| Re: [chrony-users] Trouble with PPS |
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On Tue, Apr 17, 2018 at 10:20:04AM +0200, Alvaro Gamez wrote:
> 2018-04-17 9:00 GMT+02:00 Miroslav Lichvar <mlichvar@xxxxxxxxxx>:
> > You might want to add the noselect option to the GPS refclock
> > directive to not consider it for selection at all. Otherwise, it would
> > be good to at least increase its distance with the delay option to not
> > get two falsetickers if the offset between them changes. The example
> > in chrony.conf man page uses both.
> >
> >
> But PPS by itself can't provide datetime, only second synchronization.
> What would happen if I don't let chrony select GPS clock? Would it know
> what time is it?
Yes. That's what the lock option is for. It allows a refclock to
provide another refclock with seconds, but otherwise be ignored for
synchronization.
> PPS signal comes from the same device as GPS data, so their distance
> shouldn't ever vary, right?
This is about the NTP distance. It is the maximum assumed error of the
source. If the delay of the GPS messages is slowly changing over time
(which is not uncommon), the offset between the two sources will be
changing. If you don't increase the distance, you might get two
sources that don't agree with each other (their error intervals don't
overlap) and chronyd wouldn't which one should be trusted.
--
Miroslav Lichvar
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