by 'raydir' is observed. The instrument designated
               by 'inst' is treated as if it were co-located with the observer.

               [1,c4] = size(obsrvr); char = class(obsrvr)

               Optionally, you may supply the ID code of the object as an
               integer string. For example, both 'EARTH' and '399' are
               legitimate strings to supply to indicate the observer
               is Earth.

      step     the step size to use in the search. 'step' must be shorter
      