II Morrow Inc. 920 GPS - Manual (Page 84)

Section E - Differential GPS Operation
What is
Differential
GPS? Differential GPS, or DGPS, is a method by
which the inherent degree of error in a GPS
position fix may be greatly reduced. DGPS is
used in marine applications only. Developed
largely by the United States Coast Guard and
scheduled for completion in 1996, DGPS
helps to overcome intentional position fix
inaccuracies
introduced by Selective
Availability. Using DGPS, the navigational
accuracy of a GPS receiver can be increased
from approximately 100 meters to as little as
10 meters. The Apollo handheld GPS receiver
has the capability to operate using DGPS.
How Does
Differential
GPS
Work? DGPS relies on reference stations operated by
the Coast Guard. Each reference station serves
GPS users in a particular area and contains
highly accurate and sophisticated equipment
to calculate and measure the navigational error
in received GPS signals. Navigational error is
determined by comparing the known location
of the reference station with the calculated
location using uncorrected GPS data. Once the
error is known and measured, correctional
data is made available to GPS users to
increase the navigational accuracy of their
receivers.
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