Finding location for brightest Iridium flare

Bruno Tilgner (
Sun, 19 Oct 1997 18:21:13 -0400

>One or two on this list have discussed how to find the location
>for the brighest flare of a particular Iridium pass, and have found
>that, in fact, just a few miles can make a large difference in magnitude=
>The idea is to enter different lat/longs into the programs and =

>check the results. This is, of course, rather long-winded.
>I just wonder if the problem could be tackled by the existing authors

The point where the sun light reflected by an Iridium mirror hits the
Earth's surface can indeed be calculated with a modification of the
algorithm kindly published by Randy John on his web page.

Essentially, the topocentric satellite position vector must be replaced
by the vector Sun -> satellite which is very easy to calculate.

The reflected vector (calculated in exactly the same way as the
topopocentric satellite position vector) may or may not intersect the
Earth's globe. This boils down to the mathematical problem of finding
the intersection between a line in space and a sphere.

People knowledgeable in 3D analytical geometry can probably solve it by
hand, I myself have done it with MATHEMATICA about a year ago for a
different purpose (finding the transit of a satellite in front on the Moo=
or the Sun).

The final step is to replace the sphere by the Earth's geoid. I have
developed a little algorithm for that which works by iteration.

The program, still under development, works quite well. For the time
being it gives the latitude/longitude as a function of time of the point
where the reflection angle would be zero. =

I would not attempt to calculate the visiual magnitude, at least for
the time being. The relationship between reflection angle and magnitude
is not well enough known. The data points on curves published so far have=
far too large scatter.

If someone is interested, I can give more details. For the sake of
all the readers on this list who are not intersted in this rather special=

subject, I would suggest to contact me privately.

Bruno Tilgner
Saint-Cloud, France