[ntp:questions] Problem with Trimble Placer 450 - gpsd - ntpd

Bruce Dale 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.

Problem #1:

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.

Problem #2:

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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