Generation of Thinned Star Catalogs for Onboard Applications of Indian Remote Sensing Satellites

摘要

A systematic method to obtain a thinned core star catalog for use onboard a remote sensing satellite has been presented. The requirement of attitude accuracy of remote sensing earth observation (EO) missions is very stringent. Some factors that affect attitude accuracy are (ⅰ) the number of star vectors measurement, (ⅱ) the uniform distribution of identified star images in the FOV and (ⅲ) minimum or null duration of star gaps in orbit. Additionally, the performance of the attitude and orbit control system (AOCS) is closely linked to the characteristics of onboard star catalog, specifically, the total number of stars in the catalog and their spatial distribution. Nonetheless, the natural distribution of stars in the Universe is non-uniform and irregularly concentrated. Thinned optimum catalog has to be generated from all-star database such as SKYMAP-2000 by considering the limitations of a particular star sensor used, by tailoring to suit mission requirements and varied treatment of different regions. This has to be done until the catalog is uniform in some statistical sense. In this paper, a method to generate such a catalog, suitable for an Earth Observing Satellite (EOS) is presented. The EOS has two 20° x 20° FOV CCD array star sensors mounted slightly asymmetrically in order to avoid interference from other protruding surfaces on the FOV. The sensor has been designed to detect stars up to the visual magnitude of 6, for which, the actual number including multiple and variable stars is about 5020. Before the actual thinning process, the faintest of the near neighboring stars as well as the variable stars are completely removed. While dealing with multiple stars ,the star of the highest magnitude is retained and the coincident faint stars are removed from the catalog. This step improves the uniformity of the catalog markedly. Since the particular EOS is designed for a global mission, a master catalog, more uniform over the entire celestial space, is considered. For local uniformity, a criterion that allows a maximum of 8 stars in the 20° x 20° FOV has been analysed. This condition allows at best 2 stars in a 10° x 10° FOV and thereby enforces a necessary large angular separation between the selected stars. In this study, the actual number of stars from a 10° x 10° FOV is collected and of them, only two are retained. The criterion followed again is that the two brightest of the lot only are retained. With such procedures, the number of stars in the master catalog with 6.0 magnitude range is reduced to 1289. The uniformity of the resulting thinned catalogs is superior to the randomly generated thinned catalogs. It is seen that the number of stars in a master catalog can be brought down effectively up to a certain point; further decrement worsens the uniformity since they may not meet the criteria of minimum number of star availability in a selected FOV region. The selected star catalog from this procedure is integrated into the attitude determination (AD) system and the efficacy of the catalog is studied thoroughly. We find that the global and local uniformity of the thinned catalog based on the grid generation-quadrant separation (GQ) method is very good and meets the mission specifications in its entirety.