What is AIS

Automatic Identification System (or AIS) is an automatic maritime tracking system used by vessels to broadcast their position, identification, and other information to other vessels in their vicinity, ports, and coastal ground stations. AIS had been a mandatory requirement in commercial shipping since 2004 for all vessels that weigh over 299 gross tons (GT) and operate internationally.

AIS Objectives

The primary objectives of AIS are:

  • Collision Avoidance & Maritime Situational Awareness
  • Vessel Identification & Port Traffic Management

AIS Transmissions

AIS uses the VHF band for transmission operates principally on two dedicated frequencies or Very High Frequency channels (VHF channels):

  • AIS 1: Operates on the 161.975 MHz- Channel 87B (Simplex – Ship to Ship).
  • AIS 2: Operates on the 162.025 MHz- Channel 88B (Duplex – Ship to Ground Stations).

To meet the high broadcast rate AIS uses Self Organizing Time Division Multiple Access (STDMA) technology.

AIS Data & Reports

AIS reports include the vessel’s name, MMSI number, IMO number type, size, navigational data, destination port, speed over ground, and other voyage information. Presently, satellites are employed to receive and broadcast AIS data, this makes tracking vessels highly reliable and accurate.

Vessels fitted with AIS transceivers can be tracked by other vessels with an AIS transceiver or nearby ground stations. By transmitting AIS vessel positions to coastal stations and maritime traffic surveillance centers on the coast, monitoring and guidance of ship traffic can be performed. AIS also enables coast guards and government agencies to monitor illegal fishing, and potential incursions.


  • Collision avoidance
  • Search and rescue
  • Fleet and cargo tracking
  • Maritime security
  • Fishing fleet monitoring and control
  • Accident investigations

How does it work?

AIS transponders on board the vessel continuously and automatically transmit AIS data (position, navigation & identification data) at periodic intervals via VHF. The VHF signals, containing the respective vessels AIS data is received by AIS transponders fitted on other ships or on land-based systems, such as VTS systems. This data is then displayed either on a chart plotter showing the positions of the other vessels or on a radar display.

A vessel’s positional and velocity data are obtained from the from the vessel’s GPS system while the navigational data such as heading, and course are received from the vessel’s compass. Identification data, such as the vessel’s call sign, IMO number, size, type and name, is preset and is less frequently transmitted.

AIS Transceivers Classes

AIS transceivers are classified into 2 types - Class-A and Class-B.

Class A Transceivers

Class A AIS transceivers are mandated on commercial vessels >= 300 gross tons and operating internationally. The transponder, which operates on 12 watts, transmits information every 2 to 12 seconds within a range of 20 miles. Class A transponders transmit more vessel data in comparison to Class B transponders.

Class B Transceivers

Class B AIS transceivers are generally used by smaller vessels such as yachts and speed boats. These transponders, broadcast AIS data less frequently (30 second intervals) and have a transmit range of 4-6 miles. Class B transceivers only transmit the vessel's MMSI number, current position, course, and size. The primary function of Class B transceivers is to provide the safety and navigation benefits of AIS to smaller vessels at lower cost.

AIS Data Transmitted

Static Data

  • Vessel name
  • Ship size
  • International Maritime Organization Number (IMO)
  • Maritime Mobile Service Identity Number (MMSI)
  • International Call Sign
  • Type of vessel (e.g., tanker, tug)

Dynamic Data

  • Vessel Position
  • Heading
  • Speed Over Ground (SOG)
  • Rate of Turn (ROT)
  • AIS Navigation status
  • Course Over Ground (COG)
  • Coordinated Universal Time (UTC)

Voyage Data

  • Planned arrival time (ETA)
  • Maximum Draught
  • Destination Port
  • Cargo Category

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