Re: [chrony-users] PPS time

Okay, my understanding is that the primary reason for PPS in CHRONY is to get a more accurate time synchronization from a given source. The PPS signal is sent out to mark the top of the GPS second in the time message being sent over the serial port from the GPS. Both PPS and time messages are sent out at the same time, but the time messages have more non-determinism than the PPS, which is just a hardware / electrical state transition, For time keeping / clock synchronization, PPS is useful only as long as it is in reference to something that actually provides the time, i.e. the same time source. Regardless of how accurate your clocks are, using PPS from one clock and time messages from another would tend to reduce the accuracy of the time fix. From you description it is unclear if your "high precision" clock is actually a "clock" (with year, month, day, hour, minute, second) or just an accurate "timer" which produces a pulse at precisely 1 second intervals (PPS). If it is an actual clock, you would probably be better off simply using it as a reference without adding GPS. The other alternative is to use GPS+PPS intermittently and depending on CHRONY to maintain the system and discover when GPS is available. System clocks are fairly stable. The key when using PPS for adjusting time is the PPS needs to be from the same clock as you are getting time information. PPS improves the time sync by reducing the non-determinism of data being transmitted over the serial line.

John

On Tue, Jul 16, 2019 at 4:40 PM Steve Summit <scs@xxxxxxxxxx> wrote:

John Burton wrote:
> I used information from the GPSD web site as far as set up and
> configuration: https://gpsd.gitlab.io/gpsd/gpsd-time-service-howto.html
> This provides a good introduction to setting up a GPS network clock...
> Basically you use GPSD to talk to the GPS receiver, which uses KPPS
> to handle the PPS interrupt. GPSD handles all the details of serial
> port & where the PPS signal is coming from.

Yup. That's the configuration I *used* to have, that I need to
move away from, which is why I'm working to configure /dev/pps0
and have chrony talk to it directly, *without* gpsd handling the
details.

(Why? In my environment, I have GPS reception only intermittently.
I have a high-precision reference clock to keep time, via its own
PPS output, when I have no GPS. But if I feed my reference clock's
PPS into gpsd along with the time messages from my GPS receiver,
then when I have no GPS, gpsd rightly assumes that PPS is no good
either, and stops forwarding it to chrony. Thus the need for a
direct PPS feed into chrony.)

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John Burton, Ph.D.
MTEQ, Inc.

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