[ntp:questions] Isolated Network Drift Problem

Cal Webster cwebster at ec.rr.com
Tue Nov 25 17:20:46 UTC 2008



On Tue, 2008-11-25 at 09:43 -0500, Richard B. Gilbert wrote:
[...]
> > What's the best way to determine which of our NTP servers provides the
> > best local clock?
> 
> Note the difference between each clock under test and the correct time.
> Wait seven to ten days and compare these clocks again.  The clock with 
> the least change in the original offset is the "best clock".  Note that 
> this is not fool proof; you might find a situation in which the "best" 
> clock varies wildly from hour to hour and is only the best "on average".
> 
> A more adequate test would be to set up a server with a GPS receiver and 
> compare the offset of all the local clocks under test every thirty 
> minutes over a period of several days.  But, if you are going to do 
> that, you might as well make it permanent and use it for your master clock!
> 
> Consider that the Garmin GPS18LVC has a pulse per second output and 
> costs less than $100 US.  If you can site an antenna with a good view of 
> the sky, you can have a stratum 1 server of your very own and have the 
> time accurate to within a millisecond or less.  Note that while the GPS 
> is accurate to 50ns or better the process of getting the time into your 
> computer may introduce several hundred microseconds of uncertainty.

Interesting... We'd have to run a long cable though, the computer lab
has no windows and the roof is the most logical place to put an antenna.
I don't think USB will handle that much line loss. It's a single story
section of the building so we're probably talking about 100 meters or so
to get a view of the sky. I'll need a version that can handle the remote
distance. I'll request a purchase but it could be months before we see
it. 

After a bit of googling I found an excellent write-up on how to use one
of these for an NTP server [http://time.qnan.org/ "Using a Garmin GPS 18
LVC as NTP stratum-0 on Linux 2.6"]

Thanks Richard!

./Cal




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