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  Coarse probability estimate consisting of the integral outbound of the 
  tangent between a circular avoidance region and the target's position
  probability density.
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  Form: 
  Pc = CoarseProb( dR, rAvoid, sigmaU)
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  Inputs:
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  rRel    (1,1) or (2,1)  Magnitude of or actual relative position vector between centers
                          in the conjunction plane, where [x z] are along the major and
                          minor axes of the density envelope.
  rAvoid        (1)       Radius of avoidance region
  sigma   (1,1) or (2,2)  One-sigma target probability along the relative position vector
                          or full one-sigma probability ellipse in the conjunction plane

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  Outputs:
  --------
  Pc        Coarse probability
              
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