public static function Gst06($uta, $utb, $tta, $ttb, array $rnpb) {
    $x;
    $y;
    $s;
    $era;
    $eors;
    $gst;

    /* Extract CIP coordinates. */
    IAU::Bpn2xy($rnpb, $x, $y);

    /* The CIO locator, s. */
    $s = IAU::S06($tta, $ttb, $x, $y);

    /* Greenwich apparent sidereal time. */
    $era  = IAU::Era00($uta, $utb);
    $eors = IAU::Eors($rnpb, $s);
    $gst  = IAU::Anp($era - $eors);

    return $gst;
  }