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Version 2.0
Operator’s Manual
Automatic Identifaction System
stations currently on the network. When a ship first enters the cell of another
ship, it takes an unoccupied timeslot. The AIS stations continually synchronize
their slot selections with each other.
Timeslots and time-out periods are selected on a randomized basis. When a
station changes its slot assignment, it announces to all other stations on the
channel its new location and time-out for that location. Each station continually
updates its internal “slot map” to reflect changes in occupied slots and time-
outs. Special provisions are made for automatic conflict resolution in the event
two stations end up in the same timeslot, to ensure that stations always choose
unoccupied slots. In situations of high traffic density it may be necessary to
reduce the number of ships in a communication cell, as described above. This
enables time slots used by weak stations far away, to be used also by a station
nearby. The AIS system applies very specific rules on how this reoccupation of
timeslots is done.
The key to the SOTDMA scheme is the availability of a highly accurate standard
time reference, to which all of the stations can synchronize their time slot assign-
ments, in order to avoid overlap. This time reference is supplied by the precise
timing signal in the GPS satellite message. Thus, GPS plays a critical role in AIS,
providing the universal time reference as well as positioning data for each ship.
AIS data transmissions utilize a robust 9.6 kbps FM /GMSK (Gaussian Minimum
Summary of Contents for MX420
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