[ntp:questions] high jitter on serial gps causes big time offsets

Robert Scott no-one at notreal.invalid
Fri Apr 5 14:02:02 UTC 2013

On Fri, 5 Apr 2013 11:09:29 GMT, Nickolay Orekhov <nowhere at mail.ru>

>I've got the following configuration:
>tos mindist 0.128
>tinker panic 0 stepout 60
># TSIP,PPS reference clock
>server mode 10 prefer maxpoll 3 true
>fudge refid TSIP time1 0.08
>server maxpoll 3
>fudge refid PPSI
>The main goal: I want 1 ms or better precision always, even with gps
>quality going high and low.
>When satellites appear and disappear again, serial clock could be selected
>before PPS clock. Or maybe some gps receivers will show good quality before
>PPS will appear or will be selected.
>In general, I want ntpd to skip adjustments coming from specific server or
>ref clock according to some constant precision. For example, I know that
>gps serial jitter is about 30-40 ms. So I want ntpd to do no adjustments
>lower than this value...

I don't understand how this rule of not adjusting less than 30-40
msec. offers any practical help to you.  You say you don't want to
adjust by amounts less than this for fear of adjusting based on
unreliable GPS serial timing.  But if you ever have to adjust by more
than 30-40 msec., that is an admission that you were previously off by
much more than your stated requirement of 1 msec. "always".  So the
effect of this rule is to never adjust.  In short, you can't use the
magnitude of the timing error to decide if the new GPS information is
coming from the jittery serial data or from the PPS clock.  If you
want to avoid using unreliable timing data you have to have some
independent way of deciding if the data is unreliable.  I don't know
enough about the indications from the GPS device to say how this is
done, but it seems that the GPS device itself knows if it has valid
PPS timing availble.

If you want to maintain precision through low GPS quality periods you
could fall back on your local clock, which hopefully has been
rate-trimmed over a long period of time so its drift rate is small
enough to maintain 1 msec. accuracy for as long as the GPS quality
remains low.

Robert Scott
Hopkins, MN

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