[ntp:questions] Seeking help configuring GPS refclock

Rich Wales richw at richw.org
Sat Aug 1 07:48:44 UTC 2009


Dave Hart wrote:

> The GPS refclock is using end-of-line timestamps . . . .  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.

Thanks.  I've done this, and the "clockstats" file shows it is now using
the $GPRMC sentences.  However, the offset is basically unchanged --
still around -670.

How do I change the baud rate on this device?  I tried a $PGRMC command
with the tenth parameter equal to 5 (19200 baud), but it had no effect.
I also tried connecting (via "cu") at different baud rates, and issuing
commands (thinking it might autodetect the speed of received data), but
that also didn't change anything.

> 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.

OK, I'll wait and see what it's looking like in the next day or two.

-- 
Rich Wales  /  richw at richw.org  /  richw at stanford.edu
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