David Taylor kindly added one of my old graphs to his Garmin LVC18 pages: https://www.satsignal.eu/ntp/Garmin-GSP18x-LVC-firmware-issue.htm#3.60 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 <mich...@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 > > _______________________________________________ > questions mailing list > questions@lists.ntp.org > http://lists.ntp.org/listinfo/questions > _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
