[time-nuts] looking for good description/generalized model for time adjustments

J. Forster jfor at quik.com
Wed Jul 29 17:19:15 UTC 2009


Maybe let the clocks run unadjusted all day and resync between fixed times
once a day? This is what the power companies do. You can back-figure the
precise times if you really need to from the once a day correction files.

This assumes the clock rates are just off a bit, and do not randomly
fluctuate.

-John

===============



> I'm looking for a good (short) description and/or a generalized model for
> relating a local time counter to some external reference.  Here's the
> scenario..
> A computer has a local clock that is a counter being driven by a local
> oscillator.
> Periodically, we get "time updates" from some outside source that give an
> "absolute time" and a sync.
>
> We need to use "calibrated time" to record log files and schedule events.
>
> A simple way to relate local counter to absolute time is:
> Calibrated time = (local counter / counter rate estimate) + time offset.
>
> Calculating the rate estimate from (local counter at time B - local
> counter at time A) / (absolute time at time B - absolute time at time A)
> And offset in a similar way.
>
> But this has the problem that the "calibrated time" might have
> discontinuities.  Especially if you change the rate estimate and offset in
> a step function.  Calibrated time might even go backwards.
>
> I guess an additional requirement on "Calibrated time" is that it be
> continuous and monotonically increasing.  I can see that one can get this
> by "gradually" adjusting the rate estimate and offset (i.e. if your local
> clock runs fast so your local calibrated time has gotten ahead of absolute
> time, you set the rate lower than the true rate until it has slipped back
> fast and then slowly bring the rate back to the true rate)
>
> And this works pretty well if the outside reference is "better" than my
> local clock (and is essentially what NTP does, right?), but what if the
> reverse is true.
> Consider if my local clock is high quality, but I have to discipline
> Calibrated Time to an outside time that is of much poorer quality.  (By
> the gods, it's 6AM when the sun rises and 6PM when the sun sets, so make
> everything adjust to fit that, and don't give me grief about seasonal
> variations in sunrise/set times).
>
> What's a good way to do this, hopefully that allows later reconstruction
> of what the history of transformations was (e.g. I should be able to take
> my calibrated time, and convert it back into local counter time, by using
> the log of changes in model parameters).
>
> Thanks for ideas and pointers in advance
> Jim
>
>
>
>
>
> James Lux, P.E.
> Task Manager, SOMD Software Defined Radios
> Flight Communications Systems Section
> Jet Propulsion Laboratory
> 4800 Oak Grove Drive, Mail Stop 161-213
> Pasadena, CA, 91109
> +1(818)354-2075 phone
> +1(818)393-6875 fax
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