<title>Autonomous star trackers for geostationary satellites</title>

Abstract

An autonomous star tracker (AST) is a starfield in, 3-axis attitude out device capable of determining its attitude rapidly without requiring any a-priori attitude knowledge. Following attitude acquisition, the AST switches to a track mode where it outputs its attitude and rate at typically 5 Hz. The Lockheed Martin Advanced Technology Center has developed the first version of a line of reliable, highly accurate, low cost, ASTs. This AST-201 is to be test flown on a sounding rocket and will function as a key attitude sensor aboard NASA's Small Spacecraft Technology Initiative (SSTI) Clark spacecraft scheduled for launch in 1996. The AST is of modular construction and comprises athermalized, radiation hard refractive optics, a frame-transfer CCD with a sensitive area of 512 by 512 pixels, camera electronics, a single board computer, an all-sky guide star database and a highly effective sunshade that allows operation to within 25 degrees from the Sun. Initial star identification is performed by a memory-efficient algorithm that provides an attitude acquisition success rate of better than 99.85% anywhere in the sky. The AST achieves its high accuracy through use of a moderate 8.8 by 8.8 degree field of view and by tracking 27 stars on average. On geostationary satellites a single AST can provide 3-axis attitude with greater precision, at lower cost, and with higher reliability than is possible with a combination of Earth sensors, fine Sun sensors, and a high performance gyro system. In addition, ASTs are devoid of angular range limitations, avoid Sun and Moon interference by proper orientation on the spacecraft, and enable rapid fault recovery, a capability especially important to geostationary missions where outages are usually costly. Two ASTs can provide a geostationary spacecraft with an attitude accuracy of 5 microradians (1 a). The paper contains a description of the AST, a summary of the functions enabled or improved by the device, real-sky AST test results, and accuracy statistics for an AST on a geostationary spacecraft, as obtained through realistic simulations. Keywords: star tracker, autonomy, attitude determination, pattern recognition, geostationary satellites, spacecraft attitude control.