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

 With SA off you do not need the Differential gear,Virg

 
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