[ntp:questions] Seeking help configuring GPS refclock

Rich Wales richw at richw.org
Sat Aug 1 19:03:26 UTC 2009


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 4.2.5p195 at 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 4.2.5p181 at 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 4.2.2p4 at 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  /  richw at richw.org  /  richw at stanford.edu
Wikipedia:  http://en.wikipedia.org/wiki/User:Richwales
Facebook:   http://www.facebook.com/richwales



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