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APPENDIX D - DIFFERENTIAL GPS (DGPS) BEACON
DGPS works by placing a high-performance GPS receiver (reference station) at a known
location. Since the receiver knows its exact location, it can determine the errors in the
satellite signals. It does this by measuring the ranges to each satellite using the signals
received and comparing these measured ranges to the actual ranges calculated from its
known position. The difference between the measured and calculated range is the total
error. The error data for each tracked satellite is formatted into a correction message and
transmitted to GPS users. The correction message format follows the standard established
by the Radio Technical Commission for Maritime Services, Special Committee 104 (RTCM-
SC104). These differential corrections are then applied to the GPS calculations, thus re-
moving most of the satellite signal error and improving accuracy. The level of accuracy
obtained is a function of the GPS receiver. Sophisticated receivers like the Invicta 210 series
can achieve accuracy on the order of 1 meter or less.
GPS Satellites
Reference
Station
Coupler
DGPS
Radiobeacon
Antenna
R
T
C
M
-
S
C
1
0
4
C
o
r
r
e
c
t
i
o
n
s
Differential GPS Broadcast Site
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