User's Manual

iii
SYSTEM OVERVIEW
System overview
The Automatic Identification System (AIS) was originally developed to aid the Vessel Traffic Ser-
vices (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 Multi-
ple 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 re-
ports 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 telecom-
mand received by the DSC receiver on ship.
AIS system
Not all ships carry AIS
The Officer of the Watch (OOW) should always be aware that other ships, and in particular leisure
craft, fishing boats and warships, and some coastal shore stations (including Vessel Traffic Ser-
vice centers) might not be fitted with AIS.
The OOW should also be aware that AIS fitted on other ships as a mandatory carriage require-
ment might be switched off by the master if its use might compromise the security of the vessel.
Thus, users are therefore cautioned to always bear in mind that information provided by AIS may
not be giving a complete or correct “picture” of shipping traffic in their vicinity.
VTS Center
Own ship transmits
static/dynamic data*
Transmits and receives
static/dynamic data*
Transmits and receives
static/dynamic data*
*: Static data - MMSI, Name, etc.
Dynamic data - Position, bearing, COG, etc.
VTS Center sends various
information including
weather conditions and
frequency assignments.
Own ship
Other ship1
Other ship2