XPost: sci.space.tech, sci.space.history
From: henry@spsystems.net
In article <9186edb5.0310052108.54744457@posting.google.com>,
Vincent Cate wrote:
>I found some real space atmospheric models like...
>But these need far more detail to run than I want to deal with.
>For example the last URL above says:
> > The model expects as input year, day of year, Universal Time,
> > altitude, geodetic latitude and longitude, local apparent solar time,
> > solar F10.7 flux (for previous day and three-month average), and
> > magnetic Ap index (daily or Ap history for the last 59 hours).
Unfortunately, there are *reasons* for this. The upper atmosphere is
extremely variable -- with time of day and *most especially* with solar
activity. No simple, constant model can describe it well.
And yes, this means that orbital lifetimes are also highly variable.
Predictions of Skylab's orbital life disagreed by years, largely because
they made different assumptions about solar activity.
The discussion in section 8.6.2 of Vallado's "Fundamentals of Astrodynamics
and Applications", 2nd ed, is the best readily-accessible one that I'm
aware of.
>Does anyone know a simple atmospheric model that just takes an
>altitude as an input but would be closer to reality than what I have
>now?
Vallado gives a piecewise-exponential model that varies scale height with
altitude (using a small table, it's not a single formula), and says it's
suitable for general studies. (Ah, and he says he swiped it from Wertz's
"Spacecraft Attitude Determination and Control", so that's two books to
look for.)
In his appendix B, he spends ten pages giving a more precise model.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! | henry@spsystems.net
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