SURVEYING SATELLITE INCORPORATING STAR-SENSING ATTITUDE DETERMINATION SUBSYSTEM

SURVEYING SATELLITE INCORPORATING STAR-SENSING ATTITUDE DETERMINATION SUBSYSTEM A system for surveying features of a planet includes a novel attitude determination subsystem. A satellite includes an earth imaging sensor. The sensor is alternately directed at the earth and a predetermined star field. The optical detections are converted to electronic signals by a sensor electronics module. The outputs of the sensor electronics module corresponding to the times when the sensor is directed toward earth are processed by a signal processor, the output of which is a data stream which permits image reconstruction by a ground station computer. The outputs of the sensor electronics module corresponding to the times when the sensor is directed toward the star field are directed through onboard star detection thresholding electronics. A programmable threshold device selects detections of target stars within the star field. A star data buffer stores these target star detections for transmission to earth at a convenient time. This data stored by the buffer is compact star attitude determination data required for accurate gridding of the earth image data transmitted from the signal processor. In an alternative embodiment, an earth imaging sensor is occasionally pointed at a limited portion of the star field, such special motion being controlled from the satellite. In both embodiments, compression of star field data is accomplished by means located onboard the satellite. The compressed star field data is subsequently used to accurately determine sensor attitude.