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Re: Changing AO-40's attitude



John,
You have an interesting question.  (And its on topic!)  I have thought
along the same lines myself.  As I understand it the spin rate MUST BE ZERO
before the momentum wheels can be started.  I assume this is because that
is how the controller is designed.  Turning the wheels on full blast while
the spacecraft is spinning at 17.7 RPM would probably destroy AO-40.  I
also doubt that the wheels could "absorb" a rotation rate of 17.7 RPM.
73's de Rich @ WC8J

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"John P. Toscano" wrote:

> 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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