Re: [chrony-users] trying to get Chrony to work with gpsd/PPS GPS

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William G. Unruh __| Canadian Institute for|____ Tel: +1(604)822-3273
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On Mon, 11 Sep 2017, Miroslav Lichvar wrote:

On Thu, Sep 07, 2017 at 06:49:42PM -0400, Bryan Seitz wrote:
# Garmin GPS 18 LVC
server    mode 0 minpoll 4 maxpoll 4 prefer
fudge    time2 0.600 flag1 1 flag2 0 flag3 1 refid PPS

flag3 enables the kernel PPS discipline, which uses a 1-second update
interval. If you wanted to have something similar with chronyd, you
would need to shorten the polling by adding "poll 0" to the refclock
directive. I think in most cases with a serial PPS input that is too
short, so I'd suggest to try "poll 2" first.
I believe that has the disadvantage of loosing the "popcorn" filter. Ie,
occassionally the server will server a time which is really wrong (by 10s of
usec, or msec or sec) about 40% of the times with are really off are thrown
away(really off means that they are furthest from the average.) to get rid of
those. Running with very short poll would mean that that does not work as
well. It is not clear at all why you would want to run with such a short poll

refclock SHM 0 offset -0.6 delay 0.5 refid NEMA noselect
refclock SHM 1 offset -0.6 precision 1e-9 lock NEMA refid PPS

The second "offset -0.6" should be removed. Unless a wrong edge of the
PPS signal is used, the offset is typically just a few microseconds.

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