Version 2.0
137
Automatic Identification System
Operator’s Manual
AIS Communications Scheme
AIS
messages must be updated and retransmitted every few seconds at a mini-
mum, since the usefulness of the data decays rapidly as a function of time. To
accommodate this high update requirement, AIS utilizes a unique self-organizing
time-division multiple access (SOTDMA) data communications scheme, which
uses the precise timing data in the GPS signals to synchronize multiple data
transmissions from many users on a single narrowband channel.
Each ship broadcasts its AIS messages and receives messages from all ships
within VHF radio range. The area in which AIS messages can be received is called
the ship’s “cell”. Each ship is in this way in the center of its own communication
cell.
The practical size of the cell can be varied according to the traffic density on the
AIS channel. If the number of AIS messages begins to overload the network, the
ship’s AIS system can automatically shrink its cell by ignoring weaker stations
further away in favor of those nearby.
Under the SOTDMA protocol, each minute of time is divided into 2,250 timeslots.
An AIS report fits into one or several of these 2,250 timeslots, which
are selected
automatically based on data link traffic and projections of future actions by other
Summary of Contents for MX420
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