FW: Unexplained IRIDIUM flares - now explained

Tue, 27 Jan 98 21:02:49

The author of this post has asked me to forward it to SeeSat-L for 
dissemination. I take no responsibility for injuries sustained during the 
demo ;-)

Jeff Hunt <jeff@satellite.eu.org>

--- On Tue, 27 Jan 1998 16:13:00 -0600  John Fleming-P26166 
<John_Fleming-P26166@email.mot.com> wrote:

>I saw your web page 
>  http://www2.satellite.eu.org/iridium.html
>and it mentioned that there are other IRIDIUM flares not yet explained.  
>Here now explained are one possible source of those flares.  I have been 
>working on the IRIDIUM SV's for six years, from specification through 
>operations, and done a lot of work modelling of the attitude control of 
>the IRIDIUM SV's, so I'm relatively certain of this.

>The IRIDIUM SV's solar panels are not always pointed normal to the 
>sun-line. Anytime the solar beta angle (the angle between the sun line 
>and the orbit plane) is greater than 40 degrees, the solar arrays (SA's) 
>are off-pointed from the sun line, resulting in occasional (once per 
>orbit) reflections of the sun from the glass-covered solar arrays to the 
>ground.  The maximum angle off-normal is 25 degrees.  Here is how it 

>1.  When the beta angle is between 40 and 65 degrees (or between -40 and 
>-65 degrees), the SA's azimuth gimbal is locked at +/- 40 degrees, and 
>the elevation gimbal continues to swing 0-360 degrees, maximizing the 
>incidence angle.

>2.  When the beta angle is above 65 degrees, the azimuth gimbals are 
>locked at 90 degrees, and the elevation gimbals are also locked, one at 
>zero, and one at 180 deg.  This is easily simulated by your body.  Stand 
>up, put one arm out horizontally to your front, hand vertical (thumb up, 
>pinky down).  The other arm is out behind you, horizontal, with hand 
>vertical, thumb up, pinky down.  Now your palms are the solar arrays, 
>and your feet point to the center of the earth.

>Azimuth and elevation gimbals are explained here.  Imagine a plan view 
>of the IRIDIUM SV.  As depicted below, we are facing in the -X 
>direction, toward the "front" of the SV.  In orbital motion terms, the 
>SV is coming towards us.  The
>SV's +Z axis is always pointing to the earth, to nadir (well, +/- 0.3 
>degrees of nadir).

>    +Y Azimuth  -Y Azimuth
>         Axis    Axis
>          ^       ^
>          |       |
>          |       |
> ---------   ---   ----------
>|  +Y SA  |-|   |-|  -Y SA   |  ---> Elevation axis
> ---------  |   |  ----------
>            |   |
>        \   |   |   /
>         \  |   |  /              ---> -Y
>          \ |   | /              |
>           \|___|/               |
>             ^^^                 v
>                                +Z
>Elevation rotation axis is along the longitudinal centerline of the 
>solar array panel.  Azimuth rotation axis is at the inside edge of each 
>array panel, and always parallel with the panel plane.  Thus, the 
>Azimuth axis is fixed relative
>to the panel plane;  and the elevation axis is fixed relative to the SV 
>In human body terms, in the above example your horizontal arm is along 
>the elevation axis, and the azimuth axis is vertical at your wrist.

>When the beta angle is close to 65 degrees is the time it is most likely 
>to get a see-able reflection from the solar arrays, since that is the 
>time of maximum incidence/reflection angle.  However, any time the beta 
>angle is above 52
>degrees, or below 78 degrees, there will be once per orbit reflections 
>of the sun onto the earth.  I have not yet programmed Satellite Tool Kit 
>to calculate these times of intersection, 

>Could you please disseminate this information to those people who are
>calculating reflections, and they could include this information in 
>their models
>and account for more of these uncalculated flares and glints.  If you 
>need more detailed information, contact me at the address below.
>John Fleming
>Motorola Satellite Communications
>EMail:  John_Fleming@sat.mot.com

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