RelativeOrbitRHS:

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   Continuous-time linear model of Hills equations in Hills frame.
	 Valid for circular orbits only.
       x - radial, aligned with position vector;
       y - in the direction of the velocity vector;
       z - normal to the orbit plane.
   This frame is consistent with Kaplan's convention.
		Note: Previously named "HillsPropagationH". Renamed for clarity.

   Since version 7.
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   Form:
   xDot = RelativeOrbitRHS( x, t, w, u )
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   Inputs
   ------
   x               (6,1)     [r;v] Spacecraft state in Hills frame
   t                (1)      Current time (sec)
   w                (1)      Orbital Rate (rad/s)
   u               (3,1)     Thrust vector

   -------
   Outputs
   -------
   xDot            (3,3)     [rDot;vDot] Time derivative of spacecraft state
                             in Hills frame

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