State knowledge representation in the Mission Data System

The possible states of a system, be it a spacecraft, rover or ground station, are what system engineers identify and specify, what software engineers design for, and what operators monitor and control. Many activities inside mission software are directly concerned with state, whether planning it, estimating it, controlling it, reporting it or simulating it. The cause of several mission failures can be traced to inadequate or inconsistent representations of state. Consequently, the concept of 'state' and its representation occupy a prominent role in mission software architecture. The Mission Data System (MDS), presently under development by NASA to provide multi-mission flight and ground software for the next generation of deep space systems, addresses this fundamental need. This paper describes the MDS approach to state knowledge representation, covering state variables, state functions, state estimates and state constraints, emphasizing design patterns that reduce sources of human error.