[ntp:questions] Garmin LVC 18x jitter problem

Michael Haardt michael at moria.de
Thu Jul 11 17:43:21 UTC 2019

Hello David,

I trust the clock, because with constant temperature crystals perform
pretty good.  The clock is disciplined with PPS, but no adjustments
happened during the measurement, so it runs with a constant frequency
offset.  It took a few days to reach that state.

My point is that NMEA on this device does not jitter with a gauss
distribution or similar, but in a way that causes the reasonable jitter
calculation to give too low values.

The jitter diagram shows why 0.1 as minimum distance suffices.
The problem is, that this is only a workaround, as it effects the root
dispersion.  What I need is to bypass the NMEA jitter calculation only,
or give a minimal value for that, to prevent it becoming a false ticker
when the actual jitter is larger than the estimated jitter.

In the diagrams that happens if the estimated jitter does not cover
zero, because zero is the PPS/crystal clock with 4 us jitter (both
estimated and actual).

GPS reception was fine with good average SNR and a reasonable of used
and visible satellites.  TDOP was fine.

I had this behavior before the upgrade from firmware 4.00 to 4.20,
which is recommended due to the GPS week rollover.  I did not have it
when the GPS reception was worse and I had a lower average SNR and
an overall higher NMEA jitter and worse TDOP.


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