Abstract: The invention relates to a method and device for monitoring data flow, specially to provide radar data for air traffic control systems. In order to detect data supply failures and to provide an automatic replacement supply, the data which is transmitted is computed at the output of the data transmission device at parameterable time intervals, whereupon the data flow is reproduced, the measuring values are stored during several intervals and compressed into an average value. When a new value is inputted, the oldest value is erased. The average value is used as a starting value for a sensitivity curve which generates a time window for periodic comparison of added measuring values for an error detector which signalizes a total failure of measuring values in a predetermined time unit and initiates a selection mode to access a predefined data source in an appropriate data network and the data is inputted into the respective data supply system via interfaces.