Hierarchical modeling and evaluation of phased-mission systems

This paper proposes a hierarchical and modular methodology for modeling and evaluation of phased-mission systems in which phases have constant predetermined duration, and where missions may evolve dynamically selecting the next phase to be performed according to the system state. A 2-level modeling is proposed: the higher one models the mission itself; the lower one models the various phases. A separate 'modeling and resolution of phases' and 'dependencies among phases' are considered. This methodology is applied using an example of a space application. This method is compared with previous models. The advantages of this approach are in the great flexibility, easy applicability and reusability of the defined models. It permits: (1) obtaining information on the overall behavior of the system; and (2) focusing on each single phase to detect system dependability bottlenecks. The explicit modeling of the phase changes: (1) is a neat and easily understandable representation of the interphase dependencies; and (2) allows a straightforward modeling of the mission-profile dynamic selection. General purpose tools available to the reliability community can easily manage the computational complexity of the analysis.