How to calculate an IRIDIUM flare.

RJ@RMATSC.RIEM.COM
Fri, 10 Oct 1997 10:07:07 -0700

Don Baker, a writer for a quarterly publication about IRIDIUM asked the
following:

     One question I do have, for all who have been involved, is how the 
     structure of the satellites can be represented via coding in 
     flare-prediction programs. Can Rob or anyone else comment on that,
in a way 
     that will be understandable to an audience of non-programmers
(including 
     myself)? 

I thought the answer might be of general interest.

The short answer is "No, I can't explain it to non-programmers".  It's
terribly mathematical and difficult to follow.  In fact it is not
programming that you need to know, it's mathematics.  I just don't see
how it can be of interest to your readers.  OK, OK, let's give it a
shot.

o A ray of light skimming a mirror at glancing blow of (say) 1 degree
will bounce off at 1 degree.  That's just the definition of a
reflection.
o I know where the observer is with respect to the earth (latitude,
longitude and the altitude above sea-level).
o I know where the spacecraft is with respect to the earth (from the
orbital elements).
o I know the attitude of the spacecraft with respect to the earth
(because Paul Maley asked the Iridium folks).
o I know the orientation of each MMA (Main Mission Antennas, the
mirrors) to the spacecraft (from Paul).
o I now know the orientation of each MMA to the observer (this is the
part that I have to do and it's hard).
o Now the question is:  The observer is looking at a mirror (MMA), what
point in the sky does he see?  (A little tricky but basically just the
definition of a reflection).
o How far is that point from the Sun (just the distance between two
points in the sky).

Well, that was easier than I thought.

Your question implies a belief that somehow there is a little tiny
spacecraft in my program.  I suppose that's the only way to interpret a
statement like "I modeled the spacecraft in software".  The only thing
that is modeled is the direction in which each MMA is currently pointed.
A 'direction' is called a 'vector' in mathematics.  So the rest is just
the manipulation of vectors which is fairly ordinary mathematics (OK,
it's a little advanced).

			Randy