[chrony-users] why PPS one second ahead NTP?

[Petri Hyvarinen <petri.hyvarinen@xxxxxxxxx>]

Dear list readers,

I have been trying to figure out why PPS gets approx. one second ahead of NTP servers. Sometimes even PPS is ahead of NMEA approx. one second (perhaps depending of the offset set in configuration?).

 

Here is the situation how it ends up

MS Name/IP address         Stratum Poll Reach LastRx Last sample

===============================================================================

#* PPS                           0   4   377    14  +6587ns[+7168ns] +/- 5001ns

#- NMEA                          0   4   377    13   +109ms[ +109ms] +/-  200ms

^? 109-108-30-87.bb.dnainte>     2  10   377   854   +995ms[ +995ms] +/-   48ms

^? ntp3.dnainternet.fi           2  10   377   880  +1000ms[+1000ms] +/- 6362us

^? skg.vaxxi.net                 2  10   377  1031   +997ms[ +997ms] +/-   58ms

^? hell.ruselabs.com             2  10   377    55   +993ms[ +993ms] +/-   19ms

 

This is embedded system with u-blox module onboard where I have not all details available how HW is integrated between host and GNSS receiver module.

 

Below relevant lines (in my understanding) from chrony.conf

pool 0.fi.pool.ntp.org noselect

refclock PPS /dev/pps0 lock NMEA refid PPS precision 5e-6

refclock SHM 0 refid NMEA offset 0.9999 precision 2e-1 #offset 0.15 results approx. 41..44ms last sample

leapsectz right/UTC

 

I use gpsd to serve PPS and NMEA. Its configuration looks like below:

DEVICES="/dev/ttymxc0 /dev/pps0"

GPSD_OPTIONS="-n"

USBAUTO="false"

START_DAEMON="true"

GPSD_SOCKET="/var/run/gpsd.sock"

 

I have tried to set SHM 1 or SOCK as PPS source for refclock, but in my opinion above config is most stable.

 

I need rather accurate measurements within ms deviation between devices and I don’t always have internet connection available, so therefore I have “preferred” local NMEA over NTP. NTPs are just listed in case I know my time difference with rest of the world.

 

Do you see obvious configuration issue here? If not, how I could debug root cause for PPS ahead?

 

Thanks in advance and any hint appreciated.

 

Regards,

Petri

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