[ntp:questions] Seeking help configuring GPS refclock

Dave Hart davehart at gmail.com
Sat Aug 1 06:44:44 UTC 2009


On Sat, Aug 1, 2009 at 4:41 AM, Rich Wales<richw at richw.org> wrote:
> 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.
[...]
> I've configured the unit to utter $GPRMC and $GPGGA sentences at
> 4800 baud, with a 100-msec PPS signal.
[...]
> First, why is the GPS refclock's offset so big (-655.20)?

The GPS refclock is using end-of-line timestamps, and it takes about
300 msec at 480 octets per second to transmit the two sentences
(totaling ~143 octets including CR/LF), both of which are understood
by the NMEA refclock driver, resulting in the later EOL timestamp
being used each second.  If you configure the GPS to send only one
sentence, or increase the serial bitrate, the offsets should come
closer to zero.  Rather than change the GPS to send a single sentence,
you could configure the NMEA refclock to ignore all but the first one
it sends, in this case $GPMRC, using "mode 1" on the "server
127.127.20.1" line.

> 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?

The jitter figures you quoted were larger than the corresponding
offsets for the Stanford associations.  Wait for the jitter to be
substantially less than the offset, and consider the offset to have an
error band as wide as the jitter, before comparing the Stanford
offsets with the PPS.

Cheers,
Dave Hart



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