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

[Kenyon Ralph]

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
offset from most other servers. This is clearly evident in its pool
graph:
http://www.pool.ntp.org/scores/2001:470:8683:0:219:b9ff:fe7d:e6e0

The offset seems to correspond to the PLL offset, which I have munin
graphing:
http://kenyonralph.com/munin/kenyonralph.com/gauss.kenyonralph.com/ntp_kernel_pll_off.html

Most other systems [0] (including my own) seem to hover around zero,
but this one with the GPS hovers around -2 ms.

Any ideas on how I could correct this? Could this just be a case of
bad computer hardware? It's a Dell Latitude D520, which has a serial
port. It seems to be very consistent and stable, just offset! Thanks.

$ cat /etc/ntp.conf
statsdir /var/log/ntp
statistics clockstats loopstats peerstats

leapfile /etc/ntp/leap-seconds.list

server 127.127.20.0 minpoll 3 iburst
fudge 127.127.20.0 flag1 1 flag2 0 flag3 1 time2 0.545

server darwin.kenyonralph.com iburst
server voodoo.kenyonralph.com iburst
server grunt.kenyonralph.com iburst

# stratum 1
server clock.fmt.he.net iburst
server tick.ucla.edu iburst
server time.xmission.com iburst
server ntp.exact-time.org iburst
server ntp.your.org iburst

# stratum 2
server clock.psu.edu iburst
server ntp2.jtsage.com iburst
server tick.mattnordhoff.com iburst

$ ntpq -p -c clockvar -c readvar -c kerninfo
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
oGPS_NMEA(0)     .GPS.            0 l    7    8  377    0.000    0.000   0.002
+darwin.kenyonra 127.67.113.92    2 u   55   64  377   20.557    1.124   2.819
 voodoo.kenyonra 94.78.119.192    2 u   63   64  377    0.283    0.870   0.062
 grunt.kenyonral 94.78.119.192    2 u   15   64  377    0.175    0.512   0.029
*clock.fmt.he.ne .CDMA.           1 u   27   64  377   21.668    1.524   3.522
+tick.ucla.edu   .GPS.            1 u   48   64  377   31.787    2.096   2.587
+2607:fa18::2407 .GPS.            1 u   18   64  377   42.634    2.584   2.715
+ntp.exact-time. .GPS.            1 u   55   64  377  201.559    2.013   2.477
+ntp.your.org    .CDMA.           1 u   65   64  377   72.936    1.430   2.342
-otc2.psu.edu    147.84.59.145    2 u   63   64  377  104.272    3.331   1.490
-ntp2.jtsage.com 127.67.113.92    2 u   18   64  377   60.644   -0.267   3.316
+tick.e.mattnord 129.7.1.66       2 u   56   64  377   61.508    0.707   3.306

associd=0 status=0000 no events, clk_unspec,
device="NMEA GPS Clock",
timecode="$GPGGA,221921,3259.1559,N,11714.2761,W,1,08,1.1,101.0,M,-35.3,M,,*7E",
poll=75537, noreply=0, badformat=0, baddata=0, fudgetime2=545.000,
stratum=0, refid=GPS, flags=5

associd=0 status=0419 leap_none, sync_uhf_radio, 1 event, leap_armed,
version="ntpd 4.2.7p255 at 1.2483-o Sat Feb  4 07:17:31 UTC 2012 (1)",
processor="amd64", system="FreeBSD/9.0-STABLE", leap=00, stratum=1,
precision=-19, rootdelay=0.000, rootdisp=1.105, refid=GPS,
reftime=d318c6e1.cdf93c61  Sat, Mar 24 2012 15:19:13.804,
clock=d318c6e9.9752b956  Sat, Mar 24 2012 15:19:21.591, peer=6248, tc=3,
mintc=3, offset=-0.000, frequency=-4.243, sys_jitter=0.002,
clk_jitter=0.001, clk_wander=0.000, tai=34, leapsec=201207010000,
expire=201212280000

associd=0 status=0419 leap_none, sync_uhf_radio, 1 event, leap_armed,
pll offset:            -0.000352
pll frequency:         -4.24277
maximum error:         0.005
estimated error:       0
kernel status:         pll ppsfreq ppstime ppssignal nano
pll time constant:     3
precision:             1e-06
frequency tolerance:   495.911
pps frequency:         -4.24277
pps stability:         0.00227356
pps jitter:            0.001
calibration interval   256
calibration cycles:    18526
jitter exceeded:       75927
stability exceeded:    0
calibration errors:    9

[0] https://www.google.com/search?q=munin%20NTP%20kernel%20PLL%20offset

-- 
Kenyon Ralph
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