Two computer systems have been incorporated into theUOSATspacecraft, the primary spacecraft computer being based on the RCA CDP 1802 microprocessor, and the secondary computer on the Ferranti F100 system.This paper describes the system philosophy and design consideration of the primary computer, particularly as regards its operation in a space environment where low power and reliability are essential. An overview of the general system is given and the various interfaces to the other spacecraft systems and experiments are also described.