A requirements-based, bottom-up SLOC estimate and analysis of NASA's Orion crew exploration vehicle spacecraft flight software

Estimating the size and amount of resources required to develop a large and complex software system is challenging. Early in a project lifecycle, requirements for the system may be immature and functionality defined only at a high level, yet resource profiles are necessary for appropriately funding, staffing, and developing a viable project plan. In addition, recommendations on key recurring questions related to software size and functionality (such as processor sizing, implementation phasing, allocations between the spacecraft and the ground, and possible downsizing/simplification opportunities) need to be established to aid in system design and efficient project planning. This paper presents the methodology and results of a bottom-up software size estimation and analysis study for NASA's Orion crewed spacecraft flight software, based on functional software requirements developed during the pre-Preliminary Design Review (lifecycle) phase. Results of this study are used to independently assess contractor software size estimates, better understand envisioned functionality, and better manage resources and staffing. Data compiled in this study are also used to evaluate requirements generation process success as a basis to predict processor utilization, and to aid in software simplification, migration, and implementation phasing efforts.