The Automatic Identification System (AIS) is an automatic tracking system used by ships and vessel traffic services (VTS) for identifying and locating vessels by electronically exchanging data with other nearby ships, AIS base stations, and satellites. When satellites are used to detect AIS signatures, the term Satellite-AIS (S-AIS) is used. AIS information supplements marine radar, which continues to be the primary method of collision avoidance for water transport.
AIS is intended, primarily, to allow ships to view marine traffic in their area and to be seen by that traffic. This requires a dedicated VHF AIS transponder that allows local traffic to be viewed on an AIS enabled chart plotter or computer monitor while transmitting information about the ship itself to other AIS receivers. Port authorities or other shore-based facilities may be equipped with receivers only, so that they can view the local traffic without the need to transmit their own location. All AIS transponder equipped traffic can be viewed very reliably but is limited to the VHF range, about 200 nm.
AIS transponders automatically broadcast information, such as their position, speed, and navigational status, at regular intervals via a VHF transmitter built into the transponder. The information originates from the ship’s navigational sensors, typically its global navigation satellite system (GNSS) receiver and gyrocompass. Other information, such as the vessel name and VHF call sign, is programmed when installing the equipment and is also transmitted regularly. The signals are received by AIS transponders fitted on other ships or on land based systems, such as VTS systems. 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. Data is transmitted via a tracking system which makes use of a Self-Organized Time Division Multiple Access (SOTDMA) datalink designed by Swedish inventor Håkan Lans.
The AIS standard comprises several substandards called “types” that 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:
Vessel-mounted AIS transceiver (transmit and receive) which operates using SOTDMA. Targeted at 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 will then pre-announce their transmission, effectively reserving their transmit slot. SOTDMA transmissions 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.
Vessel-mounted AIS transceiver (transmit and receive) which operates using either carrier-sense time-division multiple-access (CSTDMA)or SOTDMA; there are now 2 separate IMO specifications for Class B. Aimed at lighter commercial and leisure markets. CSTDMA 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, 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.
Shore-based AIS Base Station 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 transponders for status reports and or transmit frequency changes.