| Four approaches for calculating the visibility of threa tened area are analyzed and compared with experiments in this paper. Visibility analysis is used to analyze the finding probability of the target detected by observers in three dimensional space. The visibility analysis is the foundation of route planning and it is important for choosing a safe route. The main work o f the visibility analysis is to find hidden area which is not visible to the obs ervers, such as radar，air pre-alarm aircraft，interceptor etc. The more the fl ight route is planned to pass through hidden area, the safer the aircraft is. Th is paper discusses four algorithms: point to region algorithm, fast point to tri angle algorithm, point to triangle algorithm, hidden removal algorithm, they are compared with the experiments. |
