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

[Ed W <lists@xxxxxxxxxxxxxx>]

On 16/03/2011 10:36, Damien R wrote:
> 
> On 15 March 2011 18:26, Bill Unruh <unruh@xxxxxxxxxxxxxx
> <mailto:unruh@xxxxxxxxxxxxxx>> wrote:
> 
>     Yes, if both are synchronized to gps, then both are synchronized.
>     However if
>     one looses the connection to the gps, then it has no way of getting
>     the time.
> 
> 
> Yes and when the gps connection is lost, I want to stay on local clock
> with PPS.

Well I think the point is that if you loose gps signal then you will
loose your PPS also?

Note that chrony (and ntp) will condition the internal clock so that
it's reasonably accurate.  Then if you loose GPS fix for short periods
of time, the internal clock will continue to track real time quite
effectively.

Note that if we are talking very high accuracy then the limit is that
the internal clock drifts as a function of a couple of things, the
largest being temperature.  So if the temp will vary while GPS fix is
lost then the drift will likely increase as temp drifts.  I believe
Miroslav discussed and may have included some very basic temp regression
analysis in the git chrony - if very high accuracy is required then this
would perhaps be something worth playing with (as would an external high
accuracy clock...)

However, if you just need roughly second accurate accuracy for periods
of a few hours of relatively stable temperature, with longish periods of
gps in between, then likely you will easily achieve this?

Also I think it's unlikely you will loose gps fix often/for long if the
units are outside and near a road?

Fun project - please write up your conclusion?

Good luck

Ed W

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