An Overview of the Phase-modular Fault Tree Approach to Phased Mission System Analysis

Phased-mission systems (PMS) are systems supporting missions consisting of multiple, consecutive, and non-overlapping operational phases. System composition, structure, and behavior can change from phase to phase; as a result, components may be modeled in different modes over the course of the mission. The need to model components and relationships in different modes poses unique challenges to analysis methods. We look at how fault tree analysis (FTA), a primary means of performing reliability analysis of PMS, can meet this challenge in this paper by presenting an overview of the modular approach to solving fault trees that represent PMS. Traditional solution methods are either combinatorial or Markov-chain based. The modular approach uses both combinatorial and Markov-chain solution methods as appropriate. This approach works by initially identifying modules that are independent throughout the PMS. This is accomplished by finding the union of all the components that are interdependent in any phase. It then solves for the reliability of each module within each phase and uses a combinatorial approach for finding the PMS system reliability based on the reliability of the modules. Further for solving for the reliability of the independent modules, if they correspond to static fault trees throughout all phases, it uses a combinatorial approach suitable for static PMS’s (combined BDD’s) and if they correspond to a dynamic fault tree in at least one phase, it uses a combined Markov Chain approach, Since the