[ntp:questions] GPS NMEA offset with PPS

Nickolay Orekhov nowhere at mail.ru
Thu Dec 29 19:02:04 UTC 2011


1. If you mark clocks as "true" you somehow fool yourself :-). Because now
if the clocks are real falsetickers you won't even know about it and your
system
will be out of sync and for ex. will show low offset from some falseticker
maybe.
So you have to make clocks closer but don't mark them as "true". When
comparing them ntpd will take into account large jitter of NMEA and maybe
you may go forward
even without increasing mindist.

2. NTPD can choose 2 of 3 clock sources but he can't choose 1 of 2 clocks.
So you have to combine NMEA and PPS in one source
or add another ntp server for selection algorithm to work.

3. When starting ntpd will at first STEP time to some clock source. I can
suppose It will be NMEA. But later ntpd will choose
PPS because of low jitter and spike detection algorithm can take place. It
will wait for 15 minutes and then make another STEP. To skip this
15 minutes, set time1 to real value.

That's just a wild guess.
To be more specific, please, could you post your full config first of all,
and then ntpq output
of commands such as "pe", "rv", "cv" and so on short after restart and then
after about half an hour? I think there's some problem in configs. C

2011/12/29 Tomi Lehto <tlehto at iki.fi>

> Hi,
>
> Nickolay Orekhov <nowhere at mail.ru> wrote:
> > 2011/12/29 Nickolay Orekhov <nowhere at mail.ru>
> >
> > Yes, you are right. Your system is synced with PPS and gets seconds from
> > NMEA.
> > You can set time1 to make NMEA offset closer to reality ( and to PPS ).
> >
>
> Having NMEA offset close to zero, or leaving it at 350 ms, is there a
> difference
> how it affects the system clock if the pps pulse is working?
>
> > By the way, if you have only two clock sources, NMEA + PPS and offset
> > between them is bigger then "mindist" they
> > will be marked as falsetickers and there'll be no sync.
> > You might want to increase it by specifying: "tos mindist _value_" in
> your
> > ntp.conf.
> >
> > Default mindist is 0.001s, so as a wild guess, I can suppose that you set
> > your clock to  "true" or already increased mindist or
> > you have another one source, NTP maybe, otherwise they will be
> > falsetickers.
> >
>
> You are right; I forgot to point that I have set clocks to "true",
> otherwise they would become falsetickers which now makes sense given the
> large
> offset NMEA+PPS are only sources are available.
> I'll try 'tos mindist' later and see what happens.
>
> Next thing to figure out is when I restart ntpd, the PPS offset goes back
> to
> -350 ms and ntpd starts adjusting it to zero (and NMEA offset goes up to
> ~350 ms.
> That seems to take hours. It looks like the system clock runs away as soon
> as
> ntpd stops.
>
> Tomi
>
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