Hierarchical modelling and evaluation of phased-mission systems

In this paper, we propose a hierarchical and modular methodology for the modelling and evaluation of a class of phased-mission systems in which each phase has a Constant pre-determined duration and where missions may evolve dynamically selectìng the next phase to be performed according to the System state. To model such systems we propose two levels of modelling, the higher to model the mission itself and the lower, composed by several models, for the phases. We thus consider separate modelling and resolution of the phases and of the dependencies among them caused by the usage of the same System components in the different phases. We show the application of our methodology using the example of a space application. This includes the necessary models and some numerical evaluation of the probability to survive a mission and of the expected performability of the System. Last, the paper includes a comparison of our method with previous proposals. The dependability analysis of phased-mission systems having Constant phase duration can be carried out with our method in an exact way. The method turns out in a great flexibility, easy applicability and reusability of the defined models. Furthermore, it pennits not only to obtain information on the overall behaviour of thè System, but at the same rime to focus on each single phase to detect System dependability bottle-necks. The computational complexity of the analysis can be easily managed by many general purpose tools available to the reliability community.