9.1. Scope#

The focus of the present chapter is the selection and use of system level reliability methods. The reliability of all components (EEE, MEC, MIS) are considered in the context of the system. In early project phases (Phases 0, A and B as defined in [BR_SYS_9]) a system level reliability target for the space mission is defined. As systems are made of many interacting components, system target reliability has to be allocated to individual component target reliabilities (top-down analysis).

In later project phases (Phases C and D as defined in [BR_SYS_9]), after components are designed, system level reliability is estimated from part level reliability to assure that system reliability target is met (bottom-up analysis). Methodologies, for bottom-up system level reliability estimation are provided. If target system reliability is not met, the system can iteratively be optimized by repeating the top-down (reliability allocation) and bottom-up analyses. An overview of top-down and bottom-up system reliability analysis are provided, as well as guidelines for selecting modelling methodologies.