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   Computes the orbit position vector in the perifocal plane.
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   Form:
   r = RPQ( e, rP, nu )
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   ------
   Inputs
   ------
   e                     Eccentricity
   rP                    Perigee radius
   nu                    True anomaly (rad)

   -------
   Outputs
   -------
   r                     [xP;xQ;0]

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   References: Wiesel, W. E., Spaceflight Dynamics, McGraw-Hill, 1988, p. 63.
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