public static function Gst06a($uta, $utb, $tta, $ttb) {
    $rnpb = [];
    $gst;

    /* Classical nutation x precession x bias matrix, IAU 2000A. */
    IAU::Pnm06a($tta, $ttb, $rnpb);

    /* Greenwich apparent sidereal time. */
    $gst = IAU::Gst06($uta, $utb, $tta, $ttb, $rnpb);

    return $gst;
  }