The International Space Station's (ISS) External Active Thermal Control System (EATCS) radiator wings are rotated to provide benign thermal environments. The Radiator Goal Angle Calculation (RGAC) software determines the radiator wing orientation relative to the space station. The Thermal Radiator Rotary Joint (TRRJ) is the hardware that rotates the radiator wing on the ISS.The radiator wing orientation is determined by two opposing criteria. First is the requirement to have a cold thermal environment to reject waste ISS heat. Second is the requirement to keep the EATCS working fluid of ammonia (NH3) from freezing. The compromise between these two opposing requirements is to maintain a constant thermal environment of Tcmd = -51°C (-60°F).The actual rotation profile is based on an edge-to-sun orientation on the sun side of the orbit and an Earth facing orientation on the shadow side of the orbit. Deviations from this simple algorithm are made in an attempt to keep the thermal environment as close as possible to Tcmd.The thermal environment that the radiator wing will experience on orbit is calculated assuming that the radiator wing is a flat plate. The thermal environment is calculated including direct solar flux, albedo solar flux, and Earth Infrared () flux. The heating from other station elements are estimated using a table lookup. The accuracy of the thermal environment is required to be ±8.3°C (±15°F).