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

Richard B. Gilbert rgilbert88 at comcast.net
Thu Feb 2 00:25:50 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).
> Please see the diagrams on
> https://ntp.isc.org/bin/view/Support/HowToCalibrateSystemClockUsingNTPDev
> I wonder if this difference shows the maximum precision (i.e. 500 
> ms/day) I will achieve with these calibration procedures or if I'm 
> doing something systematically wrong.
> Cheers
> Daniel
What problem are you trying to solve?

If you want to make a one-time correction to your clock frequency, 500 
ms/day may be a reasonable objective.   As Terje pointed out, the 
frequency varies with temperature and the temperature varies with the 
time of day, season of the year, whether the heat is on or off, etc.  
The frequency will also change, slowly, as the crystal ages.

Whatever you set the frequency to, today, will probably be not quite 
right for tomorrow, next week, etc.

This is why we run ntpd on our computers to synchronize our clocks to an 
atomic clock somewhere.   The atomic clock is several orders of 
magnitude better than the undisciplined local clock and ntpd can 
generally hold your local clock within +/- 20 milliseconds of the 
correct time using servers on the internet.   With a hardware reference 
clock (GPS timing receiver) and a judicious choice of hardware and 
operating system it is possible to hold the local clock within, perhaps, 
+/-50 microseconds of the correct time.

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