he observer.  The
               intercept with the target body's surface of the ray
               defined by the observer and 'dvec' is sought.

               [3,1] = size(dvec); double = class(dvec)

               'dvec' is specified relative to the reference frame
               designated by 'dref'.

               Non-inertial reference frames are treated as follows:
               if the center of the frame is at the observer's
               location, the frame is evaluated at 'et'.  If the
      