REl

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   Computes the orbit radius and the parameter from Keplerian elements.
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   Form:
   [r, p] = REl( el )
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   Inputs
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   el                  Elements vector [a,i,W,w,e,M] where a is perigee
                       radius for a parabola

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   Outputs
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   r	                  Radius
   p                   Parameter

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