[ntp:questions] prefer in ntp.conf

Paul paul at lavender-fam.net
Thu Jun 14 14:17:31 UTC 2007

I couldn't find your post on the prefer - it sounds exactly what I am
finding. Incidently the mnea patch does listen to the nmea data (so you
could have a totally free standing ntp system. Unfortunately simple GPS
receivers don't have the capability you mention, hence the need for
sanity checks.


 On Wed, 2007-06-13 at 14:25 -0500, Jason Rabel wrote:
> Paul,
> What GPS are you using? What OS? What does your ntpq -p data look like? 
> If you get a receiver with TRAIM, then you can have it disable the PPS
> output when it detects itself getting out of sync, either by bad data or no
> lock. Otherwise most GPSes will just keep pulsing away none-the-wiser
> (except for their NMEA data which NTP doesn't check against).
> You *could* write extra code in the driver to check various NMEA sentences
> (which why it hasn't been done I'm not sure) to make sure it has a proper
> lock.
> Or as someone else mentioned, ensure that your antenna has a proper view of
> the sky so you do not loose lock.
> I wrote a post just a day or two ago about the PREFER source and how if you
> have several internet sources they can out weigh your local (more accurate)
> source and thus discard it if you do not have the PREFER flag set. So it can
> be a potential issue.
> Jason
> >I am using a GPS nmea/pps source (lets not go into the heart-ache
> >involved in getting that working), and a very recent download of ntp
> >source, with the nmea patch. Obviously I have a few internet sources as
> >well, but use the prefer keyword for the pps source. Obviously when it
> >is working the pps is about as accurate as I can get. However when I run
> >ntpq -p it isn't quite doing as I want.
> >I want to lock exactly to that pps source WHILE IT IS SENSIBLE, and to
> >drop back to the internet when it has lost satellites. However ntp
> >appears to be taking an average, loaded towards the GPS source. When the
> >GPS looses satellites and goes beserk ntp sensibly drops it as a source.
> >Your suggestions would be welcomed.
> >
> >Paul
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