[time-nuts] seeking a time/clock software architecture
magnus at rubidium.dyndns.org
Sat Sep 24 09:02:28 UTC 2011
On 24/09/11 00:21, Jim Lux wrote:
> On 9/23/11 2:24 PM, Chris Albertson wrote:
>> On Fri, Sep 23, 2011 at 12:32 PM, Jim Lux<jimlux at earthlink.net> wrote:
>>> On 9/23/11 10:50 AM, Chris Albertson wrote:
>>> Yes, in the general case, but in the spacecraft case, I think we're more
>>> concerned about smoothness and such over time spans of days, maybe
>>> weeks and
>>> More about establishing time correlation between multiple
>>> in a constellation, for instance.
>> I think better to have your system be usable in the "real world" and
>> then the spacecraft to use "real world" standards when it can. If it
>> can handle the ful general case then it will work in the spacecraft
>> too. So Chinese lunar calenders are a good mental exercise. At any
>> rate piece wise 2nd order polynomial will work in all cases I can
>> think of because you can always make the pieces really small if need
>> be to the point where it becomes a table look up.
> hmm.. 2nd order for time, or 2nd order for rate (3rd order for time). I
> keep thinking it would be nice to have the derivative of rate be
> continuous (although I confess I can't think of anything beyond gut feel
> for that). Maybe for all the "common cases" that's sufficient for a
> "predict into the future for a reasonable time"
>> Spacecraft spend a fair amount of time on the ground in testing.
>> People swap out parts. I work in telemetry and you should see the
>> database of tens of thousands of polynomial functions that must be
>> used to process data. from say a DeltaIV. It's not only clocks but
>> dozens of sensors that get changed out in the months preceding launch.
> Yes.. And there's no standard form that I've been able to discern for
> how those polynomials are specified. It's
> vehicle/spacecraft/instrument/software tool specific.
> So if you're writing a program to handle it automatically, you need to
> code up something special each time. These days, we get telemetry
> calibration in forms like .pdf files generated from a word document,
> plots from Matlab, Excel spreadsheets in some unique form, various and
> sundry import/export files from whatever program they're using to
> process telemetry, and so forth. There was an effort a few years back to
> try and standardize "mission data systems" but I don't know that it ever
> really worked. The cost to write those custom ingest routines is small
> in the context of a $150M mission every couple or three years.
> (maybe there is a standard for this.. I know there is a IEEE standard
> for sensor calibration data.. I should take a look at it again.)
Once you pulled data out of telemetry or whatever, putting it in XML
form according to a DTD fitting your needs should not be too hard, and
further processing should not take too much effort to extract data.
Essentially what RINEX does, but without the XML wrapper.
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