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   Compute clutter area.
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   Form:
   a = ClutterArea( theta, r, u, rP, t )
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   ------
   Inputs
   ------
   theta             (1,1)  Beamwidth (deg)
   r                 (3,:)  Orbit position
   u                 (3,1)  Boresight
   rP                (1,1)  Planet radius
   t                 (1,1)  Pulsewidth

   -------
   Outputs
   -------
   a                 (1,:)   Clutter area

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