Section 2. How does the Automatic Identification System work on board ships?

The Automatic Identification System (AIS), in conjunction with other shipboard equipment, contributes to the enhancement of situational awareness and safety to navigation. AIS messages and information can be broadcast from a variety of units including shipborne Class A and B stations, AIS base stations, AIS repeaters, AIS aids to navigation (AIS AtoNs), Search and Rescue (SAR) Transmitters, Man-Overboard (MOB) transmitters, and Emergency Position Indicating Radio-Beacon (EPIRB) transmitters.

Vessels equipped with AIS transponders continuously broadcast their information over the VHF maritime band. The distance of the AIS signal can be up to 20 to 40 nautical miles (approximately 37 to 74 kilometers) depending on weather conditions and/or local topography; elevated shore stations can broadcast over greater distances. Ship details may then be received and displayed on other AIS-enabled devices including Minimum Keyboard Display (MKD), ECDIS, ECS, PPU, and radar. AIS base stations also receive this information and transmit it to VTS centers for monitoring purposes, allowing for less frequent and more efficient communications. Over the last few years, the capacity of low-medium orbit satellites equipped with AIS transponders have increased significantly, contributing to the increased availability of data and global monitoring of ship traffic.

For ships to broadcast their information, they require either a Class A or Class B transceiver:

Class A
[IEC 61993-2; IEC 61162;
ITU-R M. 1371; IMO Circ. 289]
Class B
[IEC 62287-1 and 62287-2]
Class A shipborne mobile equipment is intended to meet the performance standards and carriage requirements adopted by the IMO, who’s Safety of Life at Sea (SOLAS) Convention requires that “all ships of 300 gross tonnage and upwards engaged on international voyages and cargo ships of 500 gross tonnage and upwards not engaged on international voyages and passenger ships irrespective of size shall be fitted with an automatic identification system” (SOLAS Chapter V – 1/7/02). In addition to this international law, some national authorities (e.g. some EU member states) have extended this requirement to smaller vessels (> 15 m), including fishing boats. Class A transceivers report their position (message 1, 2, 3) autonomously every 2-10 seconds dependent on the vessel’s speed and/or course changes (every three minutes or less when at anchor or moored); and, the vessel’s static and voyage related information (message 5) every 6 minutes. They are also capable of text messaging safety related information (message 6, 8) and AIS Application Specific Messages (message 6, 8, 25, 26), electronic Broadcast Notice to Mariners, and other marine safety information. Class B shipborne mobile equipment is interoperable with all other AIS stations, but does not meet all the performance standards adopted by IMO. Similar to Class A transceivers, they report every three minutes or less when at anchor or moored. Their position is reported less often and at a lower power. Likewise, they report the vessel’s static data (message 18, 24) every 6 minutes, but, not any voyage related information. They can receive safety related text and application specific messages, but, cannot transmit them. Class B AIS units are divided into two groups: those that employ Self-Organizing Time-Division Multiple Access (SOTDMA) and those that employ Carrier-Sense Time-Division Multiple Access (CSTDMA). Of the two, SOTDMA units are generally more capable and CSTDMA units are generally less expensive. In either case, Class B AIS units will be much less costly in comparison to Class A AIS units due to the differences in capabilities between them.

A third type of AIS system, “receive-only”, can be used by small boat (pleasure craft) operators who wish to benefit from a low cost alternative to Class B AIS transponders. It provides limited capability, such as seeing who else is on the water nearby, but does not have any broadcast capability. While "receive-only" systems are not subject to the same international standards as Class A and B transponders, in Canada, they are subject to Radio Standards Specifications 182 covering maritime radio transmitters and receivers in the 156-162.5 MHz band as well as the Radio Communication Act (R.S.C., 1985, c. R-2)

Next: Which Ships use the Automatic Identification System? ⇨

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