Automatic Identification System



The Automatic Identification System (AIS) is included in the International Convention for the Safety of Life at Sea (SOLAS), and large ships began fitting AIS in July 2002.

AIS transmits, automatically and at set intervals, dynamic information relating to the ships course, speed and heading; static information related to the ships name, length, breadth; and voyage related details such as cargo information and status (underway, at anchor).

AIS is a Very High Frequency (VHF) radio broadcasting system that transfers packets of data over the VHF data link (VDL) and enables AIS equipped vessels and shore-based stations to send and receive identification information that can be displayed on a computer or chart plotter.

This information can assist in situational awareness and provide a means to assist in collision avoidance. In addition, AIS can be used as an aid to navigation, by providing location and additional information on buoys and lights.

There are two classes of AIS, Class A and Class B, as well as different types of AIS used for shore stations (AIS Base Stations), aids to navigation (AIS AtoN), AIS on search and rescue (SAR) aircraft and AIS search and rescue transmitters (AIS SART).


Message types and formats

AIS employs the principle of using a ship’s speed and maneuvering status as a means of governing information update rates and ensuring the appropriate levels of positional accuracy for ship tracking. A similar process is applied to the content of ship information messages to ensure that the data being transferred is not encumbered with static or low priority information.

The different information types, identified as “static”, “dynamic” or “voyage related” are valid for different time periods and thus require a different update rate.

Information included in the various message types is:

Static information: Every 6 minutes and on request•


– IMO number (where available);

– Call sign & name;

– Length and beam;

– Type of ship; and

– Location of the position-fixing antenna on the

ship (aft of bow/ port or starboard of centreline).

–Dynamic information: Dependant on speed and

course alteration

– Ship’s position with accuracy indication and

integrity status;

– Position time stamp (in UTC);

– Course over ground (COG);

– Speed over ground (SOG);

– Heading;

– Navigational status (e.g. at anchor, underway,

aground etc. - this is input manually); and

– Rate of turn (where available).

Voyage related information: Every 6 minutes, when

is data amended, or on request

– Ship’s draught;

– Hazardous cargo (type);

– Destination and ETA (at masters discretion); and

– Route plan (waypoints).

Short safety-related messages:

– Free format text message - sent as required.


Class B transponders are designed for carriage by sub-SOLAS vessels. Each consists of one VHF transmitter, two VHF Carrier Sense Time Division Multiple Access (CSTDMA) receivers, one of which is multiplexed with the VHF Digital Selective Calling (DSC) receiver, and a GPS active antenna. Although the data output format supports heading information, in general units are not interfaced to a compass, so this data is seldom transmitted. Output is the standard AIS data stream at 38.400 kbit/s, as RS232 and/or NMEA formats. To prevent overloading of the available bandwidth, transmission power is restricted to 2 W, giving a range of about 5–10 mi.

Four messages are defined for class B units:

Message 14: Safety Related Message

This message is transmitted on request for the user – some transponders have a button that enables it to be sent, or it can be sent through the software interface. It sends a pre-defined safety message.

Message 18: Standard Class B CS Position Report

This message is sent every 3 minutes where speed over ground (SOG) is less than 2 knots, or every 30 seconds for greater speeds.

MMSI, time, SOG, COG, longitude, latitude, true heading

Message 19: Extended Class B Equipment Position Report

This message was designed for the SOTDMA protocol, and is too long to be transmitted as CSTDMA. However a coast station can poll the transponder for this message to be sent.

MMSI, time, SOG, COG, longitude, latitude, true heading, ship type, dimensions.

Message 24: Class B CS Static Data Report

This message is sent every 6 minutes, the same time interval as for Class A transponders. Because of its length, this message is divided into two parts, sent within one minute of each other.

Note that this message was defined after the original AIS specifications, so some Class A units may need a firmware upgrade to be able to decode this message.

1.         MMSI

2.         Boat name

3.         Ship type

4.         Call sign

5.         Dimensions

6.         Equipment vendor id.