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

Daniel Kabs daniel.kabs at gmx.de
Fri Feb 3 09:11:32 UTC 2006


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.


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