Re: [chrony-dev] adjusting frequency on the basis of temperature

[Bill Unruh <unruh@xxxxxxxxxxxxxx>]

On Wed, 15 Dec 2010, Ed W wrote:

> I'm also kind of surprised, but heartened that you manage to get a stable 
> enough ping speed to use chrony over it!  I can only assume that this is 
> because the number of satellite hops remains fairly fixed between your 
> location and Iridium?  If you look at the way Iridium routes most calls you 
> can see that the routing distance can change quite considerably over a few 
> tens of minutes - certainly I observe the ping speeds to be fairly variable 
> (I'm in the UK)

The main question is the symmetry of the data. Ie, does the data going out to
the ntp server follow the same path and the data going back. If the trip is
symmetric, it does not matter if the trip now takes 2 sec and the next one
takes .1 sec. However, if the two legs are highly assymmetric and variable,
then the problem is more accute. chrony probably does as well as possible with
this situation, but it will not be a highly accurate time.

So, I have a local GPS, a server 40ms away and another 4ms away. I get far more
accurate time from the one that 40ms away than from the one that is only 4ms
away. Not sure why but the assymetry of the signals to the nearby one seems to
be much worse than to the distant one. ( the GPS is of course the standard by
which I can say this).


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