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

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On Mon, 21 Mar 2011, Miroslav Lichvar wrote:

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.

Maybe an NMEA source should not be regarded as a stratum 0 clock but rather as
a stratum 1. Its error ( many ms) is far greater than the PPS. Although even
with that error that does not make up for the huge (300 ms or greater) delay
from the nmea.

Certainly pairing the nmea with the PPS is the only proper thing to do,
although apparently the GPS18x has a firmware bug that causes the nmea to come
out over a second after the PPS, which screws up that association.

William G. Unruh   |  Canadian Institute for|     Tel: +1(604)822-3273
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