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Spacecraft attitude control provides the basic stability so that sensors, solar panels, antennas and other hardware are properly oriented to perform their functions. A Horizon scanner can automatically seek the earth horizon by detecting the sharp discontinuity in InfraRed intensity at the outer edge of the Earth's mesopause for purposes of a spacecraft's orientation and control. As a satellite in a Low Earth Orbit (LEO) and three-axis stabilized, the Canadian satellite RADARSAT-1 is equipped with two horizon scanners (HS) in order to scan dynamically across the Earth's disc and to establish the attitude relative to the Earth. This paper discusses the thermal design and analyzes the on-orbit thermal performance of the HS.