Garmin GBR 21 - Manual (Page 16)

14
The Main Menu Page on the
GPS III/III+/12Map/
Navtalk. Select ‘Setup’ to
make the desired interface
settings.
From the Interface Page,
select ‘RTCM/NMEA’ to
properly interface with the
GBR 21.
DGPS:
How It Works
The DGPS Beacon Transmitter is placed at a known
location (i.e., the exact position of the site has been
previously determined). At the beacon transmitter site,
the GPS satellites are monitored using a GPS receiver.
This receiver is equipped to calculate corrections for each
satellite received. The correction is the difference between
the distance to the satellite (from the beacon site) as
measured by the GPS receiver, and the actual distance to
the satellite based on the known location of the beacon
site. These corrections are communicated to the users
GPS set through the DGPS Beacon Station and the
GBR 21. The users GPS set then uses the corrections to
remove errors from its measurements. Satellites received
by the shipboard GPS receiver, but not by the GPS
receiver at the beacon transmitter site, will not have
corresponding corrections. When three or more satellites
received by the shipboard GPS receiver have correspond-
ing corrections, the result is a highly accurate position
reading. The more satellites with corrections, the more
accurate the position.
The GBR 21 receives RTCM-SC-104 format signals
from ground based DGPS Beacon Stations operating in
the 283.5 kHz to 325.0 kHz frequency band, with MSK
modulation and data rates of 25,50,100, or 200 bits per
second. These stations are typically operated by
government agencies such as the U.S. Coast Guard.
BEACON
RECEIVER
GPS
RECEIVER
CORRECTION
(RTCM SC-104)
DGPS BEACON
SIGNAL IN
GPS SIGNAL IN
OR
GA 22/23 H-FIELD
BEACON ANTENNA GA-29 GPS
ANTENNA
STANDARD
ANTENNA
COUPLER
DATA
Figure 5: Shipboard DGPS System
Reference