Sending commands to device while gpsd is running

[Filip Kubicz]

Hi gpsd mailing list,

I have a LC29D GNSS module that talks over UART with NMEA sentences.
Receiving part is easy, gpsd parses the UART data and I can read the SKY and 
TPV in my client program.

However, I also need to send some commands over UART to configure it and then 
feed it with the information it needs (e.g. vehicle wheel ticks).

So far I assumed that the correct way to send such commands is with gpsctl -x 
"$COMMAND". But I see it behaves in a strange way, possibly trying to 
re-activate the device:

# gpsctl -x "$COMMAND" -D 5 /dev/ttyGPS0
gpsctl:INFO: SER: opening GPS data source type 2 at '/dev/ttyGPS0'
gpsctl:INFO: SER: speed 115200, 8N1
gpsctl:INFO: gpsd_activate(1): activated GPS (fd 3)
gpsctl:INFO: device /dev/ttyGPS0 activated
gpsctl:INFO: startup at 2021-12-30T10:55:48.000Z (1640861748)
gpsctl:PROG: switching to match packet type 1: 
$GNRMC,105549.43,V,,,,,,,301221,,,N,V*11\x0d\x0a
gpsctl:PROG: switch_driver(NMEA0183) called...
gpsctl:PROG: selecting NMEA0183 driver...
gpsctl:INFO: /dev/ttyGPS0 identified as type NMEA0183, 1 sec @ 115200bps
gpsctl:PROG: => Probing for Garmin NMEA
gpsctl:IO: SER: => GPS: $PGRMCE*0E\x0d\x0a
gpsctl:IO: <= GPS: $GNRMC,105549.43,V,,,,,,,301221,,,N,V*11
gpsctl:WARN: cycle-start detector failed.
gpsctl:WARN: device error, bailing out.

When I just send the command over UART, the command succeeds and the device 
sends back
$COMMANDACKOK*16

What would be the best way to send a command to a device while gpsd is running? 
The commands are NMEA-like text.
The commands are independent from the daemon operation.

Kind regards,
Filip Kubicz