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AIS (Automatic Identification System)

Introduction to AIS

Automatic Identification System (AIS) is a radio system that receives positions of other vessels utilizing the system. This, in addition to radar, make realtime navigation around vessels safer. AIS is intended to assist the normal watch keeping procedures on a vessel and allow maritime authorities to track and monitor vessel movements.

Information provided by AIS equipment, such as unique identification, position, course, and speed, can be displayed on a dedicated AIS screen or fed by a protocol (commonly NMEA 0183) to a chart plotter or computer system.

AIS systems, because they receive information on the position, course, and speed of nearby vessels can make decisions and raise alarms should a collision between the current vessel and any nearby vessel be a possibility.

AIS operates over the VHF Radio network on a dedicated channel / frequency. See VHF Frequencies for more information.


AIS transmitters automatically broadcast information, such as their position, speed, and navigational status, at regular intervals via a VHF transmitter built into the transmitter. The information originates from the ship's GPS. Other information, such as the vessel name and MMSI, is programmed when installing the equipment and is also transmitted regularly.

Class A

Class A transmitters are targeted at large commercial vessels. Class A transceivers will then pre-announce their transmission, effectively reserving their transmit slot and are therefore prioritised within the AIS system. This is achieved through 2 receivers in continuous operation. Class A's must have an integrated display, transmit at 12.5 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 AIS is aimed at lighter commercial and leisure markets. Class B transceivers listen to the slot map immediately prior to transmitting and seek a slot where the 'noise' in the slot is the same or similar to background noise, thereby indicating that the slot is not being used by another AIS device. Class Bs transmit at 2 W and are not required to have an integrated display: Class Bs can be connected to most display systems where the received messages will be displayed in lists or overlaid on charts. Default transmit rate is normally every 30 seconds. Class B equipment receives all types of AIS messages.

Transmitters and Receivers

Class B AIS units are available as receive-only units or as transceivers, which both receive and transmit. All Class A AIS units are transceivers, there are no receive-only Class A AIS units. Receive-only units are very much cheaper than transceivers, which makes them an attractive proposition when considering the additional safety of an AIS unit versus the cost. It is possible to implement Collision Avoidance on board with a receive only AIS unit, however you need to be aware that other vessels will not be made aware of your presence and so it becomes your responsibility to avoid any potential collisions alerted by an AIS collision alarm.

Some VHF Radio systems have integrated class B receivers built in. This is a simple and cost effective way of having receive only AIS on board a sailing yacht.

Using AIS for Navigation

A typical dedicated AIS display

Collision Avoidance

The primary safety features of AIS receivers is a collision alarm feature. Using the known position, speed and course over ground of your own vessel (from a GPS), and the known positions, speeds and courses of nearby vessels, it is possible for an AIS unit to raise an alarm should any nearby vessel be on a course that will intercept your own vessel's course, or pass within a programmed safety margin.

This could be a features of the AIS unit itself, or it could alternatively be a feature of your navigation software or chart plotter, in the case where AIS data is output from the AIS unit into such a device.

AIS collision avoidance functionality usually allows some programmability, for example:

  • Type of alarm to be raised (visual or audio, for example playing ship's bells or flashing the screen).
  • Distance at which to begin calculating collision alarms -- e.g. do not alarm if a vessel is over 10 NM distant.
  • Time at which to begin calculating collision alarms -- e.g. do not alarm if the collision is more than 60 minutes from happening.
  • Safety margin -- e.g. alarm if another vessel is approaching within 0.2 NM or 0.1 NM of your own vessel.

Vessel Tracking

AIS is commonly used for vessel tracking by port authorities world wide. Most ports these days have the capability to track vessel movements into, out of, and within the port thanks to on board AIS capabilities in those vessels.

Certain ports have enacted legislation mandating AIS. For example Singapore now requires all vessels entering its waters to have an active AIS transceiver on board (not just receive only AIS), or install an alternative transponder system -- see this page for details.

World wide, there are a number of tracking stations which have a simple AIS receive capability and then broadcast (on the internet) the position, course and speed of any vessel found.

A website that pulls the data from many of these tracking stations together and displays them on a world map is Marine Traffic.

Forum Discussion Topics

List links to discussion threads on partnering forums. (see link for requirements)



Books, guides, etc.


We welcome users' contributions to the Wiki. Please click on Comments to view other users' comments, add your own personal experiences or recommend any changes to this page following your visit. Personal experience?

  • I have a Simrad IS 50 class B Transceiver on my boat for over 3 years now. I wonder how I had lived without it for so long. It, and Radar, are vital for avoiding collisions. --Istioploos Greece Icon.png Sailboat favicon.png Travels with S/Y Thetis

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Names: Delatbabel

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