Mathematics – Probability
Scientific paper
Mar 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003spie.4850..108v&link_type=abstract
IR Space Telescopes and Instruments. Edited by John C. Mather . Proceedings of the SPIE, Volume 4850, pp. 108-121 (2003).
Mathematics
Probability
Scientific paper
Two redundant AST-301 autonomous star trackers (AST) serve as the primary attitude sensors for JPL's space infrared telescope facility (SIRTF). These units, which employ a 1553B interface to output their attitude quaternions and uncertainty at a 2 Hz rate, provide a 1 σaccuracy of better than 0.18, 0.18, and 5.1 arcsec about their X, Y, and Z axes, respectively. This is a factor 5.5 better than the accuracy of the flight-proven AST-201 from which the trackers were derived. To obtain this improvement, the field of view (FOV) was reduced to 5 by 5 degrees, the accurate Tycho-1 and ACT catalogs were used for selecting the 71,830 guide stars, star image centroiding was improved to better than 1/50th of a pixel, and optimal attitude estimation was implemented. In addition, the apparent direction to each guide star in the FOV is compensated for proper motion, parallax, velocity aberration, and optical distortion. The AST-301 employs autonomous time-delayed integration (TDI) to achieve image motion compensation (IMC) about its X axis that prevents accuracy degradation, even at rates of 2.1 deg/s, making it actually suitable for use on spinning spacecraft. About the Y axis, a software function called "image motion accommodation" (IMA) processes smeared images to maximize the signal to noise ratio of the resulting synthetic images, which enables robust and accurate tracking at rates tested up to 0.42 deg/s. The AST-301 is capable of acquiring its attitude anywhere in the sky in less than 3 seconds with a 99.98% probability of success, without requiring any a priori attitude knowledge. Following a description of the 7.1 kg AST-301, its operation and IMA, the methodology for translating the night sky test data into performance numbers is presented, while, in addition, the results of tests used to measure alignment stability over temperature are included.
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