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AIS System overview
Identification System (AIS) was originally developed to aid the Vessel
Traffic Services (VTS) by use of VHF transponder working on Digital
Selective Call (DSC) at VHF Channel 70 and is still in use along the UK
coastal areas and others. Afterward the IMO developed a Universal AIS using
the new sophisticated technology called Self-Organized Time Division
Multiple Access (SOTDMA) based on a VHF Data Link (VDL).
This system is synchronized with GPS time to avoid conflict among multiple
users (IMO minimum 2000 reports per minute and IEC requires 4500 reports on
The system operates in 3 modes - autonomous (continuous operation in all
areas), assigned (data transmission interval remotely controlled by
authority in traffic monitoring service) and polled (in response to
interrogation from a ship or authority). The VHF channels 87B and 88B are
commonly used and in addition there are local AIS frequencies.The shipborne
AIS transponders exchange various data as specified by the IMO and ITU on
either frequency automatically set up by the frequency management
telecommand received by the DSC receiver on ship. VHF transmit power is also
set up for 12.5 W or 2 W automatically.
broadcast Static and Dynamic information (autonomous and continuous mode).
If OS wants to know information about ship 1, OS shall send an interrogation
in polling mode; then ship 1 will transmit her response on the same VHF
channel without operator intervention
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