[ntp:questions] PPS time1 and reported offset

Dave Hart hart at ntp.org
Fri Oct 14 00:38:03 UTC 2011


On Thu, Oct 13, 2011 at 22:57, A C <agcarver+ntp at acarver.net> wrote:
>> I already had a GPS setup as PPS(1) and system offset was
>> mostly zero +/- a few microseconds.
>
> Right, I get the same thing if I don't set time1 at all.  When I do set
> time1 I get the value of time1 plus or minus some microseconds as an offset
> that is always displayed in the peer list.  It never settles out to zero
> like all the other clock drivers do even when they have a time offset set on
> the fudge line.

I am pretty sure you can fix this by disabling kernel PPS sync, that
is, by not using flag3 1.

PPSAPI provides a way to pass a fudge (offset) to be applied to PPS
timestamps, but the common PPSAPI code in ntpd used by the PPS and
NMEA drivers (among others) does not use it.  As a result, when you
enable so-called hardpps, where the kernel disciplines its clock
directly to the PPS, the time1 fudge is not used by the kernel
hardpps.

Once you verify things behave as expected without flag3 1, please file
a bug report at http://bugs.ntp.org/ noting the incompatibility of
flag3 1 and time1.

Cheers,
Dave Hart


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