Abstract: A method of determining position errors of a space vehicle, which has been rotated from a known starting position into a desired new position, by means of star acquisition and star identification, using a star sensor as well as a star catalogue. Information in the catalog concerning the known positions of, and distances between, a selected group of stars expected to fall within visual field of the star sensor when the space vehicle is in the desired new position, is compared with the same information for selected stars sensed by the star sensor, until a match is found. The position error of the space vehicle is then determined by reference to the deviation between the known and sensed positions of the stars within the selected group.