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M8T time offset

From: Denny Page
Subject: M8T time offset
Date: Tue, 21 Jan 2020 12:31:36 -0800

So, I’ve been seeing a bit of a weird problem. I have an M8T that I’m using with gpsd. The behavior I see is that gpsd oscillates every 30 minutes or so between reporting time correctly, and reporting time off by 1 second. 

The system clock is managed by chronyd, and is locked to a LeoNTP in the local network. Chronyd is monitoring several other stratum 1 devices in the local network. No shift is seen with the other devices. The attached graph shows the offset of the gpsd/M8T as seen by chronyd. Note that gpsmon reports the same 1 second PPS offset seen by chronyd.

The most interesting thing is that the issue only occurs if the M8T is in binary mode. If the M8T is in NMEA mode, gpsd reports the correct time with no shifts. I have done a config reset on the M8T, followed by a cold boot of the M8T, and the results remain consistent.

This may or may not be coincidental: I did not see this issue back when using 4.X kernel. I believe that the issue may have began with Linux 5.3, but I cannot be sure. I haven’t re-installed old kernels to find out when this began or if it is even kernel related. My current kernel version is 5.4.13.

I am using gpsd 3.19, although I have tested with current head (3.20) and see the same behavior.

Thoughts anyone?


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