# RE: Superbird A (89-041A, 20040)

From: Matson, Robert (ROBERT.D.MATSON@saic.com)
Date: Fri Dec 16 2005 - 13:11:24 EST

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```Hi Leo, Ed, Björn & other Superbird A trackers,

For years I think we've all been assuming that Superbird's large
solar wings are the source of the bright (up to ~2nd magnitude),
predictable glints that occur over a 5-8 minute window each night.
(After all, these solar wings are much larger than the spacecraft
bus itself, and offer significant surface area with which to direct
solar glints down to the ground.)

But the well-known phase shift that occurs during the flash window
each night raises a small dilemma:  clearly at least two somewhat
specular surfaces on the satellite are involved that must point in
opposite (or nearly opposite) directions.  If surface #1 causes
the glints at the beginning of the flash window, and surface #2 is
the source of those at the end of the flash window, then the pattern
of flashes is something like this:

1 1 1 1 1 121212121212 2 2 2 2 2

Currently ~22 seconds separates each pair of 1's and each pair
of 2's, with flashes from surfaces 1 and 2 alternating every ~11
seconds during the middle of the flash window.  So what's the
dilemma, you may ask?  The problem is that the flashes from surface
1 and surface 2 are the same brightness (or very close to the same
brightness), which means that either the two surfaces share the same
size and reflectivity (or much less likely, that the smaller size
of one surface is exactly compensated by a higher reflectivity).
Why is this a problem?  The satellite has two solar wings, after
all.  The problem is that there is a significant difference in
surface reflectivity between the front side and back side of each
solar wing.  Thus if the two solar wings both faced in the same
direction, then the flashes I've labeled "1" and "2" could not
be of the same brightness.  They probably wouldn't even be of
the same color.

When I presented my talk on Superbird A at Eurosom 3 in Edinburgh,
my proposed solution to this problem was simple:  the two solar
wings must be pointed in opposite directions!  (I even theorized
at the time that perhaps this was an emergency failure mode
configuration guaranteed to always have sunlight falling on one
solar panel. This asymmetric arrangement would also handily
provide a mechanism for the observed spin-up of the satellite
that continues to this day -- the Yarkovsky effect).

The problem is that I don't think spacecraft designers ever actually
do this (deliberately point the solar arrays in opposite directions)
for the very reason that the reflected energy imbalance ~will~ spin
up their satellites.  Which brings us back to square one:  how can
we get equal magnitude glints from two surfaces on Superbird A
pointed in opposite directions?  The only solution I can come up
with is that the glints come from the body of the satellite itself.
And while the solar panels must also generate solar specular
reflections, they may not ever intersect the earth -- or if they
do, they occur at a different time from the satellite body flashes.
(There ~have~ been observations of secondary flashes from Superbird A
at a different time then the main flashes -- perhaps these are from
the solar panels...)  I'll have to check my archives, but my guess
is that these secondary flashes only occur at the 22+ second period,
not 11+ seconds.

--Rob

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