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SYSTEM OVERVIEW
System overview
The Automatic Identification System (AIS) was originally developed to aid the Vessel
Traffic Services (VTS) by use of a VHF transponder working on Digital Selective Call
(DSC) at VHF CH70, and is still in use along the UK coastal areas and others. Some time
later 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).
The system operates in three 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). It 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 VHF channels
87B and 88B are commonly used and in addition there are local AIS frequencies.
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.
VTS center
Aids to
Navigation
(AtoN)
Transponder
VTS Center transmits TDMA CH
management message including
code, type, position, etc. of buoys
every 3 min, and the AtoN broadcasts
these messages for ships.
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.
Interrogation and Response
Static and Dynamic information incl.
MMSI, Name, POSN, HDG, COG, SOG
Ship 1
Own ship
The VTS center transmits a command on
frequency assignment, slots, report rate,
VHF output power, channel spacing, etc.
(Assigned mode)
AIS-fitted AtoN broadcasts its
identification, type of operation,
location, displacement, etc. at
3 min intervals or at a reporting
rate designated by the
Administration authorities.
AIS system
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