Requirements based system level risk modeling

Space exploration missions are often characterized by multiple phases and each phase in turn satisfies some objective or requirement. The success of the mission is measured by the degree to which these requirements are satisfied. Missions either aim to demonstrate a new technology, or to obtain new science data or a combination of both of these. During the mission design process, numerous trade studies are conducted between cost, performance and risk. At a very high level, the goal is to maximize the probability of achieving the most science return (or demonstrating the most technology) at the least possible cost. We consider the problem of maximizing this probability by quantifying the degree of importance of each requirement and it´s probability of being satisfied. The probability of a requirement being satisfied, in turn, is assessed by finding the aggregate of the probability of all the possible events that could prevent it from being satisfied. We assume a complete list of the requirements, the relevant risk elements and their probability of occurrence and the quantified effect of the risk elements on the requirements. In order to assess the degree to which each requirement is satisfied, we need to determine the effect of the various risk elements on the requirement. The complexity arises due to the fact that various risk elements that effect a requirement in question are not necessarily independent. Moreover, in order to compute the weighted average of the requirements, it´s important to take into consideration their dependencies. Therefore we carefully define the relationships between the elements within each category (intra-category) and the elements between the two different categories of risk and requirements (inter-category).