Automatic identification System AIS

Automatic identification System AIS transceivers broadcast vessel information, such as the course, position, speed, and navigational status, at regular intervals via a VHF transmitter built into the transceiver.

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Automatic Identification System (AIS) is an automated tracking system that displays other vessels in the vicinity. It is a broadcast transponder system which operates in the VHF mobile maritime band.

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  • Your own ship also shows on the screens of other vessels in the vicinity, provided your vessel is fitted with AIS.
  • If AIS is not fitted or not switched on, there is no exchange of information on ships via AIS.
  • AIS onboard must be switched on at all times unless the Master deems that it must be turned off for security reasons or anything else.
  • The working mode of AIS is continuous and autonomous.

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The information is derived from the navigational sensors on the vessel, GPS global navigation satellite system (GPS) and gyro compass.

Additionally other information, such as the vessel radio call sign and name, is installed in the equipment and is also transmitted as part of the “electronic” message.

Signals are received by AIS transceivers fitted on other ships, satellites and land based stations, such as harbour port control (VTS).

The received information is displayed on a screen or incorporated in a chart plotter, showing the  vessels’ positions in much the same manner as a radar display.

Automatic identification System AIS standard messages also can comprise substandards called “types” that specify individual product types. The specification can provide detailed technical information to monitor the Automatic identification System AIS system for errors and performance.

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Automatic identification System AIS Message types

1. Static Information (Every 6 minutes and on request):

  • MMSI number
  • IMO number
  • Name and Call Sign
  • Length and Beam
  • Type of ship
  • Location of position fixing antenna

2. Dynamic Information (Depends on speed and course alteration)

  • Ship’s position with accuracy indication
  • Position time stamp (in UTC)
  • Course Over Ground (COG)

3. Voyage Related Information (Every 6 minutes, when data is amended, or on request)

  • Ship’s draught
  • Type of cargo
  • Destination and ETA
  • Route plan (Waypoints)

4. Short safety related messages

  • Free format text message addressed to one or many destinations or to all stations in the area. This content could be such as buoy missing, ice berg sighting etc

Automatic identification System AIS product types are:

Class A

Vessel-mounted AIS transceiver which operates using SOTDMA. Used by large commercial vessels, SOTDMA requires a transceiver to maintain a constantly updated slot map in its memory such that it has prior knowledge of slots which are available for it to transmit.

SOTDMA transceivers effectively reserve their transmit slot and are therefore prioritised within the AIS system. Class A 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

There are now two separate IMO specifications for Class B transceivers aimed at smaller commercial and leisure vessels:

  • a carrier-sense time-division multiple-access (CSTDMA) system
  • a system that uses SOTDMA (as in Class A).

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Automatic identification System AIS in the original CSTDMA-based system, defined in ITU M.1371-0 and now called Class B “CS” (or unofficially as Class B/CS),[22] 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 B “CS” transmits at 2 W and is not required to have an integrated display: Class B “CS” units can be connected to most display systems where the received messages will be displayed in lists or overlaid on charts. Default transmit rate is approximately thirty seconds. The Class B “CS” standard requires integrated GPS and certain LED indicators. Class B “CS” equipment receives all types of AIS messages.

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Land Base station Automatic identification System AIS

Shore-based AIS transceiver (transmit and receive) operates using SOTDMA. Base stations have a complex set of features and functions which in the AIS standard are able to control the AIS system and all devices operating therein.

Ability to interrogate individual transceivers for status reports and or transmit frequency changes.


Aids to navigation (AtoN) Automatic identification System AIS

Shore- or buoy-based transceiver (transmit and receive) operates using fixed-access time-division multiple-access (FATDMA). Relay AIS messages for extended network coverage and can collect / transmit data related to sea and weather conditions.


Search and rescue transceiver (SART) Automatic identification System AIS

Used in an emergency AIS device created as an emergency distress beacon which operates using pre-announce time-division multiple-access (PATDMA), or sometimes called a “modified SOTDMA”. The device randomly selects a slot to transmit and will transmit a burst of eight messages per minute to maximize the probability of successful transmission. A SART is required to transmit up to a maximum of five miles and transmits a special message format recognised by other AIS devices. The device is designed for periodic use and only in emergencies due to its PATDMA-type operation which places stress on the slot map.


Specialist Automatic identification System AIS transceivers

A a number of government authorities have encouraged the development of hybrid Automatic identification System AIS devices. These devices seek to maintain the integrity of the core Automatic identification System AIS transmission structure and design to ensure operational reliability, also to add a range of additional features and functions to suit their specific requirements.

The “Identifier” Automatic identification System AIS transceiver is one such product where the core Class B CSTDMA technology is designed to ensure that the device transmits in complete compliance with the IMO specifications, but a number of changes have been made to enable it to be battery powered, low cost and more easy to install and deploy in large numbers. Such devices will not have international certification against an IMO specification since they will comply with a proportion of the relevant specification.

Typically government authorities will make their own detailed technical evaluation and test to ensure that the core operation of the device does not harm the international Automatic identification System AIS system.

Automatic Identification System  AIS FAQ

AIS signals are transmitted from vessels through VHF radio frequencies.

An icon appears on the navigation displays of craft equipped to receive AIS information, indicating the transmitting ship’s location.
Clicking on the icon reveals the transmitting vessel’s position and visually tracks its course while providing you with its speed, heading and, if it is on a collision course with your boat, the time it will take to reach your location.

AIS can also broadcast a vessel’s name, radio call sign, length, beam, draft, rate of turn and, often, its destination, port of origin, and even if it is carrying hazardous material.

Recreational boats equipped with a Class B AIS transponder not only receive AIS signals but also have the ability to broadcast similar position and navigation information to other boats. This means your boat will appear as an unmistakably large object to other boats in your area receiving AIS.

An audible alarm sounds when AIS-equipped vessels approach each other within an unsafe distance.

AIS (automatic identification system) gives you ability to know what other nearby boats are doing and even possible predict their movements.

Most commercial vessels are required to have operational AIS equipment before leaving port and must broadcast vital navigation information to all craft within several miles of their positions.

Recreational craft are not required to have AIS, however increasing numbers of pleasure boats are being equipped with AIS to take advantage of the many safety and navigational benefits it has to offer.

AIS  can receive a signal even when you are not able to visually see vessels  with the naked eye.

When approaching the entrance to a harbour, you may “see” boat and ship traffic entering and leaving the harbor before you reach the channel entrance.

AIS is somewhat like a radar in that it shows you boat movements that may be hidden from visual sight behind obstacles or land formations.

AIS signals are not continuous but sent out at intervals. Commercial vessels using mandatory AIS Class A transponders transmit their position and navigation information every two to 10 seconds while underway and every three minutes when at anchor.

Noncommercial Class B AIS, used more frequently by recreational boats, sends data every three minutes when at speeds of less than 2 knots and every 30 seconds at speeds over 2 knots. These transmitting intervals are for dual-channel AIS transponders. Single channel sets can take about twice the time to send signals. This is why AIS icons may appear to move in small jumping movements on your navigation screen.

AIS signals are transmitted at VHF frequencies. Class B AIS sets normally have a range of about 5 to 7 miles. Class A transponders used on commercial vessels have a more powerful transmitter and can reach out 20 miles or more.

In areas where traffic is heavy (in and around a commercial harbor, for example), your navigation display may become cluttered with AIS icons. It is possible to choose models that have a filtering function so you can filter out targets that may be moving away from you, stationary boats at anchor, and other vessels that do not pose a threat. THIS MUST BE USED WITH CAUTION.

AIS is definitely prime boating safety equipment, particularly when crossing traffic lanes in congested boating areas and when visibility is poor.


The following table gives a summary of all the currently used AIS messages.

AIS message Usage Comments
Message 1, 2, 3: Position Report Class A Reports navigational information This message transmits information pertaining to a ships navigation: Longitude and latitude, time, heading, speed, ships navigation status (under power, at anchor…)
Message 4: Base Station Report Used by base stations to indicate their presence The message reports a precise position and time. It serves as a static reference for other ships
Message 5: Static and Voyage Related Data Gives information on a ship and its trip One of the few messages whose data is entered by hand. This information includes static data such as a ship’s length, width, draught, as well as the ship’s intended destination
Message 6: Binary Addressed Message An addressed point-to-point message with unspecified binary payload.
Message 7: Binary Acknowledge Message Sent to acknowledge the reception of a message 6
Message 8: Binary Broadcast Message A broadcast message with unspecified binary payload.
Message 9: Standard Search and Rescue Aircraft Position Report Used by an aircraft (helicopter or airplane) which is involved with search and rescue operation on the sea (i.e. search for and recovery of survivors of an accident at sea). Sends out location (including altitude) and time information
Message 10: UTC/Date Inquiry Obtain time and date from a base station Request for UTC/Date information from an AIS base station. Used when a device does not have time and date locally, usually from GPS
Message 11: Coordinated universal time/date response Response from message 10 Identical to message 4.
Message 12: Addressed Safety-Related Message Used to send text messages to a specified vessel Text message may be in plain English, commercial codes or even encrypted
Message 13: Safety related acknowledge Response from message 12
Message 14: Safety related broadcast message Identical to message 12, but broadcast
Message 15: Interrogation Used by a base station to get the status of up to 2 other AIS devices
Message 16: Assigned mode command Used by a base station to manage the AIS slots
Message 17: Global navigation-satellite system broadcast binary message Used by a base station to broadcast differential corrections for GPS
Message 18: Standard class B equipment position report A less detailed report than types 1-3 for vessels using Class B transmitters Does not include navigation status nor rate of turn
Message 19: Extended class B equipment position report For legacy class B equipment Is replaced by message 18
Message 20: Data link management message Used by a base station to manage the AIS slots This message is used to pre-allocate TDMA slots within an AIS base station network
Message 21: Aids-to-navigation report Used by an (AtN) aid to navigation device (buoys, lighthouse..) Transmits precise time and location as well as the characteristics of the AtN
Message 22: Channel management Used by a base station to manage the VHF link
Message 23: Group assignment command Used by a base station to manage other AIS stations
Message 24: Static data report Equivalent of a Type 5 message for ships using Class B equipment
Message 25: Single slot binary message Used to transmit binary data from one device to another
Message 26: Multiple slot binary message with communications state Used to transmit binary data from one device to another
Message 27: Long-range automatic identification system broadcast message This message is used for long-range detection of AIS Class A and Class B vessels (typically by satellite). Same as messages 1, 2 and 3

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