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Losing GPS position

I am sending the ROV’s position to Ardusub so that I can record the lat and long along with the video. I’m sending it via TCP port 27001. After about an hour of operations the position reverts to 0 degrees lat and long. I restart the vehicle, USBL and QGC and nothing happens unless I wait about an hour, then restart and it comes good.
The USBL user interface indicates when it has connected to Ardusub by changing its status (see below).It says it’s still connected when QGC stops displaying the correct lat and long.

The position is also being transmitted to Hypack survey software and I can see that it is updating there while it has stopped in QGC.
Does anyone have any suggestions or ideas?

I don’t know if it makes any difference, but I send the NMEA packets UDP to port 27001 on the ROV, and have had no issues.
I use Cerulean Sonar USBL. NMEARouter to direct the packets to the ROV, and TimeZero NAV software.

I also vote for UDP regarding positioning.
You do not need to care about dropped packages etc.

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The USBL only has a TCP option, but I’ll try sending it to a TCP port then using NMEA router to reroute it to UDP 27001.
Thanks for the tips.

This part is exceptionally weird. If that happens again, try checking what is in the screen session (screen -r nmearx). That may help us find the issue. From a quick code inspection I couldn’t find anything obviously wrong.