[ntp:questions] Single best NTP status indicator of clock accuracy?

unruh unruh at wormhole.physics.ubc.ca
Fri Dec 11 05:39:40 UTC 2009


On 2009-12-11, E-Mail Sent to this address will be added to the BlackLists <Null at BlackList.Anitech-Systems.invalid> wrote:
> unruh wrote:
>> On 2009-12-10, kjans <kert.jans at gmail.com> wrote:
>>>   I am looking for the best single indicator that can be used to
>>>   determine/monitor the current clock accuracy.
>> Offset. That is ntp's best guess as to the difference
>>  between the time on your clock and the true UTC time.
>
> AFAICT
>
> ntpq -c rv gets the system variables, among them is a combined
>  offset (ms) from the selected truechimer peers reference times,
>  (or the offset from the preferred peer).
>  e.g. ntpq -c rv ... offset=-5.515 ...
>
> ntpdc -c loopinfo will also get that offset in a different format.
>  e.g. ntpdc -c loopinfo offset:               -0.005515 s ...
>
>
> The Root Dispersion to the primary reference clock (stratum 1),
>  would be "the difference between the time on your clock and
>  the true UTC time" (ms)
>  e.g. ntpq -c rv ... rootdisp=56.621 ...
>   {with GPS/PPS, root dispersion will likely be 5. or less.}

The root dispersion is an estimate of the error in the remote clock's time. It is not an
estimate of the difference in time between your clock and the true time.
Add to the root dispersion half the round trip time to get the max error
in your clock. 

The best estimate of the difference between your clock time and the true
time is the offset. It is designed to be the best estimate. The max
error in that time is given by the root dispersion of the soruce plus half the offset. 


>
>
><http://alumni.media.mit.edu/~nelson/research/ntp-survey99/html/>
> "dispersion of that root time: these measurements are useful
>   for determining the final accuracy of a host's clock"
>




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