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

[Miroslav Lichvar <mlichvar@xxxxxxxxxx>]

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 pool.ntp.org 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 pool.ntp.org 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.

http://i.imgur.com/5Oaoa.png

> >> (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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