SeaSpace TeraScan 1.5M Operation And Maintenance Manual page 205

Notes:
Do not use a NMEA message to supply ship heading unless:
a. The ship heading is transmitted at least once per second,
and
b. The ship heading will remain valid even if the ship is not moving.
The antenna debug environment variable can be turned on to see the messages being received by the
nmea_monitor program. To do this, make sure that no other copy of nmea_monitor is running and
% setenv ANT_DEBUG 2
% nmea_monitor
Also, the ac 1 NMEA command will show the current ship heading. The ac command can run even if
a pass is being received.
DO NOT RUN MORE THAN ONE COPY OF nmea_monitor AT A TIME. THERE IS NO INTERLOCK
TO KEEP TWO COPIES FROM TRYING TO READ THE SAME SERIAL PORT.
To change from one type of ship heading data to another, edit the file $PASSDIR/config/system.config.
TeraScan 1.5m Polar Satellite Tracking Antenna O & M Manual 189