On Tue, Aug 09, 2016 at 04:22:56PM -0400, Chris Greenman wrote:
> Ok I set it to 0.0065 offset.  Here's my chrony.conf
> 
> pool pool.ntp.org
> driftfile /var/lib/chrony/drift
> allow
> refclock SHM 0 refid GPS offset 0.065
> #refclock SHM 0 refid GPS precision 1e-1 offset 0.9999 delay 0.2
> makestep 1 -1
> 
> and here is the chrony sources output:

> ===============================================================================
> #* GPS                           0   4   377    12  +2112us[+2967us] +/-
>  325us
> ^- orchid.sidereal.ca            2   6    17    19    +11ms[  +12ms] +/-
> 40ms
> ^- static-96-244-96-19.bltmm     2   6    17    19    +11ms[  +12ms] +/-
> 40ms
> ^- hydrogen.constant.com         2   6    17    18    +14ms[  +15ms] +/-
> 32ms
> ^- host2.kingrst.com             2   6    15    19  +9078us[  +10ms] +/-
>  100ms

Ok, so now it seems the correction should be actually about 10
milliseconds smaller. This means the GPS offset is not very stable,
which is normal with message based (e.g. NMEA) samples. The value
specified by the delay option should include this interval. If you set
it to 0.2, the assumed error of the source will be +/- 0.1 seconds. As
long as the GPS time (corrected by the offset) doesn't move from the
true time by more than 0.1 seconds, the error interval will contain
the true time and the source will always agree with other good
sources.

If you enable the statistics log and run it for few days, you will
probably better see how much the offset moves. If you don't have time
for that, I think delay of 0.2 seconds should be good enough.

The recommended refclock line in this case would be:

refclock SHM 0 refid GPS offset 0.060 delay 0.2 minsamples 64

The assumed error of the GPS source will now be similar or larger than
the NTP sources, so if they are online, GPS should be ignored or
combined with them (+ symbol in the sources output). Only when the NTP
sources are offline the synchronization will use GPS alone.

-- 
Miroslav Lichvar

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