Reliability-related safety analyses for satellite navigation systems

Safety and reliability are two interrelated attributes for safety-critical systems. While the typical safety analysis focuses on preventing hazards associated with erroneous safety critical outputs, this paper introduces an equally important hazard for the loss of critical functionality, referred to as the "loss-of-function" hazard. Tradeoffs are studied among three safety/reliability measures, i.e., the probability of working correctly, the probability of generating erroneous outputs and the probability of losing critical functionality. One of the goals for this study is to assist system engineers in making correct and timely design decisions. A major problem encountered in computing the probabilities of the various safety hazards is the initial condition consideration. This is because a fault-tolerant system can have various operational conditions and a hazard can occur under any of the working conditions, each with different probabilities. To provide a reasonable estimation, a measuring method that incorporates all possible initial conditions is proposed.