SatLink VSAT User Guide
Publication no. 101557
Copyright © 2009 – STM Group, Inc.
Page 97 (160)
The GPGGA GPS string concerns the global positioning system fix data. STM supports the
following string:
$GPGGA,123519,4807.038,N,01131.000,E,1,08,0.9,545.4,M,46.9,M,,*47
Parsing out the fields sequentially in the string, we find that this string indicates a fix taken at 12:35:19
UTC, for the latitude 48 degrees, 7.038 seconds North and the longitude 11 degrees, 31 seconds East.
The “1” following the “E” indicates a selection from one of 9 possible options for fix quality, 0-8. These
options are:
0: Invalid
1: GPS Fix (SPS)
2: DGPS Fix
3: PPS Fix
4: Real Time Kinematic
5: Float RTK
6: Estimated (Dead Reckoning)
7: Manual Input Mode
8: Simulation Mode
After the fix quality option comes “08”, representing the number of satellites being tracked. The
horizontal dilution of position is 0.9. The altitude in meters above sea level is 535.4 meters, and the
height of geoid (mean sea level) above WGS854 ellipsoid is 46.9 meters. The two empty fields represent
the time in seconds since the last DGPS update and the DGPS station ID number. At the end of the string
is the Checksum data, 47.
Serial interface settings specified for use with NMEA-0183 are:
Baud Rate: 4800
Data: 8 bits
Parity: None
Stop: 1 bit
Flow Control: None
19.3 Optimising the mobile VSAT for recovering contact with network after
blockings
The “dvb tablecache” command can be used to tune the behaviour of mobile VSATs that experience short
blockings, as e.g on trains passing through tunnels. This CLI commands allows for configuring the
mobile VSATs to wait longer before deciding that the link to the SatLink Hub has been lost and start
reacquiring the forward link. The format of the command is
dvb rx tablecache <on|off> [<tunerRetryTime>]
The tunerRetryTime parameter sets the maximum tuner retry time in seconds. After running the
dvb
tablecache
command dvb rx stop and start command is needed to activate a changed time.
Below is an example with a tuner retry time of 180 seconds:
dvb rx tablecache on 180
dvb rx stop
dvb rx start
which results in that the mobile VSAT will wait 3 minutes after detecting an outage before it starts to
reacquire the forward link and re-read all DVB tables. If the forward link is found again within 3 minutes
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