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

Terje Mathisen terje.mathisen at hda.hydro.com
Thu Feb 2 21:36:14 UTC 2006

Daniel Kabs wrote:
> Hello Terje!
> Terje Mathisen wrote:
>> 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.
> Did I understand you correctly: You are insinuating that least-squares 
> fitting the time offset is getting an average value whereas the 
> frequency error (ntp.drift value) represents a "live" value.
> I expected it to be the other way round: I thought the frequency error 
> is a "slow" value that takes hours or days to converge as a result of 
> the control loop phasing in and as such can only slowly react to 
> environmental changes (e.g. change in temperature). This contrasts to 
> measuring the time offset over a short periode which gives a "snapshot" 
> of the current clock drift and as such represents current environmental 
> effects.
> What am I getting wrong here?

Afaik ntp.drift normally carries about an hour's worth of history.

It is rewritten every hour.


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

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