Spacecraft computers for scientific information systems

The use of spacecraft as vehicles for conducting investigations of the scientific properties of space has rapidly evolved into a complex of interconnected sensors and processors, both on the ground and aloft, termed an information system. Requirements of rapid response to stimuli, continuity of control for protracted periods (perhaps as great as several years), and capability of handling large quantities of data require the use of automatic control. The unknown factors and changing conditions in space require satellite instrumentation with a high degree of adaptability. This adaptability can be obtained by making the data processing and control general purpose as far as possible. An instrumentation satellite with a central computer time-shared by the various sensors and the control system, is a new type of system for this application. In this paper the factors which influence the system design are described: 1) the effect of the space environment on the computer 2) the effect of the mission (space telemetry) on the system design 3) the effect of on-board data processing on the system design 4) a comparison between general-purpose and special-purpose on-board system designs. The paper includes a description of a typical telemetry data process, and the design of a spaceborne, general-purpose micro-programmed computer is described in some detail. Quantitative comparisons are made between the alternatives. There is also a discussion of the effects of the space environment on the electronics of the processor and of means of assuring reliable operation.