Daniel,
Beam me sideways, Scotty.
I'm not making sense with your mission. The measurements you qoute are
quite reasonable in that they argree to within a fraction PPM. That's
what I would expect. However, what's with the 500 ms per day? There is
no such provision or expectation in the specification or implementation.
The maximum frequency tolerance is 500 PPM, which works out to about 43
seconds per day. Your measurements about 23 seconds per day are well
within that tolerance.
Dave
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
David L. Mills wrote:
Daniel,
Plan A
1. Run ntptime -f 0 to remove any leftover kernel bias.
2. Configure for a reliable server over a quiet network link.
3. Remove the frequency file ntp.drift.
4. Start the daemon and wait for at least 15 minutes until the state
shows 4. Record the frequency offset shown with the ntpq rv command.
It should be within 1 PPM of the actual frequency offset. For enhanced
confidence, wait until the first frequency file update after one hour
or so.
Plan B
1. Run ntptime -f 0 to remove any leftover kernel bias.
2. Configure for a reliable server over a quiet network link.
3. Start the daemon with disable ntp in the configuration file.
4. Record the offset over a period of hours. Do a least-squares fit;
the regression line slope is the frequency.
Dave
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