[time-nuts] weird Raspberry PPS+GPS NTP server behaviour

[Adam Kumiszcza]

Hi!

For several weeks I'm seeing strange behavior of my Raspberry Pi based NTP
server with Uputronics GPS PPS expansion board (Ublox MAX-M8Q chip).
Several weeks ago the mean daily jitter was below 300 ns, offset didn't
exceed ±1 µs, provided no restart or configuration change was made. Now the
offset often wanders even below 5 µs, steady change, and then comes back.
You can see it at the screenshot from my Grafana server (attached). As you
can see, the number of satellites visible and TDOP value should not be the
cause of it.

My current ntp.conf is as follows (I use ntpsec and gpsd):

driftfile /var/lib/ntp/ntp.drift
leapfile /var/lib/ntp/leap-seconds.list

statsdir /var/log/ntpstats/
statistics loopstats peerstats
filegen loopstats file loopstats type day enable
filegen peerstats file peerstats type day enable

restrict default limited kod nomodify nopeer noquery
restrict source nomodify noquery
restrict 127.0.0.1
restrict 192.168.0.0 mask 255.255.0.0

tos mindist 0.002

refclock shm unit 1 minpoll 4 maxpoll 4 flag4 1 prefer refid PPS
refclock shm unit 0 minpoll 4 maxpoll 4 time1 0.057984 refid GPS

#server nts.time.nl nts
#server nts.ntp.se:4443 nts

server tempus1.gum.gov.pl prefer
server tempus2.gum.gov.pl
server ntp.task.gda.pl
server ntp.nask.pl

server MY_GPSDO iburst

Before that I thought it was because I used NTP pool, as some time ago
similar behavior was mentioned here on the list, but, as you can see, I do
not use it anymore and the same strange behavior remained.

Any idea what could be the cause of this? Thanks in advance.

Adam
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