A safety critical approach for aerospace software architecture

Software safety and reliability are key aspects for the success of space missions. An extensive software engineering literature exists on such two aspects, anyway more recent studies highlight the needs for software engineering to bridge the gap with the reliability and safety techniques used for hardware systems. This paper, on the way of the previous study, presents some considerations deriving from the application of reliability engineering disciplines to the different phases of software life cycle on a real case study. Starting from the functional analysis, going through the architectural design until the testing, the main effort has been spent to build a logical and consistent model by applying the RAMS techniques. Methods for assessing the software reliability have been investigated trying to carry them on not only at the final phase of testing but during all the software life cycle. In this way the reliability growing of the generic x-phase becomes the basis for the next one and can give useful indications to the overall space program. The described model has been applied 'a-posteriori' to the space vehicle on board software and the relative quantitative results are detailed.