships 50,000 gt engaged on international voyages. It will be adopted by all ships of 300 gt and upwards engaged on international voyages or not, in 2008.

Above given information is not complete and should give only an overview about the AIS topic. For more details please have a look to the following links :

US Coust Guardwww.imo.orgwww.aislive.com



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