Appendix 2 - Technical description
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Statutory Documents - IMO Publications and Documents - Resolutions - Assembly - IMO Resolution A.1106(29) Revised Guidelines for fhe Onboard Operational Use of Shipborne Automatic Identification Systems (AIS) (Adopted on 2 December 2015) - Appendix 2 - Technical description

Appendix 2 - Technical description

1 AIS operates primarily on two dedicated VHF channels (AIS1 161,975 MHz and AIS2 162,025 MHz). Where these channels are not available regionally, the AIS is capable of automatically switching to alternate designated channels. However, this capability should only be considered for use in urgent, temporary situations, noting the possible adverse effects on AIS at sea.

2 The required ship reporting capacity according to the IMO performance standard amounts to a minimum of 2000 time slots per minute (see figure 1 below). The ITU Technical Standard for the Universal AIS provides 4500 time slots per minute. The broadcast mode is based on a principle called (S)TDMA (Self-organized Time Division Multiple Access) that allows the system to be overloaded by 400 to 500% and still provide nearly 100% throughput for ships closer than 8 to 10 NM to each other in a ship-to-ship mode. In the event of system overload, only targets far away will be subject to drop-out in order to give preference to targets close by that are a primary concern for ship-to-ship operation of AIS. In practice, the capacity of the system allows for a great number of ships to be accommodated at the same time.

Figure 1 - Principles of TDMA


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