The requirement for definition of the Earth's horizon develops from the need for improved attitude reference in space vehicles. It appears that horizon sensors, operating on the Earth's infrared horizon, can provide an accurate, lightweight, and reliable reference system if the horizon anomalies can be ascertained and shown to be systematic in nature to allow removal of errors for the sensor by calibration. Delineation of an effective Earth horizon defini¬tion study demands an extensive examination of present knowledge and theories regarding the Earth's radiance characteristics, techniques for analysis of radiance profile variations, and performance of radiance detector equipment. These areas were examined in depth and this report presents a comprehensive collection of this knowledge. The past activities examined include both the¬oretical and experimental programs.
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