[ntp:questions] NTP server with Garmin GPS 18 LVC

David Lord snews at lordynet.org
Mon Dec 16 13:58:46 UTC 2013

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 mode 1 minpoll 4 maxpoll 4 prefer
> fudge 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

$ tail -n 6 /var/log/ntp/stats/loop_summary

loop 5400, 14+/-45.3, rms 5.8, freq -36.39+/-0.331, var 0.093

loop 5400, 8+/-36.3, rms 4.0, freq -36.44+/-0.270, var 0.080


> Many thanks,
> Adrian

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