[ntp:questions] Maximum time2 fudge value for NMEA refclock?

Rhys user at mailinator.org
Thu Jul 22 13:11:44 UTC 2010

In article <mqilh7-0hi.ln1 at p4x2400c.home.lordynet.org>, 
snews at lordynet.org says...
> Rhys wrote:
> > In article <keqgh7-lgg.ln1 at p4x2400c.home.lordynet.org>, 
> > snews at lordynet.org says...
> >> Rhys wrote:
> >>> In article <i241ce$bu9$1 at news.eternal-september.org>, david-
> >>> taylor at blueyonder.co.uk.invalid says...
> >>>> "Rhys" <user at mailinator.org> wrote in message 
> >>>> news:MPG.26b018b8126a17a1989681 at news.optusnet.com.au...
> >>>> []
> >>>>> Sorry I should have said, GPS18x. Now that its stabilised its still 20-
> >>>>> 80ms positive out in ntpq, so either my offset didn't actually change,
> >>>>> or in the wrong direction.
> >>>> For NMEA alone, no PPS, you may get poorer performance then just with 
> >>>> LAN/Internet sync.  Just to remind me, you're somehow measuring one system 
> >>>> against another?
> >>>>
> >>>>> Seems a regular thing for it to jump crazy on startup, makes fine tuning
> >>>>> a little impossible. PPS takes 30 minutes i think to reappear, it might
> >>>>> be kernal PPSing in the background vs NTP trying to synch to NMEA.
> >>>> What do you mean "reappear"?  To be present on ntpq -p at all, or to have 
> >>>> the synced tag "o"?
> >>>>
> >>>>> I was thinking about turning on more NMEA sentences to see if it
> >>>>> stabilises NMEA, but getting to the Windows boot on that isn't easy now,
> >>>>> and this main PC has no serial.
> >>>> I would have thought that the fewer sentences the better, to be honest, 
> >>>> although I've seen one recommendation to use the sentences which include 
> >>>> the number of satellites locked (IIRC) so that you can get a measure of 
> >>>> how marginal GPS lock might be.
> >>>>
> >>>> My own tests showed that while a serial-to-USB converter made accuracy a 
> >>>> lot worse, it could still be better than a LAN connection alone.
> >>>>
> >>>>   http://www.satsignal.eu/ntp/NTP-on-Windows-serial-port.html#usb
> >>>>
> >>>> Cheers,
> >>>> David 
> >>> Using both 20 MMEA and 22 PPS references. PPS turned off on the NMEA, 
> >>> Kernel mode on the PPS. They seem to operate almost independantly. PPS 
> >>> has zero reach, with no 'when' time, for about 1/2 an hour, it may be 
> >>> waiting for NMEA to get under a certain threshold, or for NTP to stop 
> >>> thrashing around the 500 PPM mark and align. Its present, just not doing 
> >>> anything.
> >> That's what I see with separate gps and pps sources and
> >> drivers. PPS does nothing until GPS (or other prefer peer)
> >> is marked as selected. This can be an hour or more or never
> >> depending what fudge offset is set for gps although with
> >> 680ms fudge offset at moment, pps is selected and at reach
> >> 377 in under 30 minutes from starting ntpd.

Well its settled down ok now. Config NMEA offset is set around 600ms 
again, and using 'true' keyword for both as well. True on just NMEA 
causes PPS to drop out for being too far off. Noselects on all the lan 

Therefore I've got a config at last that can self align on an airgapped 
lan, thanks to those who posted up their offsets and the link showing 
the offset chart, helps a lot for this kinda stuff.

Now to figure out how to synch an LPRO...

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