Validation of Guidance Control Software Requirements Specification for Reliability and Fault-Tolerance

A case study was performed to validate the integrity of a software requirements specification (SRS) for Guidance Control Software (GCS) in terms of reliability and fault tolerance. A partial verification of the GCS specification resulted. Two modeling formalisms were used to evaluate the SRS and to determine strategies for avoiding design defects and system failures. Z was applied first to detect and remove ambiguity from a part of the Natural Language based (NLbased)
GCS SRS. Next, Statecharts and Activity-charts were constructed to visualize the Z description and make it executable.

Using this formalism, the system behavior was assessed under normal and abnormal conditions. Faults were seeded into the model (i.e., an executable specification) to probe how the system would perform. The result of our
analysis revealed that it is beneficial to construct a complete and consistent specification using this method (Z-to- Statecharts). We discuss the significance of this approach, compare our work with similar studies, and propose
approaches for improving fault tolerance. Our findings indicate that one can better understand the implications of the system requirements using Z-Statecharts approach to facilitate their specification and analysis. Consequently, this approach can help to avoid the problems that result when incorrectly
specified artifacts (i.e., in this case requirements) force corrective rework.