The Development of a Low Cost, Modular Attitude Determination and Control System

In an attempt to reduce the cost of future satellites, new technologies are being pursued to develop a modular attitude determination and control system that will provide three-axis control and cost less than ten percent of present systems. The low cost and modularity of this system make it especially attractive to a wide variety of small satellites. This paper will present the design and developmental status of this plug and play attitude control system. The general idea is to provide a complete attitude determination and control system, including sensors, actuators, and software, that can be used for a wide variety of satellites. A single, pressurized box will contain all the electronics for the system, including the command computer, power supply, reaction wheel motor drivers, and star tracker electronics. Pressurization of the box (1atm BOL, 12,000 ft. EOL) enables the use of terrestrial components, which greatly reduces cost. The box will provide its own thermal control through the use of heaters, fans, and radiators. The power for the attitude control system will be provided by the spacecraft bus, suitably conditioned by the system\u27s onboard power supply. There will be one other interface to the spacecraft to allow for downloading of telemetry data and uploading of ground commands. This independence of the attitude control system from the satellite allows for ease of integration. The system will be controlled by a PowerPC 603e processor running flight software that is already commercially available. This use of already developed software also helps to reduce cost. The processor will communicate with the star trackers and reaction wheels via a commercial serial bus. Attitude determination will be provided by custom designed CCD or CID star trackers. Unlike most other sensors, star trackers can be used for any type of mission, from Earth-orbiting to interplanetary. Gyroscopes will not be used in the first generation of this system because they are not necessary for low bandwidth control. As already mentioned, the three-axis attitude control will be provided by reaction wheels, which are powered by polyphase AX induction motors to reduce mass. In addition to presenting the detailed design of the attitude control system, the paper will also address the issues that must be overcome, including radiation tolerance, particularly susceptibility of single event upsets and latch-up. Finally, the paper will discuss the results of the in-house testing of components and the production schedule