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I have got our NASA AIS working okay but a lot of the ships are showing as '???'.

What does that signify?
It means the vessel's name has not been received by your single channel AIS receiver. Read up on the frequency of the timing and availability of AIS data here..

http://www.navcen.uscg.gov/enav/ais/what...dcasts.htm

Static information such as vessel name does not update as frequently as dynamic real time data such as speed and course.
So, would I be right in thinking that the NASA engine is a Class A receiever?

http://www.navcen.uscg.gov/enav/ais/types_of_AIS.htm