def r_polar(system='wgs84'):
    """ Defines polar radius according to a specific system

        :date: 20.10.2014

        :author: Jalal Matar

        :param system: defines the measurments
          system (iau, krassowsky or wgs84)

        :returns: polar radius
    """
    if system == 'iau':
        r_po = 6356775.0
    elif system == 'krassowski':
        r_po = 6356863.01877
    elif system == 'wgs84':
        r_po = 6356752.31425
    return r_po
