public static function Gmst06($uta, $utb, $tta, $ttb) {
    $t;
    $gmst;

    /* TT Julian centuries since J2000.0. */
    $t = (($tta - DJ00) + $ttb) / DJC;

    /* Greenwich mean sidereal time, IAU 2006. */
    $gmst = IAU::Anp(IAU::Era00($uta, $utb) +
                    ( 0.014506 +
                    ( 4612.156534 +
                    ( 1.3915817 +
                    ( -0.00000044 +
                    ( -0.000029956 +
                    ( -0.0000000368 ) * $t) * $t) * $t) * $t) * $t) * DAS2R);

    return $gmst;
  }