Earth beacon acquisition and tracking under various illumination conditions

The use of sun-illuminated Earth images has been explored to aid in meeting the stringent pointing requirements for deep space optical downlink telemetry. It has been shown that fully illuminated Earth images provide a symmetry that easily lends to the determination of the geocenter. Since more often that not, the Earth is only partially illuminated by the sun, additional work is required to meet the pointing requirements under all illumination conditions. This paper develops an algorithm for determining the geocenter of the Earth regardless of the illumination by the sun using sub-pixel scanning and a simple thresholding technique. To complete this approach the acquisition algorithm is paired with a tracking technique based on maximum likelihood estimation. Results have indicated that the algorithm works well to locate and track the position of the Earth and, thereby, a receiving station. Results of the experiments are presented and compared with other techniques.