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

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On Wed, Mar 16, 2011 at 11:36:06AM +0100, Damien R wrote:
> So with the following configuration file for chrony:
> refclock SHM 0 poll 4 refid GPS
> refclock SHM 1 poll 4 refid PPS
> chrony cannot synchronize ("Selceted source GPS, Can't synchronize: no
> majority"), but if I comment the 1st or the 2nd line chrony is able to
> synchronize with the uncommented source.

I think the problem is that the sources' error intervals
(offset +- dispersion + delay / 2) don't overlap which means they are
both marked as falsetickers and no majority is found. This is a common
problem with NMEA, the messages come with large delays, sometimes
hundreds of milliseconds.

If you want to keep both sources, you'll need to use the offset and/or
delay refclock options to shift and/or widen the error interval for
the NMEA source, so it contains the PPS interval which probably is only
few microseconds wide. The chronyc sourcestats output is useful to
determine the correct values. If you want to keep the NMEA source only
for monitoring, you can use the noselect option, so it will be ignored
in the source selection algorithm.

> In the documentation of chrony, an example which uses the SHM driver and the
> PPS driver is provided. Moreover the tutorial mentioned in my first message
> shows the same configuration. So maybe, chrony treats PPS refclock and SHM
> refclock differently ?

PPS and SHM refclocks are very different, the former provides only
subsecond time information, it needs another source to get the missing
seconds. The PPS samples received by SHM refclock are already paired
with NMEA messages and completed in gpsd, so using only the SHM 1
refclock alone is perfectly fine and I'd say maybe even recommended.

Miroslav Lichvar

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