[Server-sky] sanity check, please: orbit intersections

[Tony Rick]

On Sat, Mar 28, 2009 at 12:30 PM, Keith Lofstrom <keithl at kl-ic.com> wrote:

> Question:  The TLE examples I saw seem to give mean motion in orbits per
> day (sidereal I assume);  from that we can compute the semimajor axis,
> and with the eccentricity we can compute perigee and apogee.  I don't
> see anything in the TLE specifying apogee or perigee directly.  Those
> might pop out of one of the programs, perhaps as altitude, so the
> question is what the programs are assuming.  The value used in the
> SGP4 fortran program on wikipedia is  XKMPER = 6378.135 .   To think, I
> used to write ugly fortran like that (now I write ugly C and ugly Perl).


6378.135 km is the equatorial max, and I should be using that since  the
points
of interest are in the equatorial plane.


> Of course, the Earth is actually oblate, which causes highly inclined
> orbits to precess (that is how sun synchronous orbits work ) but this only
> shifts the angles of ascending and descending nodes, not their radii .
> Other interesting effects are tides from the sun and moon.  For actually
> computing collisions, rather than their probabilities, much more
> sophisticated
> descriptions of orbits will be needed, and orbiting bodies will need to be
> characterized with size and shape and other ballistic parameters.  The
> probabilities will tell us which subset of the 70 million will need to
> be closely scrutinized and real-time tracked for collision avoidance.<http://lists.server-sky.com/mailman/listinfo/server-sky>


I haven't scrutinized the library I'm using, but I believe several effects
are factored
in.  Instances of the cOrbit class defined therein, constructed/initialized
with a
TLE (three line format in the example and my app)  recover most, if not all,
relevant data.

- tony
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