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   Converts tangential accelerations into radial using the true anomaly.
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   Form:
   aR = CT2R( a, e, f, at, mu )
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   ------
   Inputs
   ------
   a                     Semi-major axis or perigee radius if e = 1
   e                     Eccentricity
   f                     True anomaly
   at                    Tangential vector
   mu                    Gravitational parameter

   -------
   Outputs
   -------
   aR                    Radial vector or transformation matrix

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   References: Battin, R.H., An Introduction to the Methods and Mathematics
               of Astrodynamics, AIAA, New York, 1987, pp. 489.
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