Fast Access Method for Onboard Star Catalog
G. Nagendra Rao, M. Seetharama Bhat, T. K. Alex · Journal of Guidance Control and Dynamics · 2005
A new method to retrieve the star data from the onboard catalog is presented. A query to find stars that lie within a cone of uncertainty angle of a given unit vector is answered quickly. If stars are present, a set of catalog numbers is returned along with the nearest neighbor to the center of the cone. An index to the star catalog is formed with a hash function that maps the given unit vector to an index in a hash table. The hash function preserves the stars’ closeness even after their translation to the hash table and thus facilitates the query of a star with a noisy measurement. The proposed organization of the star catalog, other data structures used, and their access methods are described. When subjected to a numerical simulation test with 10,000 random star vectors corrupted with noise, all of the queries are answered correctly. A hash table of 5773-word (16-bit) size is required for a star catalog containing 1613 stars (10,485 words). This method requires only 2.2 average catalog accesses and 2.0-μs process time per query, compared to a traditional binary search that requires 24 accesses and 3.8 μs.