



Here's a rough sketch of how the system works.  A reference station
with a very exactly known position computes the errors in the
incoming GPS signals.  These errors are due to several factors
including atmospheric distortion, SA (Selective Availability) time
dithering, etc.  The reference unit contains complex computational 
equipment to "back out" the errors in its position (since it knows
where it is already).  It then transmits these corrections on a 
broadcast which is available to any number of relatively local
receivers.  If the receivers are nearby (<300km) and are using
the same satellites as the reference unit, the errors should be very
similar for the reference unit and the receiver unit.  Thus, the
receiver unit may apply the corrections calculated by the reference
unit.

The US Coast Guard is currently (as far as I know) installing a series
of coastline transmitters for differential GPS.  These stations will
use existing radio towers.  I believe the frequency is to be approximately
305 kHz.  There are many other private corporations offering DGPS signals
on different frequencies.  For example, PinPoint ((310)-618-7076) offers
correction signals and receiver units using an FM broadcast system
which has stations all across the US.

The correction codes are usually transmitted using the RTCM 104 format.
Advertised accuracies espouse 1 to 5 meter errors.


-------------------------------------------------------------------------
John P. Baker                   | My opinions are my own.  I don't know
Johns Hopkins University        | anyone else who wants them, anyway.
Applied Physics Laboratory	|
Laurel, MD  20723               | bakerjp1@aplcomm.jhuapl.edu