             the observer is to be computed. 'et' refers to time at
                the observer's location.

                [1,n] = size(et); double = class(et)

      ref       the name of the reference frame relative to which the output
                position vector should be expressed. This may be any frame
                supported by the SPICE system, including built-in frames
                (documented in the Frames Required Reading) and frames
                defined by a loaded frame kernel (FK