Special Section

Many lessons have been learned over the last 20 years about the development of large, complex systems. Structured programming techniques used for implementation have led to improved software quality and maintainability. Compilers and other tools have been developed to improve productivity and reliability. Management procedures have evolved that improved the control and stability of the development process. However, as effective as they all are, these tools and techniques alone cannot prevent or overcome the problems that will exist if a well-structured system architecture has not been established on the front end of the development process. Such an architecture, as described in this article, has been developed by the IBM Federal Systems Division for NASA's Space Shuttle Orbiter Primary Avionics Software System (PASS). PASS performs a critical role in virtually all operational aspects of the Space Shuttle Orbiter (Figure 1). It is by far the most complex flight computer program ever developed. It employs state-ofthe-art multiple computer redundancy management concepts and incorporates a multitude of functions required to support Shuttle operations on the ground and in flight. Influenced by software engineering concepts evolved in recent years, the software organization responsible for development of this system has implemented a design architecture and executed plans and procedures that resulted in the top-down integration and delivery of incremental software releases. User schedule and capability requirements have been satisfied and numerous requirements modifications have been accommodated in a very controlled and timely manner.