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DGPS Interface (continued)
The RTCM/NMEA selection controls the GARMIN GBR-
21 differential beacon receiver. Once a RTCM setting has
been selected, the GPS 38 will automatically try to tune the
last frequency and bit rate you selected or will switch to the
default frequency of 304.0 kHz with a bit rate of 100 bps if
no previous beacon has been tuned. You may also enter
your own frequency and bit rate if desired.
To enter a DGPS beacon frequency:
1. Highlight the ‘FREQ’ field and press
E
.
2. Enter the desired frequency and press
E
.
3. Highlight the ‘RATE’ field and press
E
.
4. Select the desired transmission rate.
5. Press the
E
key to confirm your entry.
When the GPS 38 is receiving DGPS corrections from
the GBR-21, the ‘BEACON RECVR’ section of the I/O setup
page will display the beacon frequency and signal strength,
as well as the distance from the transmitter to the beacon
receiver. At the bottom of the beacon receiver field, a status
message will keep you informed of DGPS activity:
• A ‘Tuning’ message will be displayed while a beacon
signal is being tuned.
• Once the beacon signal has been tuned, a ‘Receiving’
message will be displayed.
• If a beacon signal is tuned and no corrections are
being received, a ‘No Data’ message will be displayed.
• If a beacon signal cannot be tuned, a ‘No Status’ mes-
sage will be displayed.
The GPS 38 will also display one of three alert messages
concerning DGPS operation through the message page:
No DGPS Position— not enough data is available to
compute DGPS position.
No RTCM Input— the beacon receiver is not properly
connected or the baud rates do not match.
RTCM Input Failed— DGPS data was being received
but has been lost.
DGPSInterface
Once a RTCM option is
selected, you must manually
tune the beacon re c e i v e r
from the frequency field.
The message page will alert
you to any problems relat-
ing to DGPS operation. You
may also monitor the DGPS
status from the status field
on the interface setup page.
R E F E R E N C E
DGPS Interface
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GPS 38 Man Rev B 7/23/98 4:34 PM Page 52