[ntp:questions] Re: Reading time offset from ntp variables using ntpq

David L. Mills mills at udel.edu
Fri Feb 17 22:02:25 UTC 2006


Daniel,

You might get a better understanding of the error model from the 
briefings at the NTP project page. The statistic of interest is the 
Allan variance, which describes the clock frequency stability as a 
function of averaging time.

Dave

Daniel Kabs wrote:

> Hello Professor Mills!
> 
> Apropos engineers: any engineer strives for high precision measurements 
> and thus tries to minimize uncertainty to a reasonable level. So for any 
> method of measurement the magnitude of uncertainty has to be determined 
> and valued. That's what I am trying to do currently :-)
> 
> Enter Plan B: You suggested to "record the time of day and the offset", 
> without saying how you'd record the time. So I tried different methods 
> (for "recoding the time") and found
> 
> cron   :  23.40 s = 270.83 PPM
> reftime:  23.35 s = 270.25 PPM
> org    :  23.30 s = 269.67 PPM
> rec    :  23.29 s = 269.56 PPM
> 
> Mh, maybe I should have converted the values to PPMs earlier! They look 
> much nicer now. Leaving out the "cron job", they agree within +/- 0.5 
> PPM. Of course, this should be backed by repeating the "experiment" and 
> checking how the values scatter (disperse? I'm don't know the proper 
> English word here).
> 
> You are right, this uncertainty is reasonable. So I should stop bugging 
> you about timestamps :-)
> 
> Mission accomplished! Thanks.
> Bye now,
> Daniel
> 
> David L. Mills wrote:
> 
>> Daniel,
>>
>> Have you considered what an engineer means by "almost exact?" There is 
>> always an uncertainty in any physical measurement. Yours is no exception.




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