[ntp:questions] consistent negative offset with Garmin GPS 18x LVC and FreeBSD

Chris Albertson albertson.chris at gmail.com
Sun Mar 25 16:59:56 UTC 2012

On Sat, Mar 24, 2012 at 3:34 PM, Kenyon Ralph <kenyon at kenyonralph.com> wrote:
> I'm trying to figure out why my FreeBSD 9.0-STABLE system with a
> Garmin GPS 18x LVC and ntpd 4.2.7p255 has a consistent -2 ms to -4 ms

My guess is that your GPS is correct and the internet servers are
running a positive offset.       I think NTP figures the travel time
from a pool server to you is one half the roundtrip time.  But I'm
still confused because you have enough servers that I'd think that
effect would average out.  But on the other hand all of them share the
same "last mile" of wire.

The only way to test the GPS is with some other GPSes.   Don't use NTP
to test a GPS.  Compare the PPS from each directly with a counter, a
digital scope something you rig up on a breadboard with flip-flops or
whatever you have.

Chris Albertson
Redondo Beach, California

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