Abstract: The failure detection in sensor systems of a long life satellite is needed because of the increasing demands on reliability and safety. Since the life of components which compose the system has its limitaion, we use redundant technique. The component (such as sensor) in the system has two or more redundant parts. If any of them fails, it will be replaced by another one. So the problem is how to detect the failure. A method which was called WALD SEQUENTIAL ANALYSIS in pattern recognition is presented in the paper. The use of microcomputers permits the application of the method in real time processing. The result of simulation of the method in a satellite control system shows that it is feasible to detect failure in the system.