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

Nickolay Orekhov nowhere at mail.ru
Mon Apr 8 11:50:55 UTC 2013


No I'm not a gps developer.

Imagine that I've got very precise time according to GPS+PPS for a long
period.
After that GPS and PPS quality goes low and than quality of GPS serial high
jitter clock will appear before low jitter PPS.

Before selecting PPS, ntpd will do some adjustments according to GPS. And
my highly disciplined time will be spoiled.
When ntpd will switch to PPS my time will be disciplined again but there
will be noticable period of time when GPS will spoil my local clock
discipline and maybe frequency.

That is the problem. I don't want ntpd to do small adjustments according to
GPS serial that will spoil local clock. I just want to do steps according
to GPS and do adjustments according to PPS.

2013/4/5 unruh <unruh at invalid.ca>

> On 2013-04-05, Nickolay Orekhov <nowhere at mail.ru> wrote:
> > Hello!
> > I've got the following configuration:
> >
> > tos mindist 0.128
> > tinker panic 0 stepout 60
> >
> > # TSIP,PPS reference clock
> >
> > server 127.127.8.0 mode 10 prefer maxpoll 3 true
> > fudge 127.127.8.0 refid TSIP time1 0.08
> >
> > server 127.127.22.0 maxpoll 3
> > fudge 127.127.22.0 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
>
> What do the satellites have to do with it? The internal gps receiver
> clock can freewheel over the few seconds that the sattelites come in or
> go out, and any jitter there is on the nanosecond level, not
> millisecond.
> I thought you were using pps. That is always good to at least micro
> seconds (taking interrupt jitter into account), not msec. The serial
> port gps is used only to set the seconds ( and the jitter there should
> certainly be less than a second) not the sub second.
>
>
> So, the sub microsecond PPS should ALWAYS be selected over the serial
> clock.
>
> > 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. And when PPS will appear it will be selected.
>
> Why? ntpd will do the best job it can with the output of the serial gps
> . If they jitter by 30ms, then the ntp clock will only be good to about
> that. So what. When PPS comes on line, it will be good to microseconds.
>
> I think you have developed some model of ntp and of gps which is not
> what they actually are.
>
>
> >
> > Can this be done? Or how do you solve this situation?
>
> Solve what? You have not presented a problem. You have presented a
> solution to an unknown problem and asked us to comment on it.
>
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