RE: How inaccurate are predictions made months in advance?

Bjoern Gimle (
Tue, 28 Apr 1998 06:30:41 +0200

>Regarding reliability of Iridium flare predictions:
>I took two elsets, the first April 4, the second April 16. By April 25th
>(3 weeks after the first elset) I noted:
>1. Times of maximum flares varied up to +/- 40 seconds  (no great deal)
>changing. Remember that the Sun moves its own diameter in the sky in 2
>minutes, and if in 3 weeks the timing error is already as much as 40
>'s easy to see why the predictions can change so quickly due

Looking at the situation from space, relative motion orbit plane/Sun is
about 1.5 degrees/day, slightly less at high latitudes. This is about
25 km/day at a typical Iridium range of 2000 km (or less), so essentially
the flares from a specific point in orbit hits "the same" spot on the globe
for several hours, ie all 11 satellites in one plane for a few orbits.

But the Earth rotates under this "fixed" setting, at about 0.27 km/s at
your latitude, faster near the equator. Since a flare patch is only about
25 km E-W while brighter than -4 mag, it takes only a 50 second change to
reduce it from -8 to -4 (less at lower latitudes)

(This is also the interval you can consider when you find a "spare" Iridium
is overtaking one of the operational ones every three-seven days)

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