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

Hal Murray hal-usenet at ip-64-139-1-69.sjc.megapath.net
Thu Dec 30 18:53:08 UTC 2010

In article <201012291506.oBTF6xa5021273 at heartache.daihls.com>,
 Bruce Dale <bdale at stny.rr.com> writes:

>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 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?

If the NMEA signal says no-good, gpsd should ignore the PPS pulses.
It should probably wait for several seconds of good NMEA messages
before turning on PPS.

There isn't any way for gpsd to say clock-died via the SHM interface.
All it can say is here-is-data.  ntpd should timeout if gpsd stops
feeding it data.  It may take 2 minutes.  If that's not good enough
we can look into tweaking the SHM interface.

These are my opinions, not necessarily my employer's.  I hate spam.

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