Breaking down the ABCs of AIS


The data that AIS transmits is standardized by the International Telecommunications Union and adopted by the International Maritime Organization.

That means that all AIS units communicate the same data in the same format, assuring that we are all on the same page.

AIS is available at three levels: Class A, Class B and Class C. Classes A and B require vessel-installed transponders, while Class C is limited to “receive only.” Each Class A AIS, which has been mandated for installation aboard commercial vessels, includes a 12.5-watt VHF transmitter, an integral GPS receiver, two VHF TDMA receivers, one VHF DSC receiver, and a standard NMEA 0183 interface to shipboard display and sensors.

The Class A commercial system broadcasts the vessel’s Maritime Mobile Service Identity number, along with specific navigational information, every two to 10 seconds while under way, and every three minutes at anchor. The initial broadcast contains the ship’s navigational status, rate of turn in degrees per minute, speed over ground in 1/10 resolution, latitude and longitude to 1/10,000 minute, course over ground within 1/10 degree, true heading, and the time the data was generated. Class A AIS also broadcasts a second data set every six minutes consisting of the vessel’s name, MMSI number, IMO number (a unique reference number related to the ship’s construction), and radio call sign, along with information specific to the ship’s physical characteristics, such as dimensions and draft. It also provides the type of ship, its cargo, destination, and estimated arrival (month, day, hour and minute).

Class B AIS was designed for light commercial vessels and recreational boats. It generally consists of a 2-watt VHF transmitter, a GPS receiver and two VHF receivers, one of which is used to decode DSC transmissions as well as AIS. Class B AIS transponders broadcast static vessel information every six minutes, including MMSI, radio call sign, and vessel name, length, beam and type. Position updates that include the MMSI, speed over ground, latitude, longitude, course over ground and true heading are broadcast less often than Class A transponders. Vessels traveling less than 2 knots transmit position updates every three minutes, while vessels traveling more than 2 knots transmit position information every 30 seconds.

Class C AIS is strictly receive-only. The receivers pick up broadcasts from both Class A and Class B AIS transponders, but do not transmit data. They provide the skipper with accurate positions of other AIS-equipped vessels based on their internal GPS and, as with Class A and B, make it possible to “see” behind large vessels and land masses. Class C AIS also allows the identification of aids to navigation equipped with AIS transmitters.

Since AIS uses the same frequencies as marine VHF, it has similar radio reception capabilities. Proper antenna installation is important, with a higher antenna typically providing a greater reception area. Class A-equipped vessels, with their higher transmit power and higher antenna installation, reportedly experience 20- to 30-mile reception, while Class B AIS reception is around 5 to 10 miles.

See related articles:

- AIS: the electronic eyes of navigation

- The carriage regulations for AIS

This article originally appeared in the August 2009 issue.