Using an assurance case to support independent assessment of the transition to a new GPS ground control system

We describe a specific application of assurance cases to the problem of ensuring that a transition from a legacy system to its replacement will not compromise mission assurance objectives. The application in question was the transition of the Global Positioning System (GPS) to a new ground-control system. The transition, which took place over five days, required uninterrupted control of the GPS satellite constellation while control was transferred from a 1970s-era mainframe to a distributed architecture. We created an assurance case so that the procedural documentation we had could be restructured into a form amenable to analysis. The analysis concluded that there were no major hazards; this conclusion was validated by a successful transition.