[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
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.
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,
> David L. Mills wrote:
>> 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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