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CSB200-S
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6 BACKGROUND TO AIS
The marine Automatic Identification System (AIS) is a location and vessel information reporting
system. It allows vessels equipped with AIS to automatically and dynamically share and regularly
update their position, speed, course and other information such as vessel identity with similarly
equipped craft. Position is derived from a Global Navigation Satellite System (GNSS
1 ) network and
communication between vessels is by Very High Frequency (VHF) digital transmissions.
A sophisticated and automatic method of time sharing the radio channel is used to ensure that even
where a large number of vessels are in one location, blocking of individual transmissions is
minimised, any degradation of the expected position reporting interval is indicated to the user, and
even if the unit suffers extreme channel overload conditions it will always recover to normal operation.
6.1 AIS Classes
There are two classes of AIS unit fitted to vessels, Class ‘A’ and Class ‘B’. In addition AIS base
stations may be employed by the coastguard, port authorities and other authorised bodies. AIS units
acting as aids to navigation (AtoNs) can also be fitted to fixed and floating navigation markers such
as channel markers and buoys.
Class ‘A’ units are a mandatory fit under the safety of life at sea (SOLAS) convention to vessels
above 300 gross registered tons or which carry more than 11 passengers in International waters.
Many other commercial vessels and some leisure craft also fit Class ‘A’ units.
Class ‘B’ units are currently not a mandatory fit, but authorities in several parts of the world are
considering this. Class ‘B’ units are designed for fitting in vessels which do not fall into the mandatory
Class ‘A’ fit category.
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The CSB200-S is a Class ‘B’ unit.
6.2 Information Transmitted and Received
An AIS Class ‘B’ unit will transmit:
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MMSI
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Call Sign and Name
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Length and Beam
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Ship Type
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Time
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Course Over Ground (COG)
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Speed Over Ground (SOG)
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Heading
1 GPS is the American GNSS network. There are others, notably GLONASS (Globalnaya
Navigazionnaya Sputnikovaya Sistema) which is the Russian system and BDS (BeiDoü) which is the
Chinese system