Gerald Morrison wrote:
>
..........
> It's called Differential GPS.
> -It's expensive
> -It's not real time
> -It requires a lot of computation
> -It requires specific knowledge of the ground station location
>
> No GPS application would have the accuracy required.  Furthermore, GPS is
> not real time (not to good for correcting a slide which need real time
> feedback).

Not quite correct.  Differential GPS (DGPS) commonly reffers to the
transmission of corrections by the U.S. Coast Guard (and others) in
real time.  It is based on the CA code signal and has a 2 sigma accuracy
of about 10 meters.  DGPS beacon receivers start at $500.

Postprocessing is the term applied to recording data in the field and at
a base station of known location at the same time and later combining
the
data to determine very accuratley (down to 2 mm) the field locations.
High accuracy postprocessing requires recording a variable called the
carrier phase.

There are systems on the market and more under development that
broadcast
carrier phase based corrections to allow real time kinematic
differential
corrections.  For example see the current issue of "NAVIGATION - Journal
of
the Institute of Navigation" Vol.43 No. 3 page 295. "High-Rate Precise
Real-Time Positioning Using Differential Carrier Phase"

Have Fun,
Brooke