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

David J Taylor david-taylor at blueyonder.co.uk.invalid
Tue Jul 20 13:50:49 UTC 2010

"Rhys" <user at mailinator.org> wrote in message 
news:MPG.26b04c33cb5a4ecb989682 at news.optusnet.com.au...
> 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.

Ah, I see what you mean.  With about 0.5s delay in your NMEA data, it may 
be that NTP is struggling to know which second to sync to.

> The number of satellites sounds like a useful one, although last time I
> did a 'cat gps1' it seems to drop out the NMEA to NTP, will have to
> retry. Refid PPSx/GPSx with x being number of sats anyone?

Someone mentioned that you can log the NMEA sentences with NTP, although I 
imagine you would only turn that on for debugging.

> My offsets for NMEA so far in this run (last 3 hours I think), most
> positive i saw was 20ms, most negative 80ms, so I'd put jitter of NMEA
> on GPS18x data at 100ms, if NTPs reading it correctly. WIth a fixed
> offset of 0.585, I'd be looking at 0.5-0.6 seconds. As it seems to have
> a cycle of min to max of a few hours, I'm not adjusting it till its had
> a few days, to find the new center.

So your new NMEA fudge would be -0.55s or about that?   You should check 
back, but I recall it times at the end of the NMEA sentence rather than 
the first character.

> If I get bored at work (electronic calibration lab and nothing to do), I
> might bring it into work, and run the GPS 18x like the link above
> through a few heat/cold cycles and a few power cycles, and see if theres
> any correlation in offset.

That would be interesting, and to correlate with the amount of computing 
which needs to be done.  More active satellites, more computing?  More 
active satellites, better fix, so less computing?  I don't know.

> Even if all this infos offtopic, its gonna be useful to anyone else
> trying the same thing in the future, because NTP is never predictable.

Erm .... it will be interesting to see the reaction to that remark, but 
useful to those who have a LAN for "time sync" and a PPS line for 
"precision sync", yes.


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