Modeling safety-critical system requirements with hierarchical state machine

Automatic Train Protection (ATP) system is a safety-critical system; it is widely used to ensure trains running safely. During its development lifecycle, there are many safety problems which are derived from the requirements. In order to make descriptions of the requirements accurate and consistent, we introduce requirement models in the development lifecycle. The requirement models are built based on the modified state machine with text descriptions and the introduction of SuperState. In these models, the limitations of transitions are well defined and the complexity of the models can be reduced effectively. With this approach, the requirement of train localization function of ATP system is described clearly and strictly. Besides, this requirement model is easy to understand and read for developers.