GVEGuidance:

Path: Interplanetary/AsteroidProspector

% Apply Gauss Variation Equations to determine in-plane thrust direction.
   For use with a low-thrust orbit transfer, for changing one Keplerian
   element at a time.

   Since version 2014.1
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   Form:
   uRSW = GVEGuidance( el, elT, index )
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   Inputs
   ------
   el            (1,6)    Orbital elements [a,i,W,w,e,M] in radians 
                          (distance units arbitrary). Can also accept
                          dimensions (6,1)
   elT           (1,6)    Target orbital elements. Can also accept
                          dimensions (6,1)
   index         (1,1)    Index of element to change, in the order above,
                          from 1 to 5.

   -------
   Outputs
   -------
   uRSW          (3,1)    Unit thrust vector, with 
                          x,y,z components in the radial, along-track, and
                          orbit-normal (or cross-track) directions,
                          respectively.  

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