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

Lux, James P (337C) james.p.lux at jpl.nasa.gov
Wed Jul 29 18:03:00 UTC 2009


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On Behalf Of J. Forster
Sent: Wednesday, July 29, 2009 10:19 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] looking for good description/generalized model for time adjustments

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

---

If it was just a matter of "post processing" the data to figure out from recorded times when things actually happened, it would be easy. You'd do everything in "local time", record the time updates you receive, timestamped with local time, and figure it all out later.  But we need to be able to "schedule" events to occur at predetermined times in the future, so you need some sort of clock model.

We also need better than once a day updates. A typical oscillator might be good to, say, 10ppm (variability over the short term), and we need "synchronization" to 1 millisecond.  So, in 100 seconds, you'd have drifted out of spec, if you assume a fixed "calibration." 

The real problem is that the "master" clock might be a crummy 50ppm XO, and all us "slaves" all have TCXOs that are good to a few ppm.  It's almost the inverse problem from disciplining a XO with GPS.

Jim



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



> 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.
>
s there.



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