[ntp:questions] Re: HowTo calibrate system clock frequency using NTP

David L. Mills mills at udel.edu
Fri Feb 3 14:57:49 UTC 2006


Daniel,

Yes.

Dave

Daniel Kabs wrote:

> Hello!
> 
> David L. Mills wrote:
> 
>> I'm not making sense with your mission. The measurements you qoute are 
>> quite reasonable in that they argree to within a fraction PPM. That's 
>> what I would expect. 
> 
> 
> You are referring to Plan B, I guess:
> My tests show that acquiring the time offset using either ntpdate 
> (remotely) or the offset value (locally) and then least-squares fitting 
> both data sets gives a slope that agrees within the sub-PPM regime.
> 
> 
>> However, what's with the 500 ms per day?
> 
> 
> That's the 6 PPM difference I get comparing the results from Plan A 
> (have ntpd record the drift file) and Plan B (use the time offset and do 
> a least-squares fit). I wonder if this is the expected precision in 
> measuring the system clock's frequency error.
> 
>> There is no such provision or expectation in the specification or 
>> implementation. The maximum frequency tolerance is 500 PPM, which 
>> works out to about 43 seconds per day. Your measurements about 23 
>> seconds per day are well within that tolerance.
> 
> 
> What's the "frequency tolerance" you are referring to? I reckon that's 
> the system clocks's maximum frequency error that ntpd can compensate.
> 
> 
> Cheers
> Daniel




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