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

[Ed W <lists@xxxxxxxxxxxxxx>]

Hi


On 05/04/2011 12:44, Miroslav Lichvar wrote:
> 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

This is really very interesting - many thanks for sharing!



>>>> (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.

Just to be clear - I think the GPS modules are quite accurate as
standard, but the 20Hz version is one model, the version with highly
accurate PPS is a separate model and not known to put out a 20Hz PPS signal.

However, I agree that in principle given that the largest error should
be the pulse edge detection (and random measuring delay) then you
potentially have yourself an error which is largely gaussian and
additional datapoints are beneficial

Bill, is the GPS sawtooth not at a level of around the 10s of ns?

Cheers

Ed W

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