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

Adrian P ntp at pascalau.ro
Mon Dec 16 11:16:15 UTC 2013


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?

Many thanks,
Adrian


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