[ntp:questions] Garmin LVC 18x jitter problem

William Unruh unruh at invalid.ca
Fri Jul 12 10:32:16 UTC 2019

On 2019-07-11, 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.

Do not use nmea as a time source. It it has a long delay ( hundreds of
ms) which depnds on the speed of connection, on length of the nmea
sentance, etc. EAch character takes from 100 of usec to ms to be
delivered, and the lengthof sentences is variable, and the gps sends out
a sentence only when it is not terribly busy ( which depends on how many
sattelites it is trying to handle, etc) It is good for setting the
seconds (sometimes-- if you ask it to send too many sentences it can be
out by more than a second totally messing up your timing). 
One sentence (RMA) and a high baud rate make it useful for setting the

Ie, do not use it as a time source if you want accuracy that is better
than a second. Its jitter, because fo the above, is also going to be far
from Gaussian. (eg, the sentense on some dates and times is a few bytes
longer than the previous run and suddenly the time delivered is late by
a ms. more than before. )

> 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.

Or not use nmea as a time source. 

> 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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