I'm not a "GPS Guy" but I understand that there is a method of impoveing accuracy using a technique of "Differential".  It is my understanding that a GPS receiver is installed on a known surveyed point.  It receives the GPS signals and compares where the GPS says it is, to where it "knows" it is.  The difference is then transmitted by radio (local area, short range) to other GPS systems equipped to receive the error differential.  The differential equipped GPS takes out the "error" to give a better location.

The differential equipped systems give accuracy to moveing equipment.

I understand Trimble has a "differential service" that it charges for.  I also understand that the FAA is installing "differential" facilities near airports.  Additionally, I think I remember that some other gov'n agency is taking over some "differential" radio towers near cities and/or harbors.

By the way, I've seen a system installed on a helicopter that uses two receivers and generates its own "differential".

My $.02 worth
Rob Gaddy

 
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