[ntp:questions] A Christmas puzzler - intermittent offset oscillations with a PPS source

David Taylor david-taylor at blueyonder.co.uk.invalid
Fri Dec 21 09:55:26 UTC 2012

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:

Current data:

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:


Your comments welcomed!
Web: http://www.satsignal.eu

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