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Re: Non-GPS derived timing sources (was Re: NTp sources that work in a datacenter)
From: "N. Richard Solis" <nrsolis () aol net>
Date: Mon, 2 Jun 2003 10:22:55 -0400


You can go out and get yourself a nice Cesium source (like the ones in orbit) that will give you an accurate clock 
pulse with Stratum 1 quality.  Cost: about 45K.  Of course, this wont help you with Time Of Day information.  It will 
only keep you completely accurate in case of a loss of outside reference.

NTP should properly detect a loss of all outside sync sources.  In other words, if all the clocks in the world that you 
were syncing to all of a sudden said it was tommorrow NTP should properly disregard them as all being out of sync and 
refuse to sync to them.  If you try and start up NTP with your local clock too far out of sync, NTPD will not start up. 
 If they all go away or go out of sync after you have properly disciplined your LO, it should detect the fault and 
continue to provide accurate time but drift to the degree your local clock isn't of a Stratum 1 quality.  The net 
result is that you will stay accurate as long as your local reference oscillator is of Stratum 1 quality (or better).

As an aside, you can check the accuracy of your GPS signal by comparing it to a local Cesium.  That's how the 
government does it.  You'll only be able to tell if they agree or not though, not which one is right.

-Richard

 
Sean Donelan wrote:
 

On Sun, 1 Jun 2003, Marshall Eubanks wrote:
Every major time service and most national standards labs maintain a
set of clocks of comparable accuracy - US, UK, France, Germany, Russia,
Japan, Australia, etc., so there is no shortage of timing info to
compare it with.

Actually my question wasn't so much about other national standards labs,
but that almost every major Internet backbone worldwide seems to trace
their time source to GPS.  Maybe not that surprising for US/North American
providers, but even non-american backbones seem to use GPS.

To be clear, I'm not talking about individuals syncing things to lots of
different clocks.  Clock.ORG has lots of clock sources around the world.
I'm talking about what network providers use.

It was just one of those midnight projects a month or so ago, when I
noticed my carefully balanced selection of tickers had slowly over the
last few years all changed from other time sources to GPS.  Probably
not critical, but national standards labs have accidentily flipped
the wrong switch in the past and done strange things to their time
broadcasts. Yes, lots of people noticed, and it was fixed quickly.  NTP
has all this great logic for sanity checking time sources, but if they
all come from the same origin, what happens?





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