[ntp:questions] Garmin GPS18x LVC 5m - Centos 5.5 - Kernel 2.6.18 - gpsd

David J Taylor david-taylor at blueyonder.co.uk.invalid
Tue Dec 28 15:00:26 UTC 2010


"David Lord" <snews at lordynet.org> wrote in message 
news:6utou7-ut8.ln1 at p4x2400c.home.lordynet.org...
[]
> I think it's dependent on the ntpd version as to best
> settings. I was using "fudge 127.127.20.0 time1 0.650"
> and checking stats to make sure NMEA offset remained
> low. That was with ntpd 4.2.4p6-o.

Thanks, David, much appreciated.

As I never needed fudge before, why should I suddenly start to need it? 
And 0.650s sounds rather larger than the measurements would suggest.

  http://www.dangl.at/ausruest/gps18lvc_e.htm
  http://www.dangl.at/ausruest/gps_osz21.gif

(or is the timing from the end of the NMEA, not the beginning?).  As the 
offset showed both +1s, 0 and -1s I'm not 100% convinced that the GPS puck 
was working correctly.  Removing the NMEA and relying just on the type 22 
PPS/ATOM driver seems to have fixed the problem, although it's not my 
preferred solution.


> Other versions of ntpd handle fudge factors in different
> (better?) ways. I'm moving to ntpd 4.2.7p98 and fairly
> sure method for setting up is different.

The PC was running:  4.2.5p181-o, but is now running ntpd 4.2.7p97-o. 
Since I haven't fudged, I haven't compared the methods.  Changing ntpd 
versions didn't seem to affect the problem.


>> As to PPS offset, I can't measure it accurately here but it appears to 
>> be under a millisecond, and perhaps under 100 microseconds.
>
> On NetBSD I've had periods when rms offset as from
> peer_summary, has remained below  10us for many days.
> Mean offset is usually near 0.000ms. I don't have
> reliable reception so would  expect lower offset with
> a higher aerial position.
>
>
> David

I can't compare the /real/ offset, but the values reported by ntpq -p 
typically show well under 7 microseconds, with the largest transients when 
the heating starts in the morning!

  http://www.satsignal.eu/mrtg/pixie_ntp.html

Cheers,
David 




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