Re: [chrony-users] gpsd, pps and chrony

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On Tue, 15 Mar 2011, Damien R wrote:

On 15 March 2011 11:06, Ed W <lists@xxxxxxxxxxxxxx> wrote:

On 15/03/2011 09:07, Damien R wrote:

I want to use chrony to synchronize the system clock between two
computers (with no connection between them), each equipped with a gps
(gps emmit a pps).

I guess it's just a mis-phrasing, but if there is no connection between
the two machines, how are you hoping that they will communicate between
each other to be synchronised?

No it's not a mis-phrasing, I hope both computer are synchronized with the
gps clock, and so both computer should be synchronize.

Yes, if both are synchronized to gps, then both are synchronized. However if
one looses the connection to the gps, then it has no way of getting the time.

Is it not possible to add some connection between the two machines?

No, it is impossible.

Why? Are they connected to the net in any fashion?

However, yes I believe that as long as you configure gpsd also then SHM
should work.  I have something like this in my chrony.conf:

refclock SHM 1 poll 4 offset 0.0 refid GPS1

Note I'm not currently using a gps, but your question was asked in
January on this list by Michael Ashley. Perhaps have a look through that?

I have already looked at this post but, although PPS is mentioned, this post
does not say if chrony is able to retrieve the PPS with shm reflclock.

Yes, it is.

But if it worked for you with
refclock SHM 1 poll 4 offset 0.0 refid GPS1
it looks like that chrony supports pps with gpsd.

Yes. The biggest problem is that all that the PPS gives you is the sub-second
-- it delivers no information about this second that pulse is assocated with.
You can use the nmea output of the gps receiver to recover that, but it is
delayed. Unfortunately, there is apparently a bug with the GPS18x that that
nmea time delivery is sometimes over one second later than the pulse. This
means that the pulse gets associated with the wrong second. Ie, you have to
make sure that the system knows that the nmea is often really late.

Also, it's in the back of my mind, but some older versions of gpsd
benefited from a patch by Miroslav to smooth PPS? If you are on some
distribution which only has antique gpsd versions then try getting up to
date?  Half remembered advice though...

I did not use a recent version but I will try with a new one.

I have not found this to be a problem. The pps pulse comes in with a scatter
of at most about .00001 sec ( <<10us) with no big excursions.

Best regards,
Damien R.

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