What is AIS?

The Automatic Identification System (AIS) is an automatic tracking system that allows vessels to share their identity and update position, speed, course and other information with similarly equipped vessels and AIS base stations. The system is designed to improve navigational communication between vessels to help avoid collisions even in bad weather, poor visibility or congested waterways.

AIS information supplements marine radar which continues to be the primary method of collision avoidance. Nevertheless, AIS promises one of the biggest advances in marine navigation and safety procedures since the introduction of GPS.




How it Works

Position and associated data are derived from a GPS device and communication between vessels is by VHF radio transmissions on channels specifically allocated within the marine VHF range. Coverage is similar to other VHF equipment, typically 20-30 nautical miles but AIS’s ability to ‘see’ around corners can be a benefit over radar if nearby landmass does not hinder transmissions.

Information such as the vessel's name and VHF call sign is programmed when installing the equipment and is sent when the data is transmitted. the received information can be displayed on a screen or chart plotter showing the other vessels' positions in much the same manner as a radar display. AIS enabled vessels can determine risk of collision, locate and identify each other and share information with shore-based authoriteis. It can also provide an exact location to rescue authorities. While AIS won't replace radar as a colliosn avoidance tool, it remains a significant boost to safety at sea.

AIS Types

The AIS standard comprises several sub-standards or 'types' which specify individual product types. The specification for each product type provides a detailed technical specification which ensures the overall integrity of the global AIS system within which all the product types must operate. The major product types described in the AIS system standards are:

Class A

Class A has been mandated by the International Maritime Organisation (IMO) for vessels of 300 gross tonnage and upwards engaged on international voyages, cargo ships of 500 gross tonnage and upwards not engaged on international voyages, as well as passenger ships (more than 12 passengers), irrespective of size. Class As must have an integrated display, transmit at 12 W, interface capability with multiple ship systems, and offer a sophisticated selection of features and functions. Default transmit rate is every few seconds. AIS Class A type compliant devices receive all types of AIS messages.

Class B

Class B provides limited functionality and is intended for non-SOLAS vessels. It is not mandated by the International Maritime Organisation (IMO) and has been developed for vessels such as work craft and pleasure craft. Class Bs can be connected to most display systems which the received messages will be displayed in lists or overlayed on charts. Default transmit rate is normally every 30 seconds, but this can be varied according to vessel speed or instructions from base stations. The Class B type standard requires integrated GPS and certain LED indicators. Class B equipment receives all types of AIS messages.

Receive Only

Where collision avoidance is the major concern, AIS Class B - Receive Only units will pick up transmissions but they won’t transmit any information about your vessel. So, you will ‘see’ other vessels but you won’t ‘be seen’.


AIS Base Station

Base Stations are provided an ‘aids to navigation authority’ to enable the ship to shore / shore to ship transmission of information. Networked AIS Base Stations can assist in providing overall maritime domain awareness.


AIS Aids to Navigation (AtoN)

AIS AtoN provide an opportunity to transmit position and status of buoys and lights on an electronic chart, computer display or compatible radar.



Search and Rescue Radar Transmitters (SART) using AIS can help determine the location of vessels in distress.


AIS on Search and Rescue (SAR) Aircraft

Search and Rescue Aircraft can use AIS to assist in their operations.



All AIS equipment requires connection to both a VHF and GPS antenna. For maximum range, VHF radio aerials are usually mounted as high as possible – typically on mastheads or suchlike. To avoid interference, and to be in accordance with IMO regulations, the AIS antenna should be mounted so as to avoid the same horizontal plane of VHF radio transmissions – usually at a lower position on a mast’s spreader for example. 
For ease of installation, VHF splitters are also available and may be a cost effective and convenient choice as this allows sharing AIS with an existing VHF radio aerial. However, decreased performance of each may be a consequence, as only one unit can transmit at a time. In addition, AIS and VHF radio frequencies differ slightly, so opting for a dedicated AIS tuned antenna will almost certainly boost performance.

Please see: www.marineraffic.com/ais/ for worldwide, real time and actual vessel positions and course details.

© Great Circle Marine 2012