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   Estimates rate from measurements of period
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   Form:
   x = SpinREst( accel, K, x, T, pulsewidth )
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   Inputs
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   accel          (1,1)   Thruster angular acceleration per unit time
   K              (1,1)   Gain
   x              (1,1)   Rate estimate
   T              (1,1)   Measured period
   pulsewidth     (1,1)   Thruster pulsewidth (duration of the acceleration)

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   Outputs
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   x              (1,1)   Rate estimate

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