[ntp:questions] PPS time1 and reported offset

A C agcarver+ntp at acarver.net
Thu Oct 13 22:57:18 UTC 2011


On 10/13/2011 05:16, David Lord wrote:

> NetBSD-5 ntpd version would be one of 4.2.6p2, 4.2.6p3
> or 4.2.7p98.

The stable release is 4.2.4 I believe, but I compiled 4.2.6p3 locally 
for my own use.

> 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.

>
> My experiment was with a watch xtal oscillator divided
> down to 1 pps with a pulse width of about 100ms. This was
> configured as PPS(2) with 'noselect' and fudge time1 set
> so that the initial offset of PPS(2) was within a few
> microseconds of PPS(1).
>
> As far as I'm concerned the fudge time1 behaved as I'd
> expected and for a short period PPS(2) remained within a
> few microseconds of PPS(1) but then drifted with
> temperature of the xtal module. Lowest drift rate was a
> few 10s of milliseconds/day but come the hotter weather
> it was drifting several seconds/day and I decided it
> wasn't worth continuing.


What I'm observing is inconsistent with the operation of the other clock 
drivers and the offset that they report.  They all report offsets after 
time1 is considered during the calculations.  But the PPS driver seems 
to report the time1 plus whatever other offsets occur.


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