[ntp:questions] Garmin LVC 18x jitter problem

Steven Sommars stevesommarsntp at gmail.com
Fri Jul 19 21:14:33 UTC 2019

David Taylor kindly added one of my old graphs to his Garmin LVC18 pages:
The graph is from an email exchange with Garmin ~seven years ago and shows
timing of the NMEA version 3.60 firmware.  This version suffered from drift
and high jitter, see below.  The change log mentions:
                         Changes made from version 3.60 to 3.70:  Improved
NMEA output timing stability.
Version 3.64 (unreleased) had an offset of around 600 msec, with no
significant drift.

ntpd was happy with 3.70, I haven't looked at later firmware version.
(Garmin is up to version 4.20 now.)

The graph shown in http://www.moria.de/~michael/tmp/offset.svg also shows
high drift & jitter, though not as severe as the old
release I used.

Some of the later firmware versions may have NMEA timings that are poor
fits for NTP usage.

On Fri, Jul 12, 2019 at 9:31 AM Michael Haardt <michael at moria.de> wrote:

> 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.
> Michael
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