|Subject:||[gpsd-users] Sudden shift in time?|
|Date:||Sat, 22 Oct 2011 10:12:24 -0700|
|User-agent:||Mozilla/5.0 (Windows NT 5.1; rv:7.0.1) Gecko/20110929 Thunderbird/7.0.1|
I'm running tests with ntpd on one system so I'm constantly watching the peer status. This system has seven internet peers, a PPS input (not from gpsd) and the SHM(0) input from gpsd. The SHM(0) clock had been reporting time to within about +/-50 ms of the clock tick and then out of the blue it was reporting -680 ms (+/-10ms). The jitter reported was of the same magnitude (opposite sign) for the very first cycle where the large offset appeared. It settled back to a more normal jitter of about 10ms
I didn't do anything else, I just watched it to see what would happen. On another machine I query this instance of gpsd's satellite info and all looked good there. Ten satellites were visible, eight used in the fix, HDOP was 1.2.
The offset managed to fix itself with no intervention from me. It was a sudden step correction again just like when it went haywire. Again the jitter reported was huge for the first cycle after the step change occurred and then settled back to a normal value.
The system clock did not change during this process. The SHM(0) clock was labeled a false ticker and kicked out of the calculations. The rest of the internet clocks and the PPS signal did not change their offsets or show any other unusual behavior so it was confined to just gpsd.
I'm just not sure what would cause such a sudden, huge offset. Most of the time the offset wanders around in small increments but stays within the +/-50 ms window.
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