[ntp:questions] Garmin LVC 18x jitter problem

David Taylor david-taylor at blueyonder.co.uk.invalid
Thu Jul 11 04:42:18 UTC 2019


On 10/07/2019 22:15, Michael Haardt wrote:
> I use a Garmin LVC 18x with a Raspberry Pi, process NMEA with gpsd
> (SHM driver) and acess PPS by GPIO (kernel PPS driver).  gpsd allows
> monitoring and passing PPS right into ntpd avoids gpsd conversion of PPS.
> 
> Sometimes this works, but then there are times where both clocks are
> thrown out as falsetickers.  I believe that's due to the strange jitter
> behaviour of the NMEA data.  If I understood the clock selection right,
> then basically the measured jitter forms an interval around the offset
> and the intersection interval is the root dispersion.  Should the NMEA
> clock have no intersection with the used PPS clock, it is a falseticker,
> but since PPS depends on it, PPS is so as well.
> 
> I graphed offset and jitter from two days peerstats where I was locked
> on PPS (tos mindist 0.1) and the system ran stable without the need
> to adjust the crystal frequency at constant environment temperature.
> The first day was mostly fine, but in the middle of the second day, the
> GPS jittered really bad and there are many occasions where the jitter
> interval did not intersect with 0.  The ntpd jitter estimation works as
> expected for a normal distribution, but the distribution is clearly
> different:
> 
> http://www.moria.de/~michael/tmp/offset.svg
> http://www.moria.de/~michael/tmp/jitter.svg
> 
> Using the tos mindist extends the intersection interval, but that
> effects the root dispersion.  That's correct if it is needed to have an
> intersection between different clocks of low jitter, but in my case the
> problem is a wrong (too low) jitter estimation of a clock I only need
> as PPS reference.
> 
> Is there any way to specify the precision manually, like fudge minjitter?
> Clearly the jitter suffices to keep the PPS clock running and I would
> like to have PPS determine the root dispersion, because the PPS clock
> has a jitter of 4 us.
> 
> This problem seems to have come up a number of times in the past, but
> I never saw the root dispersion impact of tos mindist mentioned and I
> suspect in a number of cases configuring a minimal jitter would have
> been a better solution.
> 
> Michael

Michael,

I'm not an expert in this, but the timing seems to be the same in both 
graphs, the problem occurring around 8000.  Could that be due to someone 
with a GPS jammer parking nearby?

I always try and have more than just the NMEA source as a "prefer" 
server if possible, something from the Internet will be much more 
reliable than the serial data!  Try adding a "pool" directive or some 
known good servers.

-- 
Cheers,
David
Web: http://www.satsignal.eu



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