Periodic spike in offset for pps

[Doug Curtis]

I apologize if this has been answered previously.

I've setup a few ntp servers on raspberry Pis using the ntpsec howto with
clockmaker.  Currently, I'm using a pi2, pi3 and pi4 with 2 of them using
the Adafruit gps hat and one of them using a "Gowoops GPS Module
<https://www.amazon.com/Gowoops-Module-Antenna-Arduino-Microcomputer/dp/B01MRNN3YZ/ref=cm_cr_arp_d_bdcrb_top?ie=UTF8>".
I setup ntpviz to run on a separate machine and I'm noticing a regular
offset spike for shm(1) in my graphs on all of them.

After changing the ntpviz period to 2 hours, it appears they are all
spiking after 34 minutes.  https://world-mail.org/ntpgraphs/gps3/
Here is a 24 hour period of one of the others:
https://world-mail.org/ntpgraphs/gps2/

Since I've been seeing this issue on all three of them, I designated one as
the test pi so I've gone through and stopped all non-essential services
that I had enabled (ntplogtmp, ntploggps).  I disabled the
/var/log/ntpstats nfs mount.  I changed it from dhcp to static ip
assignments as well.

Since they were all setup on different days, the version of gpsd is only
slightly different but here is the version of the test pi:
3.20.1~dev (revision release-3.20-695-g2de5ac5de)

Here is the ntpd version:
ntpd ntpsec-1.1.8+ 2020-05-04T14:44:13Z (git rev 574d871ed)

Here is what the ntp.conf looks like:

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

logfile /var/log/ntp.log

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

restrict default kod limited nomodify nopeer noquery
restrict -6 default kod limited nomodify nopeer noquery
restrict 127.0.0.1
restrict -6 ::1

refclock shm unit 1 maxpoll 4 refid PPS flag4 1 prefer

server rolex.usg.edu iburst
server ntp3.mattnordhoff.com iburst

refclock shm unit 0 maxpoll 4 time1 0.138 refid GPS flag4 1

I can't think of anything else to look for or try.

Thanks,

Doug
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