Taking Satellite AIS to the next level

Posted by Peggy Browning on Jun 5, 2013 1:12:00 PM

There has been some discussion to what makes a great Satellite AIS system so I wanted to throw in my opinion on the topic.  AIS is designed for collision avoidance for ships, an undisputed fact.  But once we start collecting AIS signals from space from all those thousands of ships, means we’ll have LOTS of slot overlap and signals competing with each other.  The two methodologies for processing AIS signals are On-board processing (OBP) which can not separate AIS messages that collide in a slot and Spectrum Decollision Processing (SDP) which can.  Research has shown that OBP just can not process the signals like SDP, especially in dense shipping areas with more than 1,000 ships in the vicinity.  With SDP, Satellite AIS systems achieve better First Pass Detection rates, every time, maximizing the number of ships detected in every satellite pass.

OBP vs SDP resized 600

There is a school of thought that states that if you have enough satellites doing OBP passing in close sequence then it is the cumulative total of all the satellites that matters and would provide an operational detection level.  I suppose this is true, IF each satellite actually detected the ships the one before it missed.  However, this just isn't the way AIS detection from space works.  With OBP you will only hear the strongest signal in any given slot ... this is HIGHLY likely to be the same ships over and over as the output power is the determining factor for reception at the satellite.  With many ships in the satellite field of view you are guaranteed slot collisions, and so no matter how many satellites pass over, if they are only capable of OBP you will still only hear those with the strongest signals, the "shouters".  Basically the same picture being painted over and over ...

SDP is the only technology that is able to "peel" the layers of messages in any given slot ... the output power is not of concern as now we can look beyond the "shouters" to even hear the "whisperers".

Topics: Satellite AIS, AIS, Satellite AIS data, Spectrum Decollision Processing, On-board processing

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