def r_equatorial(system='wgs84'):
    """ Defines equatorial radius according to a specific system

        :date: 20.10.2014

        :author: Jalal Matar

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

        :returns: equatorial radius
    """
    if system == 'iau':
        r_eq = 6378160.0
    elif system == 'krassowski':
        r_eq = 6378245.0
    elif system == 'wgs84':
        r_eq = 6378137.0
    return r_eq
