Iridium 20 rotation axis

ROB MATSON (ROBERT.D.MATSON@cpmx.saic.com)
16 Jun 1998 15:18:33 -0800

Hi All,

I've done some analysis of the flash data for Iridium 20 (aka Iridium 18 by
USSPACECOM) [#24871] based on observations by Ed Cannon, Frank
Reed and Don Gardner from June 9 - June 16.  I have 10 data points so
far, and I've established a prelimary inertial rotation axis that is a crude
fit to 9 of those points:

RA 4.5h, DEC +29 deg (or the opposite pole, RA 16.5h, Dec -29 deg,
which happens to be close to Antares).  The data would seem to indicate
that the normal of one of the MMAs is tilted about 38 degrees relative to
this axis -- in other words, the MMA normal spins in a cone with a
half-angle of 38 degrees centered on RA 4.5 h, Dec +29.  If so, then the
rotation axis is at least 12 degrees offset from the long axis of the
satellite.  However, keep in mind that this data is very rough right now -- a
12-degree error would not surprise me.

But 38 degrees in the "other" direction is only 2 degrees away from
an axis which corresponds to the satellite flipping end-over-end.  I believe
this would be a stable rotation axis.

The bottom line is that more measurements are needed.  For each data
point I only require the following information:

1.  your site coordinates (lat, long) (altitude if you know it)
2.  date and time (UTC) of peak flashes

The more accurate you can estimate #2 the better.  30-second accuracy
is not good enough -- I really need it to within +/- 5 seconds.  If multiple
consecutive flashes appear to be of equal brightness, use the time in
the center of the measurements.

If anyone has accurate observations prior to June 9, please post them.
I also have no data for June 11 or 12.  Thanks all!  --Rob