| RE: [chrony-dev] pps source is still marked as synchronised when no absolute clock sources are available |
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On Wed, 20 Jan 2010, Hattink, Tjalling (FINT) wrote:
From: Bill Unruh
Sent: Wednesday, January 20, 2010 16:18
Are those separate GPS cards or the same card?
They come from the same GPS card.
The daemon providing the SHM interface only provides timestamps when
the GPS card is locked and its time is accurate. The PPS interface
is
always providing sub-second timestamps.
When chrony is started it does not discipline the system clock until
the SHM interface is marked synchronised. After the SHM source is
correct it starts to look at the PPS clock and after a while the PPS
clock is used as main source. This is correct behaviour. Now I
disconnect the antenna from the GPS card and the SHM interface stops
providing timestamps, as the GPS clock is unreliable. You can see in
the sources list that the SHM clock is marked unreachable. But the
PPS
pulses are still provided and Chrony keeps using the PPS as
synchronisation source. I think this is not correct behaviour.
Chrony
should check if any absolute time source is reachable and
synchronised
every time when a PPS signal comes in. If not it should also mark
the
PPS sources unreachable. This behaviour is also shown by ntpd.
I disagree. chrony is able to keep time to an accuracy of 1/2 sec
between
the pps pulses, even if some are missed. The local clock is assumed
not
be jumping around by seconds at a time, or nothing, chrony, ntp, you,
will every be able to discipline it. Anyway, you should always have
some
other fallback source from the internet listed to make sure that the
the
system does not suddenly jump. Ie, the theory is that once the time is
within 1/2 sec then the pps clock can discipline the local clock,
using
the ability of the local clock to discipline the time.
I agree with you that chrony will be able to keep time between the pps
pulses. But in my situation I cannot blindly trust the incoming PPS
signal
being the correct time. When the GPS card has no lock it keeps
generating
This is a hardware problem, external to chrony. If your gps card is providing
PPS without those PPS haveing anything to do with accurate time, that is an
external problem to chrony, or any time discipline. Chrony has to trust its
sources in the absense of evidence to the contrary. You are trying to solve a
hardware problem-- an unreliable gps card, by chrony or ntp, and that is not
their jobs.
If you are really going to rely on the time put in three gps cards of sources
( the Garmin 18x is only about $60. Buy 5, and use them if you are really
going to rely on the time for that server).
PPS pulses from its own oscillator, but it will be free running and
drifting
fast as no GPS corrections come in. So somehow I have to tell chrony not
to
trust the PPS source anymore when GPS lock is gone. I used to do this
That should be a job for the software. for example gpsd uses both the NMEA
time and the PPS. It should be its job to decide if that gps receiver has gone
crazy, for example because its nmea time has stopped working. That should not
be a job for chrony or ntpd.
for
ntpd by making the SHM clock unreachable, but it works differenty in
chrony.
If you would like to always trust any incoming PPS signals, even if no
absolute reference source is available, maybe I can solve it by
implementing a chronyc command to disable specific reference clocks.
There is already such a command. Look at the chronyc commands ( eg offline).
Also the setup I use chrony in does not have internet access. It is an
isolated network where the server running chrony is providing ntp time
to all
the other connected devices. So I cannot use a backup NTP source, the
only
source is the GPS reference time.
Use multiple gps sources then.
Best regards,
Tjalling Hattink
--
William G. Unruh | Canadian Institute for| Tel: +1(604)822-3273
Physics&Astronomy | Advanced Research | Fax: +1(604)822-5324
UBC, Vancouver,BC | Program in Cosmology | unruh@xxxxxxxxxxxxxx
Canada V6T 1Z1 | and Gravity | www.theory.physics.ubc.ca/
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