[ntp:questions] Problem with Trimble Placer 450 - gpsd - ntpd
bdale at stny.rr.com
Wed Dec 29 15:07:01 UTC 2010
I'm new to this list, if this subject belongs in a different list,
please let me know.
I have a Trimble Placer 450 GPS receiver generating a PPS signal and
NMEA sentences to gpsd (2.95), which passes that information to ntpd
(4.2.6p2) over the shared memory interface (reference clock driver type 28).
When the Placer 450 has a consistent 3D fix on several satellites,
ntpd selects SHM(1), the PPS signal, as the syspeer. No problem.
The problems occur when the Placer 450 loses satellite fix. In my
environment, this happens multiple times a day.
The Placer 450 does not report loss of satellite fix as expected in
the NMEA sentences. I have been working with the gpsd folks to work
around this problem and they are happy to do so.
The time server is a system consisting of the Trimble GPS receiver,
gpsd and ntpd. The gpsd folks are happy to implement a fix to the
problems with the Placer 450, but how are problems between gpsd and
ntpd resolved? Specifically, how should a GPS receiver report a loss
of satellite fix to ntpd over the shared memory interface? Should
ntpd ignore the PPS during the interval of no fix? And what is the
correct behavior of ntpd when this happens?
I have been experimenting and have found that inhibiting the PPS
signal during the interval of no fix minimizes the number of
reference clock switch overs. This also minimizes the perturbations
in the loopstats time and frequency offset.
Thanks in advance,
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