[ntp:questions] Seeking help configuring GPS refclock

Rich Wales richw at richw.org
Sat Aug 1 04:41:57 UTC 2009


I recently bought a Garmin GPS 18x LVC and am trying to set it up
as a reference clock, but I'm running into some strange results.

Specifically, the GPS time seems to be way off, and the PPS is
several milliseconds different from what I thought was a pretty
accurate local stratum-1 server.

I've hooked up the GPS signal lines as described here:

         http://www.satsignal.eu/ntp/FreeBSD-GPS-PPS.htm

(I'm pulling the required +5VDC power from a USB port.)

I've configured the unit to utter $GPRMC and $GPGGA sentences at
4800 baud, with a 100-msec PPS signal.

I'm using ntpd 4.2.5p195 on an 800-MHz Dell box running FreeBSD
7.2 (using a custom kernel with PPS support enabled).  This system
is really not doing anything else other than running ntpd; "top"
shows it to be 99+% idle, using no swap and almost no RAM.

The relevant lines from ntp.conf are as follows.  (For the moment,
I'm intentionally specifying a high stratum to prevent any host on
my LAN from syncing to this experimental system.)  The 10.0.229.*
hosts are on my LAN; they're all Ubuntu 9.04 (Jaunty) boxes running
ntpd 4.2.5p181 and syncing to Stanford University's NTP servers.

server 127.127.20.0             minpoll 4 maxpoll 4 version 4
fudge  127.127.20.0             refid GPS stratum 9
server 127.127.22.0             minpoll 4 maxpoll 4 version 4
fudge  127.127.22.0             refid PPS stratum 8
peer   10.0.229.29              minpoll 4 maxpoll 6 key 2 xleave
peer   10.0.229.53              minpoll 4 maxpoll 6 key 2 xleave
peer   10.0.229.114             minpoll 4 maxpoll 8 key 2 xleave

Now . . . here's some output from "ntpq -p" on my experimental box.
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
xGPS_NMEA(0)     .GPS.            9 l    5   16  377    0.000  -655.20   2.257
xPPS(0)          .PPS.            8 l    3   16  377    0.000    5.844   0.004
+liberation.rich 10.0.229.53      5 u   15   64  377    2.208   -0.652   0.769
+whodunit.richw. 10.0.229.114     4 u   16   64  377    2.219   -0.195   0.665
*iknow.richw.org 171.64.7.89      3 u   41  128  375    8.147    0.756   2.690

I'm confused about two things here.

First, why is the GPS refclock's offset so big (-655.20)?

And second, why is the PPS offset off by several milliseconds from
the Stanford-synced hosts?  Does this mean that Stanford's campus NTP
infrastructure is misconfigured and measurably off, and I'm right?
Or is it possible that I'm somehow set up wrong?

In case it might help, here is a bit of sample output from my GPS
(from a "cu" session).  If I'm interpreting this right, it looks
like I'm seeing 8 satellites and ought to be getting high accuracy.

$GPRMC,013257,A,3726.2063,N,12210.8019,W,000.0,000.0,010809,015.0,E*65
$GPGGA,013257,3726.2063,N,12210.8019,W,2,08,0.9,38.1,M,-32.4,M,,*45
$GPRMC,013258,A,3726.2063,N,12210.8019,W,000.0,000.0,010809,015.0,E*6A
$GPGGA,013258,3726.2063,N,12210.8019,W,2,08,0.9,38.0,M,-32.4,M,,*4B
$GPRMC,013259,A,3726.2063,N,12210.8019,W,000.0,000.0,010809,015.0,E*6B
$GPGGA,013259,3726.2063,N,12210.8019,W,2,08,0.9,38.0,M,-32.4,M,,*4A

Any thoughts?

-- 
Rich Wales  /  richw at richw.org  /  richw at stanford.edu
Wikipedia:  http://en.wikipedia.org/wiki/User:Richwales
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