Automatic Identification System

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An automatic identification system (AIS) is an electronic system for improving maritime safety, applying many of the principles of air traffic control to the sea. AIS equipment automatically transmits information on its own identity, speed and heading, and other basic data that could predict a potential collision. This information is received by all AIS-equipped vessels, search and rescue agencies, and, in busy areas, vessel traffic systems. The navigational information is derived from onboard GPS receivers, preferably with the enhanced accuracy of differential GPS or the wide area augmentation system.

Larger ships are required, by the Safety of Life at Sea convention, to have AIS; there are several versions of the technology, with the lower end being affordable for recreational boats and commercial fishing vessels in busy waters. Depending on the AIS implementation, the information may be presented on a radar-like screen, or sent from the AIS receiver, via NMEA 0183 marine electronic interface, to a chartplotter that merges an electronic chart with AIS and other navigational aids. The various systems allow an operator to select a specific vessel within range, and obtaining its name, course and speed, classification, call sign, registration number, MMSI, and other information. Maneuvering information, closest point of approach (CPA), time to closest point of approach (TCPA) and other navigation information, more accurate and more timely than information available from an automatic radar plotting aid, could also be available. Costs continue to drop, especially when the AIS unit does not need its own display, but is one more input to a PC-based chartplotter.

Air traffic control principally depends not on radar or voice contact, but on automated transponders that continuously broadcast the key information. AIS is a shipboard broadcast system that acts like a transponder, operating in the VHF maritime frequencies. VHF is a short-range radio technology, which is advantageous in this application — vessels only hear information from other vessels that are close enough to affect them. Much like cellular telephony, the system capacity benefits from transmissions staying local to a reasonably sized "cell". Unlike cellular telephony, however, AIS does not depend on cell towers, but on the equipment distributed among vessels, controlled by an automatic networking method that allows handling well over 4,500 reports per minute and updates as often as every two seconds. The method, called Self-Organizing Time Division Multiple Access (SOTDMA) technology, is an example of what the Internet Engineering Task Force terms mobile ad hoc networking (MANET).

Information transmitted by AIS

The equivalent of transponders for ships is the automatic identification system (AIS). The "fast broadcast' is sent every 2 to 10 seconds while underway, and every 3 minutes while at anchor at a power level of 12.5 watts. "Slow broadcasts" repeat every 6 minutes.[1]

Fast broadcast datum Source Slow broadcast datum source
Maritime Mobile Service Identifier (MMSI) Administratively assigned and manually programmed Maritime Mobile Service Identifier (MMSI) Administratively assigned and manually programmed
Navigation status Standard codes such as "at anchor", "not under command", etc. International Maritime Organization (IMO) number Administratively assigned and manually programmed
Rate of turn Specific rate of turn indicator or autopilot Radio call sign Administratively assigned and manually programmed
Speed over ground Navigation computer, possibly autopilot Cargo type Defined in IMO table; manually programmed
Position accuracy GPS type (e.g., regular, DGPS) and if RAIM[2] is in use Name of ship Manually programmed
Longitude GPS Dimensions of ship Manually programmed
Course over ground Fluxgate compass or GPS compass computer Location on ship where reference point for position reports is located Manually programmed
True Heading Gyro input Type of position fixing device Manually programmed (e.g., GPS, undefined)
Time stamp GPS Draught of ship Manually provided
Destination Manually provided
Estimated time of Arrival at destination Manually provided


AIS information is used both by mariners and by maritime safety organizations.

A mariner would not have to make vague calls such as "ship on my starboard bow", but, assuming both vessels have VHF radios with digital selective calling (DSC), which is required for any vessel required to have AIS, it could be called by its specific "phone number" in the AIS message. AIS also provides the capability to send and receive short safety-related text messages.

Port safety

Busy ports that use the positive control of vessel traffic service build that on top of AIS. Just as private aircraft without full instruments and transponders cannot enter high-traffic airspace, security and safety requirements may bar non-AIS boats from busy waterways and harbors.

AIS is important to maritime security.

Fishing vessels

AIS does not replace vessel monitoring systems (VMS), but does complement VMS. While they do handle some of the same information, it goes to different audiences, at different update rates, and with different confidentiality requirements.


  1. United States Coast Guard Navigation Center, What AIS broadcasts
  2. Research and Innovative Technology Administration (RITA). U.S. Department of Transportation (US DOT), Integrity Monitoring