Search-Less Algorithm for Star Pattern Recognition
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Search-Less Algorithm for Star Pattern Recoqnltlon' Daniele Mortarf Abstract A fast and robust star identification technique (which does not use a searching phase) is presented for a spacecraft equipped with a wide field-of-view star tracker. The algorithm, which identifies stars within a large star catalog using stars' angular separation only, consists of two identification processes: the K-vector Star-Pair Identification Technique (SPIT), which, for any observed angular separation, selects an admissible star-pair set from the star catalog; and the subsequent Reference-Star Star-Matching Identification Technique (SMIT), which performs the identification process based on a subset of all of the admissible star-pairs. The K-vector SPIT, which uses an appropriate devised vector of integers, does not require a searching phase. The best-fitting SPIT, which uses a best-fitting criterion to reduce the searching phase, is also presented. The resulting robustness of the algorithm is such that, after spikes are deleted, at least three true stars are still available. An overall software block diagram of the proposed system is depicted as well as the results obtained by extensive tests.
Introduction Star observations are widely used by spacecraft as a primary means of attitude determination. Currently, star tracker sensors based on charge coupled devices (CCD) allow us to obtain the best spacecraft attitude estimation because of the high precision data provided. Three-axis attitude estimation algorithms need the knowledge of at least two different directions; therefore, a narrow field-of-view (FOY) star tracker cannot be used in a standalone configuration but must be used together with other less accurate sensors. On the contrary, wide FOY star trackers can be used autonomously and, therefore, provide a high precision dataset to the attitude estimation algorithm. Figure 1 is a sketch of a spacecraft-mounted wide-FOY star sensor. Obviously, when a wide FOY star tracker is.used, speed and reliability of the star identification technique are of capital importance just as speed and accuracy are important for the attitude estimation algorithm. This paper satisfies the above as Paper AAS 96-158 at the AAS/AIAA Space Flight Mechanics Meeting, Austin, Texas, Feb. 11-15, 1996. 2Assistant Professor, Aerospace Department, Universita degli Studi "La Sapienza," di Roma, Via Salaria 851,00138 Rome (Italy). Member AAS. E-Mail: [email protected]. 1Presented
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FIG. 1. Sketch of a Wide-FOV Star Sensor.
requirements by proposing the fast and reliable Search-Less Algorithm (SLA) for star pattern recognition which identifies the observed stars, seen by a wide FOV star tracker, within an overall large star catalog. This task is accomplished using stars angular separation only. SLA consists of: 1) the search-less K-vector StarPair Identification Technique (SPIT), and 2) the Reference-Star Star-Matching Identification Technique (SMIT). The proposed SLA working in conjunction with the ESOQ [1] (currently the
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