 The position points from the observer's location at
            'et' to the aberration-corrected location of the target.
            Note that the sense of the position vector is
            independent of the direction of radiation
            travel implied by the aberration correction.

            Units are always km.

            Non-inertial frames are treated as follows:
            letting LTCENT be the one-way light time between
            the observer and the central body associated
            