def check_transmitter_frequency_in_header(header):
    """Check that the central frequency in the header is in MHz.
    Correct if not.

    :param header: Header dict containing 'TransmitterFrequency' key.
    :type header: dict
    :return: Checked and modified header
    :rtype: dict
    """
    if header['TransmitterFrequency'] > 1E5:
        header['TransmitterFrequency'] /= 1E6

    return header