Re: [chrony-users] chrony losing sync with GPS reference

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I selected the last part of the refclocks.log.1 file.  The proceeding lines looked uninteresting (to me).

The 3 hour gap is either loss of GPS signal, or GPS data (e.g. gpsd).  I did see chrony come good when gpsd was
restarted, but I can't recall if it was for this unit/logfile.

Very strange chrony will give up if the offset is too large ( and 10 years is definitely
too large) I cannot remember what the value is.

It is really had to figure out whyin the world you would suddenly have a 10
year offset. Something seriously wrong either with the gps or with gpsd.

The 1 second offset could be due to the the "offset 0.9999" in the conf file (see below).  I'm not sure why it is
set to this.  Probably from defaults or taken from other sources on the internet.  From memory I've also seen config
settings of "offset of 0.5".

That should be adjusted to the time that it takes the gps unit to send out the
signal. It is certainly less than 1 sec-- and is probably more like .3 sec or
so. You need something else (like a remote ntp server) to tell you what a
reasonable value for the offset is.

Is the offset setting meant to be tuned for each deployed location, or for each device type (using a Quectel L76)? 
All our systems will have the "0.9999" setting.

Usually devices will have some average time delay, but it also depends
crucially on how many NMEA sentences the GPS is asked to send out.The fewer
the better.

Initially NTP servers were being used, but we found that chrony could stop tracking them (and never every try them
again).  We figured moving to GPS would be more reliable (especially if our celluar internet connection was flaky or
down temporarily).

You could use them to determine your system's average offset.

Here is the `chrony.conf` file (with the comments removed)

      makestep 1 -1

      refclock SHM 0 refid GPS precision 1e-1 offset 0.9999 delay 0.2

      keyfile /etc/chrony/chrony.keys

      commandkey 1

      driftfile /var/lib/chrony/chrony.drift

      log measurements statistics tracking rtc refclocks tempcomp

The statistics and tracking might give more clues. NOt sure why tempcomp is
there as it is really only useful if you a PPS source.

      logdir /var/log/chrony

      maxupdateskew 100.0

Now that could be problematic. An NMEA source could really be worse than that.
The default is 1000 and I would advise that for an NMEA source.


      dumpdir /var/lib/chrony

      local stratum 10

You have this there why?

      logchange 0.5




On 6/12/18 1:30 am, Bill Unruh wrote:
      As Miroslav says, it would help to see the chrony.conf file as well. It is
      bizarre that it sits there with a 1 second offset without trying to reduce it.
      What is the 3 hr gap in the data? Did you select output or was this a
      contiguous part of the data? offset sits there reliably at 1 second with no
      hint that the system is trying to fix it.(mind you this is only a 10 sec piece
      of data) Then there is suddenly a 10 year jump in the offset. It looks to me
      like your GPS is unit is toasted since there is no indication in the actuall
      system time that anything strange has happened. Ie, either gpsd or the gps
      unit itself is misbehaving badly.

      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 ______|_

      On Tue, 4 Dec 2018, Bill Unruh wrote:

            On Wed, 5 Dec 2018, Brendan Simon (eTRIX) wrote:

                  I'm running Debian Linux (Jessie based with a few backport packages) on some
                  embedded systems that use a GPS
                  receiver to set the system clock.

                  The problem I have seen is that the chrony loses sync with the GPS reference
                  after some time, and the system clock
                  starts to drift and never recovers.  This is disastrous as I need to have the
                  clocks synchronised.

                  gpsd was restarted and then chrony started synchronising the system clock again.

                  gpsd is 3.16 (jessie-backports) and chrony is 1.30 (jessie)

            I think more information is needed. Make sure that the "refclocks" logs are
            enabled, and look at them when the clock loses sync to see what is happening.
            Are you using PPS or just the UTC time stamos from theGPS?

                  Is there anything in chrony 1.30 that could be known to cause this loss of sync
                  with no recovery ever?

                  Is there anything in gpsd 3.16 that could be known to cause this loss of sync
                  with no recovery ever?

                  I'm looking at moving to Debian Buster (still in Testing but to be released
                  early 2019).  It has gpsd 3.17 (but
                  hopefully might move to 3.18 before release) and chrony 3.4.

                  Does anyone know if this latter combination of gpsd/chrony is likely to solve
                  the issues/symptoms described above?

                  gpsd changelog isn't too informative with fixes.  I see the following:

                  3.18: Fix several buffer issues.
                        Too many other bug fixes and improvements to mention.

                  3.17: Fix a SiRF driver bug that occasionally confused NTP.


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