The COBE (cosmic background experiment) satellite required an attitude sensor so that it could aim its telescope relative to the earth's position. Such an attitude sensor is known as an horizon sensor. We provided services including proposal writing that got the job, and system architecture and data processing methods for the COBE horizon sensor. The sensor employed a motor driven conical scanner, an infrared detector with a low noise preamplifier, a three-phase motor driver, an integrating A/D converter, a microprocessor and control logic, a self-contained power supply, and output interfaces. COBE was flown successfully in 1989.