Built-in online and offline test of airborne digital systems

This paper studies the effects of both online and offline test during flight critical missions where safety is a major issue. The online test, in this context, is a test performed on a digital airborne system during some specified windows in time while it is still performing its intended task. An offline test is a test that is performed on the digital system once it is taken offline because of a suspected failure. Both the online and the offline tests are performed during flight. The difference between the two is that the offline test can be made more effective than an online test due to the longer amount of time available for testing. Moreover, the offline test may be designed to have diagnosis and repair capability built in. Upon successful repair, the faulty processor may be reconfigured back into the system. Even though both tests are important, we show that the online test is more important than the offline test in enhancing the mission reliability. This is a counter-intuitive result since system repairs are conducted offline.