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How do transmitted AIS signals get attributed to correct vessels in busy areas?

Class A AIS transponders (usually onboard SOLAS-compliant vessels), make use of the SOTDMA protocol - the name stands for Self Organised Time Division Multiple Access.

This is how SOTDMA manages to organise transmissions:

  • All AIS transmissions are based on GPS derived UTC time. This enables for accurate determination and global synchronisation of the start time of each transmission.
  • Each vessel pre-occupies certain time-slots so that no transmission from other vessels sailing on the same group is made on the exact same time (ie using the same slot):
  • Each UTC second can be divided to up to 2250 such slots. This means that one AIS data transmission can be made every 26.67 milliseconds.
  • Each vessel occupies certain slots depending on her moving status, speed and course - for example, a vessel sailing faster than 14 knots while changing course would be transmitting AIS signals every 2 seconds. Upcoming transmissions will make use of the time-slot used before (pre-occupied).
  • Vessels within a self-organised group automatically avoid making use of slots that are already marked for use by another vessel.
  • When a vessel is moving from an area to another, it is possible to came across another vessel that uses the same slot. In such a case, the station will modify its own slot allocation looking for a free slot.

The result is a completely autonomous system able to self organise based on temporal and spatial criteria. Even in very busy areas where all available slots are occupied, SODTMA is able to manage the situation using a slot re-using approach by prioritising the allocation of slots to stations that are closer to each other.

This feature of Class A transponders ensures that information is managed in a dynamic way:

  • The transponder scans for available slots
  • Occupies and reserves those that it will be using
  • Starts transmitting according to its selection. The slot used is also transmitted (UTC seconds) so that other nearby vessels can do the same (scan, occupy, transmit).

Class B AIS transponders (used by non-regulated vessels) make use of the CSTDMA (Carrier Sense Time Division Multiple Access) protocol which interweaves with Class A transmissions in a way that does not cause mix-ups by giving priority to SOTDMA transmissions. The overall logic remains the same:

  • The transponder scans for an available slot
  • If it is found, it makes use of it
  • If no empty slot is found, the transmission is delayed awaiting for a free slot
  • Once the transmission is made, the same process repeats

 

Note that AIS transmissions may get scrambled and/or lost when received by satellites (Sat AIS). This can happen because satellites have a wide reception footprint which includes multiple groups of AIS-equipped vessels that transmit different data packets using the same slots - this situation is know as "packet collision".

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