I have two Raspberry Pi card computers both synced to GPS/PPS sources.
#1 is using a u-blox 6M "navigation" GPS, and #2 a Trimble resolution
SMT "timing" GPS. Every now and then (perhaps one day on ten?), the
"navigation" synced device develops spikes in the offset of about 8
microseconds in duration, as shown here:
Graphs for December 20:
http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html#oscillations
Current data:
http://www.satsignal.eu/mrtg/performance_ntp-pn.php
Although the offset appears to have a 1.25 hour period from the MRTG
graphs, examining the loopstats directly shows that the actual period is
just over about 5/3 minutes - just over 100 seconds. I don't know
what's happening. I would have put it down to poor GPS strength, except
that the effect lasts almost a whole day, and GPS missing usually
contributes bigger offset spikes. I would not expect it to be the
navigation mode of the GPS as the excursions are larger than I would
expect between navigation and timing modes, but I don't have a lot of
experience in that area. One difference in the configuration is that #1
- the one with the offsets - runs and uses gpsd for the coarse seconds,
whereas #2 relies on the rest of the network. This seems to causes a
higher CPU usage in #1, shown in there graphs:
http://www.satsignal.eu/mrtg/performance_raspi-1.php
http://www.satsignal.eu/mrtg/performance_raspi-2.php
Your comments welcomed!
--
Cheers,
David
Web: http://www.satsignal.eu
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