Software Requirements Specification and Analysis Using Zed and Statecharts

This paper presents a prototypical study, of an embedded system requirement specification, used to establish the basis for a complete case study. We are interested in comparing different specification methods that accommodate the notion of state. A partial modeling of a NASA provided Guidance and Control Software (GCS) development specification was employed. The GCS describes, in natural language, how software is used to control a planetary landing vehicle during the terminal phases of descent. Our ultimate goal is to develop a complete software requirement specification based on the IEEE Standard 830-1998. The first step in the study was to derive a Zed description for a small portion of the system (Altitude Radar Sensor Processing [ARSP]). The ARSP module reads the altimeter counter provided by the radar and converts the data into a measure of distance to the planet surface. In the second step, Statecharts were developed to model and graphically visualize the Zed specified ARSP. Using Statemate we analyzed the specification for completeness and consistency. This was accomplished through the generation of activity-charts and simulations. We present the results of this work and discuss the issues associated with comparing the two methods in terms of their ability to ascertain consistency and completeness of the final products. A more comprehensive assessment of tools publicly available for the specification, modeling and analysis of embedded systems is envisioned.