A surveying system characteristics of a planet includes a new subsystem attitude determination. A satellite comprises a detector of the terresrte picture (106). The sensor is alternately directed at land and predetermined star field. The optical detections are converted to electronic signals by an electronic sensor module (332). The outputs of the electronic sensor module corresponding to the times when the sensor is directed onto earth are processed by a signal processor (336) whose output is a stream of data that allows the image reconstruction by a ground station computer . The outputs of the electronic sensor module corresponding to the times when the sensor is directed onto the star field are directed through the electronic elements of the star sensing threshold boarded. A programmable threshold device (342) selects rating target detections in the star field. A stellar data buffer (344) stores these star-target detections to transmit to ground the right time. The data recorded by the buffer are star attitude determination compressed data required for accurate grid data on the image of the ground passed by the signal processor. In another embodiment, an earthly image sensor is occasionally directed at a limited part of the stellar field, this special movement being controlled by the satellite. In both embodiments, compression of star field data is accomplished by devices mounted on board the satellite. The compressed data on the star field are subsequently used to accurately determine the attitude of the detector.
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