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.

norm2 wrote:

Check out the info on the Starfire receiver system (in the precision farming
section) at the www.deere.com website.

<<<John Deere 10-channel engine allowing it to track as many as 10
satellites at once;
10-inch average accuracy with dual-frequency DGPS signal;
30-inch average accuracy with single-frequency DGPS signal;
6-foot average accuracy with free Wide-Area Augmentation System (WAAS)
signal controlled by the U.S. Federal Aviation Administration.* >>>

Not bad for a farm tractor.

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

These would need to be receiving the correction signal over the satellite
(like the Starfire system).  You can't get a differential signal from two
units mounted to a moving chassis.  You can get redundancy.  To make
differential work, you need to have one unit in a known, fixed location.

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