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

David Taylor david-taylor at blueyonder.co.uk.invalid
Mon Dec 16 17:35:41 UTC 2013

On 16/12/2013 11:16, 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?
> Many thanks,
> Adrian

In addition to what the others have suggested, does you GPS 18 LVC have 
a good view of the sky?  It's nothing like as sensitive as the newer 18x 
LVC (note the "x").

If it helps to compare, the data from my own Intel Atom FreeBSD 8.2 
system is here - PC Pixie:


As it's plotted with MRTG it's only sampled at 5 minute intervals, and 
the droop in the yearly graph is an artefact of MRTG's summarising, it's 
not real.  My GPS 18 LVC is mounter on the roof, sloping towards the 
west at an angle of about 22 degrees and slightly sheided by adjacent 
houses, so not a 360 degree view, but not inside either.
Web: http://www.satsignal.eu

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