The main necessary functions to manage the traffic on the movement area and the airspace around an airport are surveillance, control, routing and guidance.1 Routing is often also termed planning. Today these functions are carried out manually by the controller in the tower and in the apron. But automatic assistance is becoming more and more important with increase of traffic flow in order to reduce the controllers’ workload from routine work. The basis for assisting systems is normally a system to estimate the actual traffic situation.