[ntp:questions] PPS versus serial offset

detha detha at foad.co.za
Wed Aug 28 08:18:43 UTC 2013

I have a test setup with a RaspberryPi and a SiRF/gpsd module. All
working quite well, but one thing bugs me. Looking at the ntpq -p output
the serial port readings seem to drift away slowly but steadily from the
PPS, see <http://detha.co.za/ntp/ntpmon.20130828.jpg>.

Done the 'let it run for 24 hours, take average offset between PPS and
serial reading, and use that as time1 for the gpsd line in ntp.conf. After
that, offset between PPS and serial stays within 15ms for a few days, and
then starts creeping slowly, -15 +/- 20ms thus far. Nothing obvious I can
see on the raspi, in /var/log or things like memory usage or CPU load (all
it does is ntp and logging room temperature). 

Relevant part of ntp.conf:

tos mindist 0.020

# Kernel-mode PPS ref-clock
server minpoll 4 maxpoll 4
fudge  flag2 1 flag3 0  refid PPS

# GPS serial data
server iburst burst minpoll 4 prefer
fudge time1 0.308 refid NMEA minpoll 8 stratum 10

server stratum1.neology.co.za iburst noselect

I'm wondering what instrumentation I could put in place to see what causes


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