Superbird A from SoCal

Rob Matson (Rob_Matson@cpqm.mail.saic.com)
1 Oct 1996 13:41:35 -0800

                      Subject:                              Time:  11:44
  OFFICE MEMO         Superbird A from SoCal                Date:  96/10/01

Well you can add me to the list of people who have now observed Superbird A. 
Fortunately I set up for this satellite early (8pm local, 3h UTC 10/1/96)
using stellar references and a tripod-mounted pair of 8 x 56 binocs.  Limiting
magnitude was 7.0 at the time I set up.  The reason that it was fortunate that
I set up early is that starting at 8:15pm, coastal clouds began to interfere. 
By 8:20 I had lost my reference stars altogether and could see nothing but
clouds.  But the clouds weren't that thick, and a bit patchy, so I was hopeful
I would catch a few holes.  My perseverance paid off.

The first flash I could see occurred around 8:27pm PDT (3:27 UTC).  I started
counting in my head waiting for the next flash roughly 12 seconds later, but
saw nothing.  But at 23 seconds later I saw my second flash and started my
stop watch.  Again, saw no secondary flash, but then NO STARS were visible
during the entire flash observation period.  This tells you how lousy the
seeing was, and how bright the primary flashes are!  At my third primary flash
I hit my lap timer, and from that point on the secondary flashes were also
visible.  I accurately timed the next 6 flashes, and then decided to look away
from the binocs to see if the flashes were visible naked eye.  They were -- no
problem.  Based on the appearances of eta Oph (2.6) and delta Oph (3.0) thru
the clouds, the flashes were at least mag 2.0. Yes -- that's not a typo. 
Magnitude 2.  Incredible considering the range.  The last flash I was able to
see was around 8:33:30 (3:33:30 UTC).

Now that the initial excitement of seeing this bird has past, I can
concentrate on recording the exact UTC times of the flashes.  My feeling is
that this satellite is rotating about an axis roughly parallel to the earth's
current velocity vector.  This would cause the specular reflection to rapidly
sweep either NNW to SSE or SSE to NNW over the earth, while slowly walking
eastward with each sweep.  (At this time of year, from the satellite's
perspective at the time of flashing, the sun is to the left of the earth, and
the earth's axis is tilted around 20 degrees counterclockwise from vertical.)

The specular point sweeps over the earth at over 20,000 km/sec, so observers
in southern and northern California (and for that matter Colorado too) will
have virtually identical times recorded for the flashes.

I measured the half-period of the satellite to be 11.665 seconds (93.32
seconds divided by 8 flash periods), or 23.33 seconds for a full rotation. 
The specular reflection sweeps 180 degrees in a quarter-period so the angular
sweep velocity is 30.86 degrees/second.
Assuming a perfect mirror reflection, the specular reflection subtends 0.5
degrees -- so each flash only lasts .016 seconds.

For my location, the range to Superbird A was about 38,800 km.  If the
projected area of the solar array causing the flashes is 2 sq-meters, with a
reflectivity of say 70% and a solar flux of 177 W/m^2, the specular intensity
would be:

(2 x 0.7 x 177 W) / 5.981e-5 sr = 4.14 megawatts/sr.  Dividing this by the
range squared, the irradiance is:

(4.14 x 10^6 watts/sr) / (3.88 x 10^7 m)^2 = 2.75 x 10^-9 W/m^2.  That
would correspond to a visual magnitude of around +0.4.  This is around 4.4
times brighter than magnitude 2.0, so perhaps the projected area of the solar
panel is smaller, or the reflectivity lower, or the quality of the reflection
is poorer (spread into more than a half-degree cone).  Or perhaps the flashes
are indeed this bright, but because of their short persistance the eye
"registers" them dimmer.  --Rob