<br><br><div class="gmail_quote">On Sat, Mar 28, 2009 at 12:30 PM, Keith Lofstrom <span dir="ltr"><<a href="mailto:keithl@kl-ic.com">keithl@kl-ic.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
Question: The TLE examples I saw seem to give mean motion in orbits per<br>
day (sidereal I assume); from that we can compute the semimajor axis,<br>
and with the eccentricity we can compute perigee and apogee. I don't<br>
see anything in the TLE specifying apogee or perigee directly. Those<br>
might pop out of one of the programs, perhaps as altitude, so the<br>
question is what the programs are assuming. The value used in the<br>
SGP4 fortran program on wikipedia is XKMPER = 6378.135 . To think, I<br>
used to write ugly fortran like that (now I write ugly C and ugly Perl).</blockquote><div><br>6378.135 km is the equatorial max, and I should be using that since the points<br>of interest are in the equatorial plane. <br>
<br></div><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<br>
Of course, the Earth is actually oblate, which causes highly inclined<br>
orbits to precess (that is how sun synchronous orbits work ) but this only<br>
shifts the angles of ascending and descending nodes, not their radii .<br>
Other interesting effects are tides from the sun and moon. For actually<br>
computing collisions, rather than their probabilities, much more sophisticated<br>
descriptions of orbits will be needed, and orbiting bodies will need to be<br>
characterized with size and shape and other ballistic parameters. The<br>
probabilities will tell us which subset of the 70 million will need to<br>
be closely scrutinized and real-time tracked for collision avoidance.<font color="#888888"><a href="http://lists.server-sky.com/mailman/listinfo/server-sky" target="_blank"></a></font></blockquote><div><br>I haven't scrutinized the library I'm using, but I believe several effects are factored<br>
in. Instances of the cOrbit class defined therein, constructed/initialized with a<br>TLE (three line format in the example and my app) recover most, if not all,<br>relevant data.<br><br>- tony<br></div></div><br>