  `abcorr', can be computed with:

                  obspos = spoint - srfvec

               To transform the vector `srfvec' from a reference frame
               `fixref' at time `trgepc' to a time-dependent reference
               frame `ref' at time `et', the routine <a href="cspice_pxfrm2.html">cspice_pxfrm2</a> should be
               called. Let `xform' be the 3x3 matrix representing the
               rotation from the reference frame `fixref' at time
               `trgepc' to the reference fram