VASSAR: Value assessment of system architectures using rules

A key step of the mission development process is the selection of a system architecture, i.e., the layout of the major high-level system design decisions. This step typically involves the identification of a set of candidate architectures and a cost-benefit analysis to compare them. Computational tools have been used in the past to bring rigor and consistency into this process. These tools can automatically generate architectures by enumerating different combinations of decisions and options. They can also evaluate these architectures by applying cost models and simplified performance models. Current performance models are purely quantitative tools that are best fit for the evaluation of the technical performance of mission design. However, assessing the relative merit of a system architecture is a much more holistic task than evaluating performance of a mission design. Indeed, the merit of a system architecture comes from satisfying a variety of stakeholder needs, some of which are easy to quantify, and some of which are harder to quantify (e.g., elegance, scientific value, political robustness, flexibility). Moreover, assessing the merit of a system architecture at these very early stages of design often requires dealing with a mix of: a) quantitative and semi-qualitative data; objective and subjective information. Current computational tools are poorly suited for these purposes. In this paper, we propose a general methodology that can used to assess the relative merit of several candidate system architectures under the presence of objective, subjective, quantitative, and qualitative stakeholder needs. The methodology called VASSAR (Value ASsessment for System Architectures using Rules). The major underlying assumption of the VASSAR methodology is that the merit of a system architecture can assessed by comparing the capabilities of the architecture with the stakeholder requirements. Hence for example, a candidate architecture that fully satisfies all critical stakeholder requirements is a good architecture. The assessment process is thus fundamentally seen as a pattern matching process where capabilities match requirements, which motivates the use of rule-based expert systems (RBES). This paper describes the VASSAR methodology and shows how it can be applied to a large complex space system, namely an Earth observation satellite system. Companion papers show its applicability to the NASA space communications and navigation program and the joint NOAA-DoD NPOESS program.