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AIS Class A – What is SOTDMA?

The operation of AIS depends on the Self Organizing Time Division Multiple Access (SOTDMA) data communication technology, which was developed in the 1980’s.

It allows for large numbers of transmitters to share one single narrow band radio channel, by synchronizing their data transmission to an exact timing standard. Under SOTDMA each minute of time is divided into 2250 time slots or 26.67 ms each time slot. With a transmission speed of 9.6 kbps this translates into 256 Bits/time-slot, sufficient for one AIS report.

The exact timing signal of the GPS receiver is essential to synchronize the time slots of communicating AIS ships, as well as providing the position data for each ship. When a ship first “talks” to another ship, it takes up an unoccupied time slot, and it reserves a time slot for its next contact, depending on the status of the vessel according to the standards

For example a ship sailing at 23 knots updates its information every two seconds and therefore “reserves” a time slot 75 slots on from the first contact (2250/30=75) and so on).

The range of the system is the VHF horizon for each AIS-ship, with the ship in the centre of its own communication “cell”. The size of this cell will adjust to the traffic density, if slot capacity starts to run out, the system will automatically discard targets at a greater distance and assign those time slots to targets of greater importance.

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