The Automatic Identification System (AIS) is an automated tracking system used on ships and by Vessel Traffic Services (VTS) for identifying and locating vessels by electronically exchanging data with other nearby ships and VTS stations. AIS information supplements marine radar, which continues to be the primary method of collision avoidance for shipping. The International Maritime Organization's (IMO) International Convention for the Safety of Life at Sea (SOLAS) requires AIS to be fitted aboard international voyaging ships with gross tonnage (GT) of 300 or more tons, and all passenger ships regardless of size.
Information provided by AIS equipment, such as unique identification, position, course, and speed, can be displayed on a screen or an ECDIS. AIS is intended to assist the vessel's watchkeeping officers and allow maritime authorities to track and monitor vessel movements. AIS integrates a standardized VHF transceiver with a positioning system such as a LORAN-C or GPS receiver, with other electronic navigation sensors, such as a gyrocompass or rate of turn indicator. Ships outside AIS radio range can be tracked with the Long Range Identification and Tracking (LRIT) systems.
The AIS technology
The AIS system functions by ships transmitting at short intervals short data messages about their identity (ship’s name, call sign, physical size), navigation (position, course and speed) and voyage (port of destination, expected time of arrival, draught). This information can be displayed on the electronic information systems of a ship. Especially important buoyage (lights or buoys) can also be fitted with AIS and will then transmit data messages known as 'Aids-to-Navigation reports'.
Shore-based AIS stations Shore-based AIS stations transmit ‘Base Station Reports’, which can be used by ships as time references for synchronizing their AIS transmissions. Furthermore, the shore-based stations can transmit short text messages to ships in the area around the AIS station, for example a navigation-regulating signal corresponding to an ‘Aids-to-Navigation report’ from a buoy. These signals are also referred to as electronic buoys.
AIS of classes A and B
AIS information is divided into two classes – classes A and B – depending on the AIS transponder transmitting the AIS information. These classes are of great importance to the capabilities of your AIS. There is a great difference between the two classes, both in terms of extent, complexity and price.
AIS information from a class A transponder will always be prioritized and, thus, be shown to other ships in the area, whereas AIS information from a class B transponder will not be shown until or if there is room on the AIS channel.
AIS of class A In order to avoid that the ships’ AIS systems all speak at the same time, large ships use an AIS system of class A, which is called SOTDMA (Self-Organized TDMA). An algorithm ensures that the AIS transmitter of a ship first notices how other ships transmit their messages and, subsequently, adjusts its own transmission pattern to that of the others. In case there are more ships fitted with AIS of class A in an area than the capacity of the band width, the system will automatically limit the area of coverage so that the remotest ships in the area are discarded.
AIS of class B Small vessels fitted with AIS, such as recreational craft, can use a less expensive AIS station of class B, which transmits less frequently. This system is called CSTDMA (Carrier Sense TDMA). A class B station will listen for a couple of milliseconds to hear whether a large ship is transmitting before it transmits its own message. Some rather old AIS stations of class A can see only the position, but not the identity of these class B stations – and large ships can choose not to show AIS stations of class B on their displays if their displays become confused because of too many yachtsmen.
Thus, the class B information will be shown to other ships in the area only if or when there is room on the AIS channel. On an AIS transponder of class B you can see whether the information transmitted is prioritized and thus displayed on other ships.
An AIS-SART (Search And Rescue Transponder) is a small, battery-powered AIS transmitter that is only a little larger than a cell phone. When activated, it transmits a special localization signal that is displayed on all ships fitted with an AIS receiver.
The AIS-SART can provide great certainty for being localized fast, but it can, however, not replace a distress call as such.
The AIS-SART has been approved by the IMO.