Dependability and safety issues for aerospace software

Some considerations derived applying software verification and validation techniques to support on board and on ground segment software development for aerospace applications are shown. The purpose is to increase dependability and safety levels for unmanned space vehicles. For such systems the scientific community addresses the European regulations (ECSS standards) to two main disciplines: the RAMS for the hardware components and the Verification & Validation (V&V) for the software components. Nevertheless a complete and quantitative knowledge of the overall system reliability is never obtained as the V&V practice doesn't allow any estimation about the software reliability. It has become evident that "highly reliable hardware" does not imply that 'the whole system is reliable'. Furthermore the software dependability and safety requirements are very difficult to handle due to the criticality of the autonomous on-board software components and their complexity and context variability, as well as to the financial and time constraints. The described experience concerns the effort in deriving from the classic Verification and Validation plan a proper process (the IISRE) to improve the dependability and safety properties for the on board software and its implications on the whole system. The first section details the aerospace safety and reliability issues. The second one describes the tailored framework and the obtained results.