SURVEYING SATELLITE INCORPORATING STAR-SENSINGATTITUDE DETERMINATION SUBSYSTEMABSTRACT OF THE DISCLOSUREA system for surveying features of a planetincludes a novel attitude determination subsystem. Asatellite includes an earth imaging sensor. The sensoris alternately directed at the earth and a predeterminedstar field. The optical detections are converted toelectronic signals by a sensor electronics module. Theoutputs of the sensor electronics module correspondingto the times when the sensor is directed toward earthare processed by a signal processor, the output of whichis a data stream which permits image reconstruction by aground station computer. The outputs of the sensorelectronics module corresponding to the times when thesensor is directed toward the star field are directedthrough onboard star detection thresholding electronics.A programmable threshold device selects detections oftarget stars within the star field. A star data bufferstores these target star detections for transmission toearth at a convenient time. This data stored by thebuffer is compact star attitude determination datarequired for accurate gridding of the earth image datatransmitted from the signal processor. In analternative embodiment, an earth imaging sensor isoccasionally pointed at a limited portion of the starfield, such special motion being controlled from thesatellite. In both embodiments, compression of starfield data is accomplished by means located onboard thesatellite. The compressed star field data issubsequently used to accurately determine sensorattitude.
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