Aeronautical Surveillance System Air traffic is growing at a significant rate. There is also increasing demand for more operating flexibility to improve aircraft efficiency, and to reduce the impact of air travel on the environment. Improved tools are required to safely manage increasing levels and complexity of air traffic. Aeronautical surveillance is one such important tool in the air traffic management process. In most cases, an aeronautical surveillance system provides its user with knowledge of “who” is “where” and “when”. Other information provided may include velocity, other identifying characteristics, or intent. The required data and its technical performance parameters are specific to the application that it is being used for, such as air-ground or air-air applications. As a minimum, the aeronautical surveillance system provides information on aircraft or vehicles at a known time. Besides, the surveillance system also serves to close the gap between expectations of aircraft movements based on clearances or instructions issued to pilots, and the true trajectories of these aircraft. In this way the surveillance function provides an indication of any unexpected aircraft movements which provides an important safety function. Accurate surveillance can be used as the basis for automated alerting systems. The ability to accurately track aircraft enables ATC to be alerted when an aircraft is detected to deviate from its assigned altitude or route, or when the future positions of two or more aircraft are predicted to fall below minimum acceptable standards. Alerts may also be provided when the aircraft strays below the minimum safe altitude or enter a restricted area. The existing fixed route structure provides increased certainty of aircraft movements, making it easier for controllers to manage air traffic. With improved navigation performance on board aircraft, airspace users are demanding greater flexibility to determine the most efficient routes to satisfy their operating conditions. There is a push for restrictions associated with flying along fixed routes to be lifted. In such an environment, accurate surveillance is required to assist controllers in the detection and resolution of any potential conflicts associated with the flexible use of airspace, which will result in a more dynamic environment. (357 words)