[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


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.

Web: http://www.satsignal.eu

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