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Changing AO-40's attitude



There's an old saying, "Ignorance is Bliss".

At the risk of demonstrating myself to be frankly euphoric with
ignorance, I would like to pose a (satellite-related) question to the
list.  I'll start with a short discussion of my baseline assumptions,
in case I am seriously mistaken in one or more of them, thus making
my whole question irrelevant.

With AO-40 in its current, spin-stabilized operation mode, the gain
antennas point to a fixed location in space, irrespective of the
position in the orbit around the Earth.  In crude ASCII graphics
(fixed-width font), this looks something like this:


            o>       o>
       o>                    o>
     o>     _                   o>
    o>     (_)                   o>                      x the point!
     o>                         o>
       o>                    o>
            o>       o>

...where o> is AO-40 and the arrow points in the direction of the
antennas, and the bigger "circle" is the Earth.  I realize that I've
drawn in with a squint angle of zero at perigee, which is NOT the
current attitude, but there's only so many characters in the ASCII
character set.  And it gets worse (see next picture).

In the future, when the momentum wheels are energized and the
spacecraft is in 3D-stabilized mode, the gain antennas point towards
the center of the Earth throughout the orbit, which means that the
spacecraft actually rotates 360 degrees per revolution, like the
Moon does...

            o        o
       o    v       /         o
     o> \   _                  <o
    o>     (_)                  <o
     o> /                      <o
       o    ^       \        o
            o        o

This implies to me that it could be possible to re-orient it in any of
the 3 axes, by any amount, in one orbit or less, or even to change the
current rotation speed, by judicious use of the momentum wheels.  Given
the current (undesirable) spacecraft attitude, and given the fact that
magnetorquing is slow now because of the unfavorable attitude, this
might speed commissioning efforts.  Even if we were not in a position
to place the spacecraft into full 3D-stabilized mode yet, which
requires the momentum wheels to run continuously, plus the use of the
magnetorquers to "dump momentum" when the wheel RPM's get close to
their upper limit.

Is this thinking incorrect?  Or is this a "Catch-22" situation, where
it is unsafe to attempt use of the momentum wheels to change attitude
until more TLM data are available, and TLM data are hard to come by
because the attitude is unfavorable?  (I also realize that there is
another issue with the lack of a sun sensor reading making the
re-orientation difficult, which would also be true with my suggestion,
but perhaps someone far more expert than me in orbital mechanics could
estimate the amount of momentum wheel use needed to re-position the
spacecraft as desired, and then try running the wheels for a small
fraction of that time and seeing if the change is close to what was
expected.  Maybe the first goal would be to simply slow the rotation
rate to reduce signal fading, because the effect could be monitored
by measuring spin rate via Doppler on the beacon, even if the TLM is
not reliable.  Or maybe an early goal would be to turn the spacecraft
by an amount and direction that would allow re-acquisition of the Sun
by the sensors.)

Just curious.
John P. Toscano, KB0ZEV
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