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

Terje Mathisen terje.mathisen at hda.hydro.com
Wed Feb 1 19:57:33 UTC 2006

Daniel Kabs wrote:
> Hello Professor Mills,
> I tried both of your suggestions and the results differ slightly:
> Plan A)
> After running NTP daemon for two days, the frequency converges to 268.3 
> PPM, i.e. 23.2 seconds per day.
> Plan B)
> Running NTP daemon using "disable ntp", I recorded the offset of the 
> associated peer periodically for a couple of hours. A least-squares fit 
> gave a slope of 23.7 seconds per day. (At the same time I recorded the 
> offset using deprecated ntpdate and got 23.8 seconds per day).

This is well within the expected precision for such an experiment, the 
final ntp.drift value (23.2 or 268.3) probably reflects the current 
drift rate, not the average.

These two values are different because the environmental temperature 
varies, often diurnally, so if you log the changes in ntp.drift then 
you'll probably notice that the average corresponds closely to the 
23.7/23.8 numbers.

- <Terje.Mathisen at hda.hydro.com>
"almost all programming can be viewed as an exercise in caching"

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