On 24/05/2013 21:28, Paul G wrote:
I have four GPS receivers into four different computers. I've assumed that I
should be able to get small offsets between them but to do so I need
to set time1. If that's the correct approach how do I pick the "right"
system to be the benchmark. So far I've just been assuming that
the purpose built M. Tharp server should have the lowest latency but I'm not
sure -- in any case, at present, I can't adjust it.
Each machine in the truncated table below (except 210) has a time1
value derived in an ad hoc fashion.
l r refid st delay offset jitter
==================================================
1 o22.0 .PPS. 0 0.000 -0.001 0.001
2 +244 .PPS. 1 0.068 0.000 0.006
2 *210 .GPS. 1 0.514 -0.008 0.136
2 +192 .PPS. 1 0.455 0.008 0.032
I feel as if this topic should be clearly explained somewhere but I've been
unable to find it.
Paul,
Those PCs with PPS feed should have zero offset, and time1 should be set
to zero unless you /know/ that the GPS PPS signal is offset (e.g. you
are feeding it via several microseconds delay of cable, which is
unlikely). What NTP is actually syncing to is an internal PPS (if you
like), i.e. the external PPS signal delayed by whatever processing is
taking place inside the PC, but any external things you measure will be
subject to the same delay. What happens with the fudge set to zero?
Please show the table for each machine.
For the PC with just GPS sync, you might get it a PPS feed somehow, use
PPS as the reference, set GPS/NMEA to noselect in the ntp.conf, and
record the peersstats. I wrote this up with help from others here:
http://www.satsignal.eu/ntp/Garmin-GSP18x-LVC-firmware-issue.htm#analysis
and although the results in that case are wrong, the method is, I
believe, correct.
--
Cheers,
David
Web: http://www.satsignal.eu
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