Statutory Documents - IMO Publications and Documents - Resolutions - Assembly - IMO Resolution A.917(22) – Guidelines for the Onboard Operational Use of Shipborne Automatic Identification Systems (AIS) – (Adopted on 29 November 2001) - Annex - Guidelines for the Onboard Operational use of Shipborne Automatic Identification Systems (AIS) - Description of AIS
Description of AIS
5. Shipborne AIS (see Figure 1):
- continuously transmits ship's own data to other vessels and VTS stations;
- continuously receives data of other vessels and VTS stations; and
- displays this data.
AIS System Overview
6. When used with the appropriate graphical display, shipborne AIS enables provision of fast, automatic information by calculating Closest Point of Approach (CPA) and Time to Closest Point of Approach (TCPA) from the position information transmitted by the target vessels.
7. AIS operates primarily on two dedicated VHF channels. Where these channels are not available regionally, the AIS is capable of being automatically switched to designated alternate channels by means of a message from a shore facility. Where no shore based AIS or GMDSS sea Area A1 station is in place, the AIS should be switched manually.
8. In practice, the capacity of the system is unlimited, allowing for a great number of ships to be accommodated at the same time.
9. The AIS is able to detect ships within VHF/FM range around bends and behind islands, if the landmasses are not too high. A typical value to be expected at sea is 20 to 30 nautical miles depending on antenna height. With the help of repeater stations, the coverage for both ship and VTS stations can be improved.
10. Information from a shipborne AIS is transmitted continuously and automatically without any intervention or knowledge of the OOW. An AIS shore station might require updated information from a specific ship by "polling" that ship, or alternatively, might wish to "poll" all ships within a defined sea area. However, the shore station can only increase the ships’ reporting rate, not decrease it.
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