A method for determining the attitude error of a space vehicle by means of star acquisition and identification, after having rotated the space vehicle from a known starting attitude into a desired new attitude using a star sensor on board the space vehicle which has a lens system as well as a two-dimensional photosensor arrangement situated in the focal plane of this lens system and defining a visual field of the star sensor, as well as using a catalogue of stars which contains the known positions of a number of stars relative a system of coordinates, wherein the attitude error is determined by the following steps:
- a) defining a reduced visual field which is decreased with respect to the visual field of the star sensor by edge areas having width which corresponds to a maximum expected attitude error following completion of said rotation;
- b) selecting from along the catalogued stars which, in a case of a precise alignment of the space vehicle in the desired new attitude, are expected to be within the reduced visual field, at least three reference stars in known positions, one of which is designated as a guide star;
- c) determining expected positions within the reduced visual field for each of said reference stars, based on their known positions relative to the system of coordinates and on the desired new attitude of the space vehicle;
- d) determining the positions of the respective brightest stars within a two-dimensional first window having dimensions corresponding to twice the maximum expected attitude error and centered around the expected position of the guide star, by analyzing the output signals of the photosensor arrangement,
- e) determining the deviation of the position of a first one of the stars defined in step d) relative to the expected position of the guide star,
- f) determining corrected expected positions for the remaining two selected catalogued stars by applying an adjustment which corresponds to the deviation determined in step e)
- g) determining the position of the brightest star within a two-dimensional second window centered around each of the corrected expected positions, by analyzing the output signals of the photosensor arrangement, the dimensions of the second windows being selected such that no more than one star comparable in brightness to the catalogued stars can be present in it, taking into account stellar density present in the visual field of the star sensor,
- h) determining the distances of the positions of the thus determined brightest stars from one another and from the position of the guide star and comparing said positions with the respective distances between the known positions, of the stars selected in step b);
- i) if the distances which are compared with one another coincide, the attitude error of the space vehicle is determined from the difference between the positions of the two brightest stars defined in the two second windows and the respective expected positions of the remaining two catalogued stars,
- j) and if the distances which are compared with one another do not coincide, steps e) to i) are carried out on the basis of a different one of the stars defined within the first window.