[ntp:questions] Isolated Network Drift Problem

Calvin Webster cwebster at ec.rr.com
Wed Nov 26 13:31:21 UTC 2008


On Tue, 2008-11-25 at 20:06 -0500, Richard B. Gilbert wrote:
[...]
> The most important thing to do or attempt is to get a primary source of 
> time.  This can be an atomic clock (extremely expensive, extremely 
> stable, extremely accurate), or something directly connected to an 
> atomic clock.  A GPS timing receiver will capture signals from four 
> satellites and solve a system of four equations in four unknowns: time, 
> latitude, longitude, and elevation.  Each of the satellites carries an 
> atomic clock!  The GPS timing receivers sell for ~$100 and up.  If you 
> can site such a receiver so that it has an unobstructed view of the sky, 
> this is a very good source of time.
> 
> If it is not possible for you to have your own stratum 1 time server, 
> there are a fair number of them on the internet.  Stratum 2 servers are 
> even more plentiful.  Stratum one servers have direct access to an 
> atomic clock or equivalent; e.g. GPS receiver.  Stratum two servers get 
> their time from stratum 1 servers.  Stratum 3 gets time from stratum 2. 
> . . .   If you can't use internet servers and you can't set up your own 
> stratum one server, you are SOL.
> 
> It is possible to "fake it" using a server that has a good clock but 
> performance will not be as good as having an atomic clock at the root of 
> the tree.  That rock solid beat of one second per second is easy to get 
> in step with and easy to keep step with.
> 
> An unsynchronized clock WILL drift and will be more difficult to get in 
> step with and keep step with.
> 
> I can tell you from experience that a GPS receiver will give you rock 
> solid time and that clients will synchronize easily with it.  So will an 
> atomic clock but that costs $50,000 or $100,000 more than the GPS receiver!

Thank you for the detailed explanation. I appreciate that. :-)

./Cal




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