AIS (Automatic Identification System) is the mariner’s most significant development in navigation safety since the introduction of radar. The system was originally developed as a collision avoidance tool to enable commercial vessels to ‘see’ each other more clearly in all conditions and improve the helmsman’s
information about his surrounding environment. AIS does this by continuously transmitting a vessels
identity, position, speed and course along with other relevant information to all other AIS equipped vessels within range. Combined with a shore station, this system also offers port authorities and maritime safety bodies the ability to manage maritime traffic and reduce the hazards of marine navigation.
Due to the great safety benefits offered by AIS, this technology was made compulsory throughout the world in 2002 for all passenger ferries and vessels over 300 gross tonnes.

AIS graphic


The Automatic Identification System was developed to allow ships and coastal stations to accurately locate and identify one another. An AIS transceiver uses VHF radio and GPS technology to communicate with other nearby ships.

An AIS transponder determines its own position, speed and course using a built in GPS receiver. This
information is combined with other important navigation information and automatically communicated between AIS equipped vessels without any user interaction.

AIS transponders on other vessels and coast stations receive this information and use it to build up a live graphical display of traffic in the area. The transponder can be connected to many types of chart plotter or PC charting software to give a RADAR type display of vessel positions. AIS does not require a radar, but can offer similar capabilities and even enhance a radar image if a radar has already been fitted to the vessel. The range or coverage of the system is similar to a VHF radios. The system also has the advantage that VHF radio signals will travel around bends and over islands giving better coverage than RADAR or enhancing a RADAR picture when used together.

How AIS works Diagram


  • See and be seen: A Class B transponder continuously receives information form all Class A and Class B equipped vessels around you and displays this information on your standard chart plotter or PC. At the same time your Class B will transmit your position to all AIS equipped vessels automatically
  • Safety at night and in poor weather conditions: The class B AIS is a vital navigation tool in poor visibility conditions. The information received from other AIS ‘targets’ provides the user with vital navigation information and the position transmission alerts other vessels to the users location
  • Combined with radar, AIS gives you the best possible picture of your situation in all conditions
  • Safety in high traffic / commercial shipping areas
  • Position transmission to authorities / nearby vessels in case of emergency
  • Best possible picture of a dynamic environment (moving vessels)
  • Graphically view your position in relation to other vessels when connected to suitable display
  • Ability to ‘see around the corner’
  • Track vessels of interest such as friends/ colleagues “See and be seen”


Two types of AIS transponder are available, Class A and Class B.

Class B transponders have been developed to provide the safety and navigation benefits of AIS to smaller vessels with lower cost and simpler installation when compared to Class A. As the Class B system was developed after the introduction of Class A it was designed to be compatible whilst protecting the safetycritical operation of the Class A system for big ships.

AIS receivers are also available; these units will pick up broadcasts from both Class A and Class B AIS transponders but don’t transmit their own position. A receiver allows you to see the position of other vessels however your position won’t be visible to them.

The following diagram shows what information each type of transponder will receive from the other. Please note that in the last example ‘Class B- Class A’ there are some instances when a class A user may not see the name, call sign and vessel type- however the users vessel will still be displayed in it’s correct position. In this circumstance, the receiving vessels display may default to showing the MMSI rather than the vessels name.


A Class B AIS transponder can be fitted to any non-SOLAS vessel. A Class A transponder is only required on ships in excess of 300 gross tons on international voyages.

Receive only AIS units are available however these lack the ability to broadcast your vessels position to others. Receive only units aren’t subject to international standards so don’t necessarily have the same ability to receive all AIS messages on all channels.

AIS Comparison Table

* Some Class A transponders and receive only devices may require software update to receive the latest Class B vessel details messages.

** All Comar AIS receiver products have 2 parallel channels and can receive messages on both channels simultaneously.