Adrian P wrote:
Hi,
In an old IBM thinkpad T22 laptop that runs FreeBSD 9.2, I have
configured a NTP server that gets the PPS signal from a Garmin GPS 18
LVC, as described here:
http://www.satsignal.eu/ntp/FreeBSD-GPS-PPS.htm
The /etc/ntp.conf contains the following generic NMEA GPS receiver
driver configuration:
server 127.127.20.0 mode 1 minpoll 4 maxpoll 4 prefer
fudge 127.127.20.0 flag1 1 flag2 0 flag3 1 refid PPS
I let it run for a few days, however I still have every now and then
huge peaks (between -50 and +30 us) of the clock offset as seen in the
following graph: http://goo.gl/JpSyeO
So I am wondering: why those repeating huge clock offsets? They
allays starts with a negative peak, followed immediately by a positive
one. Is this because of the faulty laptop internal clock, that drifts
away with 30-40 us every now and then? Any thoughts?
On my system I usually have at least one or two spikes each
evening which correspond to nightly cron jobs. I'm guessing
localised motherboard temperature increases faster than
ntpd+pps can correct. The following peak is usually of opposite
sign.
$ tail -n 6 /var/log/ntp/stats/loop_summary
loopstats.20131214
loop 5400, 14+/-45.3, rms 5.8, freq -36.39+/-0.331, var 0.093
loopstats.20131215
loop 5400, 8+/-36.3, rms 4.0, freq -36.44+/-0.270, var 0.080
David
Many thanks,
Adrian
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