At 04:38 PM 7/13/2011, Michael Eder wrote...
We have looked at our GPS on a scope, the PPS it is dead on and the
NMEA
(just one sentence) is also reliable with about a 680 ms latency and
10 ms
jitter....
ce000:~/current# ./ntpstatus | tee ntplog.txt
remote refid st t when poll
reach delay offset
jitter
============================================================================
==
xSHM(0) .NMEA. 0 l 13 16 377 0.000 -74.683
68.862
*SHM(1) .PPS. 0 l 15 16 377 0.000 -0.034
0.061
...
ce000:~/current# ./ntpstatus | tee ntplog.txt
remote refid st t when poll
reach delay offset
jitter
============================================================================
==
xSHM(0) .NMEA. 0 l 1 16 377 0.000 -101.07
65.086
xSHM(1) .PPS. 0 l 4 16 377 0.000 -0.036
0.061
As I read this, ntp is seeing more like 70 ms of jitter, not 10, and
sees a -75 to -100 ms offset, so the 680 in the config doesn't seem
correct.
So maybe something like:
tos mindist 0.080
-and-
fudge 127.127.28.0 time1 0.593 refid NMEA
-or maybe-
fudge 127.127.28.0 time1 0.767 refid NMEA
(Trying to split the 75-100 offset difference to 87 - I can never keep
the signs right, so use trial and error)
What I've seen with the Thunderbolt GPSDO is that the offset for the
serial output slowly walks around, maybe +- 7 ms, centered around ~50
ms offset. If your Garmin does anything similar, even though you saw
680 ms on a scope, if you came back in an hour and measured again it
might be different. You may be able to analyze the peerstats file to
see how it changes on yours.
So, what I did was first widen the mindist, then tweaked the offset for
the serial time so it was centered around the PPS (which if things are
locked, shouldn't wander much at all). Finally, I reduced mindist to
about 15 ms (0.015 - about twice the actual variation I see).
oPPS(0) .gPPS. 0
l 7 16 377 0.000 -0.016 0.016
*GPS_PALISADE(0) .GPS. 2
l 5 16 377 0.000 4.092 0.604
oPPS(0) .gPPS. 0
l 3 16 377 0.000 -0.019 0.025
*GPS_PALISADE(0) .GPS. 2
l 1 16 277 0.000 3.808 0.695
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