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 the 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 RX Lock Signal
The SatLink 2900 outputs a RX lock signal on connector J1 to signal to external units when it is locked to
the Forward Link. The output signal is a 3.3V TTL signal, where "low" signals that the RX is locked and
"high" signals that the RX is not locked. See Figure 42 where the lock signal is found on pin 3 and uses
the common GND on pin 6.
19.4 Optimizing the Mobile VSAT for Recovering Contact with the Network after
Blockings
The dvb tablecache command can be used to tune the behaviour of mobile VSATs that experience
short blockings, as on trains passing through tunnels. This CLI command 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, the dvb rx stop and start commands are 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
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, the mobile VSAT
will simply resume communication with the Hub immediately and assume the outage was just caused by a
short blocking.
The example below disables the optimized tuning for mobile VSATs.
dvb rx tablecache off
It should be noted that the tablecache command is only recommended for use with mobile VSATs as the
standard configuration is optimized for fixed VSATs. Note also that one generally wants to increase the
TunerRetryTime to recover two-way communication faster after a blocking, but setting this parameter too
high will cause longer outages in case of a real outage where Forward Link re-acquisition might be
required to recover.
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