ref="vnorm_c.html">vnorm_c</a> to obtain the
               distance between the observer and `spoint':

                  dist = <a href="vnorm_c.html">vnorm_c</a> ( srfvec );

               The observer's position `obspos', relative to the
               target body's center, where the center's position is
               corrected for aberration effects as indicated by
               `abcorr', can be computed via the call:

                  <a href="vsub_c.html">vsub_c</a> ( spoint, srfvec, obspos );

 