Re: [chrony-users] Synchronize system clock to GPS time

["Ryan Govostes (he/him)" <rgovostes@xxxxxxxx>]

Does the PPS ref clock actually provide a time of day or anything, though? I thought it merely told us when to tick the second hand forward. Or is GPSD combining the two and sending a full timestamp over the socket? In which case, do I need the GPS refclock at all?

Or if I need both, how does one calibrate the offset?

> On Mar 3, 2022, at 23:36, Bill Unruh <unruh@xxxxxxxxxxxxxx> wrote:
> 
> Teh two sources GPS and PPS have vastly different uncertainties. so the system
> would tend to choose the PPS and ignore the GPS. The GPS will also tend to be
> much more than a few standard deviations away from the PPS. So it will not
> combine the output of the GPS with the PPS. It is like if you had one server
> which gavae you the year and wandered allo f  +- half a year, and another
> which gave you nanosecons. You would not want those two averaged to give the
> time.
> 
> You need to look at the offset of the GPS with respect to the PPS to get a
> better idea of how much of an offset you should put in for the GPS. .9999 is
> one second, which is almost certainly wrong.
> 
> 
> 
> 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
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> On Fri, 4 Mar 2022, Ryan Govostes (he/him) wrote:
> 
>> [CAUTION: Non-UBC Email] Hi Adrian,
>> Thanks for the pointers.
>> asdf.xyz was indeed just a throwaway host instead of my real NTP server. I removed that server line but it
>> did not seem to have an effect.
>> Then I tried removing the offset 0.9999 from the GPS ref clock — this was recommended in some tutorial
>> somewhere and obviously is not the correct value to use, and when I removed it, the system clock updated
>> right away.
>> Though now the GPS says it is “not combined” while the PPS is “current synced.” Perhaps that is the expected
>> behavior?
>> Ryan
>> 
>>      On Mar 3, 2022, at 22:52, Adrian Murphy <amurphy@xxxxxxxxxxxx> wrote:
>> Hi Ryan,
>> Can I ask:
>>  You have ‘prefer’ option added to the 'server asdf.xyz’ line.
>>  Could that be stopping chronyd from trusting the GPS?
>> Also, asdf.xyz does resolve to an NTP server for me, but maybe you are hiding the real hostname.
>> On Ubuntu Raspberry Pi4 I use:
>>  refclock SOCK /run/chrony.pps0.sock refid PPS1 poll 5 prefer
>>  refclock SHM 0 refid GPS0 precision 1e-20 offset 0.036790 delay 0.0
>>  server Aeternitas iburst
>>  …
>> Also the end of your conf file was garbled. Is that real?
>> Cheers, Adrian
>> 
>>      On 4 Mar 2022, at 7:07 am, Ryan Govostes (he/him) <rgovostes@xxxxxxxx> wrote:
>> 
>>      I have a GPS receiver with PPS connected to an embedded Linux system. GPSD feeds Chrony
>>      time updates from the GPS receiver over the Chrony socket.
>> 
>>      The system clock is behind by a few months, and Chrony does not want to update the clock to
>>      the time reported by GPSD.
>> 
>>      I have read the FAQ and a few threads on the same topic but the solutions presented
>>      don’t work for me. I have tried setting maxdistance to an absurdly high number, and
>>      allowing  makestep 1 -1.
>> 
>>      The “chronyc sources� command suggests that Chrony thinks that the GPS is in error.
>>      There is no console message that says that Chrony is attempting to adjust the clock.
>> 
>>      Can I force Chrony to accept the time from the GPS?
>> 
>>      Thanks,
>>      Ryan
>> 
>>      $ chronyc sources
>>      210 Number of sources = 3
>>      MS Name/IP address         Stratum Poll Reach LastRx Last sample               
>>      ===============================================================================
>>      #x GPS                           0   4   377    11   -2002h[ -2002h] +/-  100ms
>>      #x PPS                           0   4   377    10   -2002h[ -2002h] +/- 2919ns
>>      ^? asdf.xyz                      0   9     0     -     +0ns[   +0ns] +/-    0ns
>> 
>>      $ chronyc tracking
>>      Reference ID    : 00000000 ()
>>      Stratum         : 0
>>      Ref time (UTC)  : Thu Jan 01 00:00:00 1970
>>      System time     : 0.000000041 seconds fast of NTP time
>>      Last offset     : +0.000000000 seconds
>>      RMS offset      : 0.000000000 seconds
>>      Frequency       : 404.716 ppm fast
>>      Residual freq   : +0.000 ppm
>>      Skew            : 0.000 ppm
>>      Root delay      : 1.000000000 seconds
>>      Root dispersion : 1.000000000 seconds
>>      Update interval : 0.0 seconds
>>      Leap status     : Not synchronised
>> 
>>      $ cat /etc/chrony/chrony.conf
>>      server asdf.xyz iburst prefer
>> 
>>      # This directive specify the file into which chronyd will store the rate
>>      # information
>>      driftfile /var/lib/chrony/chrony.drift
>> 
>>      # Log files location
>>      logdir /var/log/chrony
>> 
>>      # Stop bad estimates upsetting machine clock
>>      #maxupdateskew 100.0
>> 
>>      # This directive enables kernel synchronisation (every 11 minutes) of the
>>      # real-time clock
>>      rtcsync
>> 
>>      # Step the system clock instead of slewing it if the adjustment is larger than
>>      # one second, but only in the first three clock updates
>>      makestep 1 -1
>> 
>>      #
>>      maxdistance 100000000
>> 
>>      refclock SOCK /run/chrony.ttyCOM2.sock refid GPS precision 1e-1 offset 0.9999
>>      refclock SOCK /run/chrony.ppsCOM2.sock refid PPS precision
>>      1e-7N‹§ēæėrļ›y隊W!Ū‰ōšĮŦē·Šđë-rčŸ+nÅöĶŠ\ĻŪ "ķ§ēæėrļ›z)í
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