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AIS System overview

The Automatic 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 two channels).

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.

All ships 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