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

[Tony Rick]

Can anybody check my approach here?

parameters:
    omega = argument of periapsis (clockwise angle from line of ascending
node to semi-major axis)
    a = semi-major axis
    e = eccentricity
    ta = true anomaly (counter-clockwise angle from semi-major axis to
object position on orbit)

- omega is provided in TLE (as argument of perigee)
- a is recovered by norad lib (cOrbit instance)
- e is provided in TLE

Assuming that the true anomaly of interest is on the line of the ascending
node,
    ta = 360 - omega

Earth center is the primary focus of the orbit ellipse.

Radius from a focus to a point on the ellipse is given by
    r = (a * (1 - e^2)) / (1 + (e * cos(ta)))

Critical region is 12289  >= r >= 13289

Applying this to Keith's 'interesting' TLE data yields no critical
intersections.

Side note: norad lib appears to provide orbital distance from surface of
Earth instead of
center in some cases.  Perigees of sample TLEs are reported as in the range
200 - 5000 km.
I'm adding mean Earth radius 6371km, but I'm not sure I have this right.

- tony
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