def get_data_keys(operator):
    """
    function to return keys for data columns for the
    :param operator: one of 'COSMOS-US', 'COSMOS-UK', or 'NMDB'
    :return:
    """
    # get the keys based on the network operator
    if "COSMOS-US" == operator:
        keys = ['MOD', 'UNMO']
    elif "COSMOS-UK" == operator:
        keys = ['CTS_MOD']
    elif "NMDB" == operator:
        keys = ['counts']
    else:
        raise KeyError('%s is not a valid operator' % operator)

    return keys