def __write_rnx3_header__(ver=3.03, typ="O"):
    """
    Write the first header line of the Rinex file
    """

    TAIL = "RINEX VERSION / TYPE"

    PAD = ' '

    res = "{0:9.2f}{1:11s}{2:1s}{1:19s}{3:1s}{1:19s}{4}\n".format(ver, PAD, typ, 'M', TAIL)

    return res