Re: [chrony-users] gpsd, pps and chrony

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On Mon, Apr 04, 2011 at 09:25:01PM +0100, Ed W wrote:
> > Since an NMEA ntp client is a stratum 1 and since NMEA has an accuracy
> > around
> > 10ms (yes, ms, not us) there is no such requirement. Ie, the accuracy of
> > stratum 1 clocks varies wildly.
> By implication are you suggesting that you expect a decent proportion of
> current stratum 1 servers are only synced to within some
> few ms of "correct"?  Seems a bit sad?

I recently did some statistics on historical data kindly provided by
the maintainer. From the offsets collected by the
monitoring server it seems that most of the pool servers are 1-10ms
off. This includes all strata, as stratum is not stored in the
database. Also, results for servers farther from the monitoring server
may be negatively affected by larger delay.

> >> (They also have a 20hz GPS with decent sensitivity and a GPS/GLONASS
> >> hybrid board...)
> > 
> > And exactly what would a 20Hz buy you?

20 times more samples could improve the stddev of the calculated mean up to
factor sqrt(20). It would also allow shorter polling interval.

Miroslav Lichvar

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