Automatic Identification System (AIS) is a system used by ships and vessel traffic systems (VTS) principally for identification of vessels at sea. AIS helps to resolve the difficulty of identifying ships when not in sight (e.g. at night, in fog, in radar blind arcs or shadows or at distance) by providing a means for ships to exchange ID, position, course, speed and other ship data with all other nearby ships and VTS stations. It works by integrating a standardized VHF transceiver system with a GPS receiver and other navigational equipment on board ship (Gyro compass, Rate of turn indicator, etc.).
The IMO SOLAS requires AIS to be fitted aboard all ships greater than/equal to 300 gross tons for international voyages.
Applications and limitations
AIS is used in navigation primarily for collision avoidance. Because of the limitations of radio characteristics, however—and because not all vessels are equipped with AIS—the system is meant to be used primarily as a means of lookout and to determine risk of collision in accordance with COLREGS rather than as an automated collision avoidance system.
When a ship is navigating at sea, the movement and identity of other ships in the vicinity are very important to navigators to make decisions to avoid collision with other ships and dangers (shoal or rocks). Visual (eye, binoculars) / audio means (whistle, VHF radio), radar and ARPA were primarily used for this purpose. However, a lack of positive identification of the targets on the displays, and time delays and other limitation of radar for observing and calculating the action and response of ships around, especially on busy waters, sometimes prevent possible action in time to avoid collision.
While AIS provides only a very basic graphical display, the data obtained can be integrated with ECDIS or a radar, providing most navigational information on a single display.
However for navigators to know the full traffic situation in the vicinity of their ships is very difficult in busy waters, although VTS may provide traffic management services if available in some harbor or river areas. AIS is a very useful aid for navigators to know a much better traffic situation of what kind of other ships and their movement with identities of targets around much earlier and accurate than just by other methods.
There are also many new ideas for the application of AIS, e.g. virtual AIS for aids to navigation and supplement to the on scene coordination for search and rescue.
Use of virtual AIS for marking important navigational information provides significant improvement to indicate the position of new wrecks, other new dangers or even navigational warning in relation to the ship's position. Virtual marking of dangers can be transmitted by virtual AIS stations (e.g. VTS) and ships can know the position of dangers on the AIS display much quickly, accurate and easier than physical navigational marks and traditional safety message broadcasted by VHF Radio, Navtex, SafetyNet or Notice to Mariners. Of course such virtual aids would only be visible to AIS equipped ships but it is most important for large ocean going ships.
For coordinating resources on scene of marine search & rescue operation, it is important to know the position and navigation status of ships in the vicinity of the ship or person in distress. With the aid of AIS, which could provide much better picture of the resources for on scene operation even the AIS range is limited to VHF radio range, usually about 30 to 50 nautical miles (60 to 90 km).
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(s)", "not under command", etc
- 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
In addition, the following data is broadcast every 6 minutes:
- MMSI number - vessel's unique identification
- IMO number - number remains unchanged upon transfer of the ship to other flag(s).
- 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
- Type of positioning system - usually GPS or DGPS
- 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
More about AIS
The Complete Guide to AIS Automatic Identification System (PDF, 1610Kb)
US Coast Guard Navigation Center: AIS Overview