- and z-components of the target's 
               position. 
 
               Units are always km.

               `ptarg' points from the observer's location at `et' to 
               the aberration-corrected location of the target. 
               Note that the sense of this position vector is 
               independent of the direction of radiation travel 
               implied by the aberration correction. 
 
               Non-inertial frames are treated as follows: letting
               ltcent be