OK, I reset my GPS to 19200 baud and configured it to emit only $GPGGA
sentences, along with a 100-msec-wide PPS. I also changed ntp.conf to
poll GPS and PPS every 8 (instead of 16) seconds.
It's been running for about an hour now, and the jitters are pretty low.
Here's a current result of running "ntpq -p".
remote refid st t when poll reach delay offset jitter
==============================================================================
xGPS_NMEA(0) .GPS. 9 l 7 8 377 0.000 -604.42 11.346
xPPS(0) .PPS. 8 l 6 8 377 0.000 4.354 0.017
+liberation.rich 10.0.229.53 5 u 15 16 376 2.121 -4.407 0.527
+whodunit.richw. 10.0.229.114 4 u 15 64 376 2.142 -4.146 0.345
*iknow.richw.org 171.64.7.55 3 u 56 64 376 8.176 -4.002 1.650
Even with the baud rate increase, the GPS offset is still almost as big
as it was before.
And the difference between my local PPS's offset and that of one of my
systems (iknow) which is on our campus network and is synced to one of
our campus stratum-2 servers still seems significant, even after letting
my server run long enough that the jitter is really small.
In case it might help, here's the "rl 0" output for my test server:
associd=0 status=00f8 leap_none, sync_unspec, 15 events, no_sys_peer,
version="ntpd 4.2.5p...@1.1934-o Tue Jul 28 05:47:52 UTC 2009 (1)",
processor="i386", system="FreeBSD/7.2-RELEASE", leap=00, stratum=13,
precision=-19, rootdelay=0.000, rootdisp=5.000, refid=127.0.0.1,
reftime=ce1f0d49.d72897dd Sat, Aug 1 2009 11:51:53.840,
clock=ce1f0e0a.62d9013b Sat, Aug 1 2009 11:55:06.386, peer=0, tc=6,
mintc=3, offset=-4.244, frequency=91.117, sys_jitter=1.548,
clk_jitter=5.218, clk_wander=0.441, tai=34, leapsec=200901010000,
expire=200912280000
and a peer listing from my system iknow.richw.org (mentioned above):
remote refid st t when poll reach delay offset jitter
==============================================================================
liberation.rich 10.0.229.53 5 u 2 16 0 7.388 0.785 0.000
whodunit.richw. 10.0.229.114 4 u 70 256 376 6.857 -0.267 0.779
sixeyes.richw.o 127.0.0.1 13 u 19 64 377 8.129 3.979 1.684
+Avallone.Stanfo 171.64.7.87 2 u 249 1024 377 0.808 1.907 0.932
*caribou.Stanfor 171.64.7.87 2 u 284 1024 377 0.952 0.456 1.109
+dusk.Stanford.E 171.64.7.87 2 u 806 1024 377 0.816 1.740 0.897
and the result of "rl 0" from iknow.richw.org:
associd=0 status=0615 leap_none, sync_ntp, 1 event, clock_sync,
version="ntpd 4.2.5p...@1.1881-o Tue Jun 16 21:48:24 UTC 2009 (1)",
processor="i686", system="Linux/2.6.28-13-generic", leap=00, stratum=3,
precision=-21, rootdelay=1.303, rootdisp=70.961, refid=171.64.7.55,
reftime=ce1f0bc0.d5482987 Sat, Aug 1 2009 11:45:20.833,
clock=ce1f0f62.afaa1862 Sat, Aug 1 2009 12:00:50.686, peer=63920,
tc=10, mintc=3, offset=0.456, frequency=-120.794, sys_jitter=1.109,
clk_jitter=1.432, clk_wander=0.073, tai=34, leapsec=200901010000,
expire=200912280000
and a peer listing from our campus stratum-1 server (171.64.7.87,
grandfather.stanford.edu):
remote refid st t when poll reach delay offset jitter
==============================================================================
*TRUETIME(1) .GPS. 0 l 46 64 377 0.000 -1.435 1.402
+bigben.cac.wash .USNO. 1 u 480 1024 377 21.624 9.756 0.045
+clock.isc.org .GPS. 1 u 488 1024 377 4.736 3.071 0.122
and the result of "rl 0" from grandfather.stanford.edu:
associd=0 status=0444 leap_none, sync_uhf_radio, 4 events, freq_mode,
version="ntpd 4.2....@1.1585-o Sun May 10 16:52:30 UTC 2009 (1)",
processor="i686", system="Linux/2.6.18-6-686", leap=00, stratum=1,
precision=-20, rootdelay=0.000, rootdispersion=5.384, peer=11864,
refid=GPS, reftime=ce1f0d2b.007d3962 Sat, Aug 1 2009 11:51:23.001,
poll=10, clock=ce1f0d53.390640e8 Sat, Aug 1 2009 11:52:03.222,
state=4, offset=-1.871, frequency=91.237, jitter=1.968, noise=1.791,
stability=0.000, tai=0
--
Rich Wales / ri...@richw.org / ri...@stanford.edu
Wikipedia: http://en.wikipedia.org/wiki/User:Richwales
Facebook: http://www.facebook.com/richwales
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