public static function Atio13($ri, $di, $utc1, $utc2, $dut1, $elong, $phi,
          $hm, $xp, $yp, $phpa, $tc, $rh, $wl, &$aob, &$zob, &$hob, &$dob, &$rob) {

    $j;
    $astrom = new iauASTROM();

    /* Star-independent astrometry parameters for CIRS->observed. */
    $j = IAU::Apio13($utc1, $utc2, $dut1, $elong, $phi, $hm, $xp, $yp, $phpa,
                    $tc, $rh, $wl, $astrom);

    /* Abort if bad UTC. */
    if ($j < 0)
      return $j;

    /* Transform CIRS to observed. */
    IAU::Atioq($ri, $di, $astrom, $aob, $zob, $hob, $dob, $rob);

    /* Return OK/warning status. */
    return $j;

    /* Finished. */
  }