Iridium Flare prediction pr

ROB MATSON (ROBERT.D.MATSON@cpmx.saic.com)
23 Sep 1997 14:28:31 -0800

Greetings All,

I've been following the Iridium flare discussion from the beginning, but have
been too busy with other "pet projects" to contribute anything until now.  I
think everyone would agree that the primary purpose/goal of everyone's efforts
to report flare sightings is to be able to predict when and where they will
occur in the future.  These flares are caused by specular reflection, of
course, so each flare observation actually reveals the direction of the
surface normal of the responsible planar reflecting surface ~at the time of
the flare~.  Now, if each Iridium satellite has an orientation that behaves in
a predictable way, then it becomes possible to predict the times/locations of
future flares.

I partially solved this problem for the Superbird A satellite.  A solution in
this case was possible because the 23.5-second period spin of the satellite
serves to inertially fix the axis of rotation (like a gyroscope). 
Unfortunately, gyroscopes precess, and Superbird A is no exception, but the
precession rate is slow on this satellite which makes it possible to predict
when/where it will flash, even weeks in advance.

But to my knowledge, the Iridium satellites are not spinning.  I suspect that
in order to "do their job", these satellites must maintain an orientation
relative to their nadir point.  In other words, the same part of the satellite
always faces "down".  Similarly, the solar arrays probably have to face in the
general direction of the sun most of the time.  To meet both of these
requirements, the solar arrays must be in constant motion unless the satellite
orbital plane happens to be very close to perpendicular to the sun-earth
vector.  This would be true for a sun-synchronous orbit that follows the
terminator, but for the Iridium network to work, its orbital planes must be
evenly distributed around the globe.  Only one plane would meet this
requirement.

Now if the solar arrays are the source of the specular reflections, and they
always pointed *exactly* at the sun, then the only times you would see flares
would be when the satellite-observer-sun angle was close to 180 degrees.  This
is not what people are seeing.  However, I doubt that the arrays need to point
exactly at the sun.  An array normal could point away from the sun by as much
as 26 degrees with only a 10% loss in power generating ability.  The
corresponding phase angle for an observer would be 128 degrees (SkyMap) or 52
degrees (QuickSat).  This is probably closer to what people have been seeing.

This allowance for ~slop~ in the solar array pointing direction really makes
it hard to predict when flares will occur.  My questions for someone out there
are these:

1.  How much and how often do Iridium solar arrays articulate?

2.  Is each Iridium maintained in a particular orientation, either to the sun,
earth or stars?

Given an observer's location, the date/time of a flare, and the orbital
elements of the satellite that caused the flare, I can provide the coordinates
(RA and Dec) that the surface normal causing the flare was pointed -- AT THAT
TIME.  These coordinates, in turn, could be converted into a local satellite
az-el coordinate system, where elevation 90 is satellite zenith, elevation -90
is satellite nadir, and azimuth 0, elevation 0 is the approximate direction of
satellite motion.  Perhaps a pattern will reveal itself in this final
coordinate system.  --Rob