PRECISION STAR TRACKER FOR THE ADVANCED LAND OBSERVING SATELLITE (ALOS)

摘要

The Advanced Land Observing Satellite (ALOS) is a JAXA's flagship for the high-resolution Earth observation. Characterized by global data collection with 2.5m resolution, the ALOS mission addresses a set of stringent pointing requirements to provide precise geometric accuracy for observed images. To meet these requirements, the ALOS's Attitude and Orbit Control System (AOCS) is given stringent specifications: attitude determination accuracy (on-board: 3.0 x 10~(-4)deg, and ground-based: 1.4 x 10~(-4) deg), attitude stability (3.9 x 10~(-4) deg/5sec), and attitude control accuracy (0.095 deg). A precision star tracker (STT) was developed for achieving these attitude requirements. With the random error of 9arcsec (for 6mag) and the bias error of 0.74arcsec (both in 3σ) for single star position, this star tracker offers the highest star position accuracy. The accuracy is accomplished through the introduction of new designs that include precise thermal regulation, low thermal distortion structure, and low-noise high-resolution CCD. ALOS STT has 3 optical heads and uses 2 optical heads simultaneously. Each optical head has the field of view of 8 x 8deg and provides in every second the position and brightness for 5 stars in the tracking mode and for 10 stars in the acquisition mode. These data are used on-board with gyro's data for Kalman filtering in AOCS, and are also downlinked for ground-based attitude determination. In addition, this star tracker is capable of transmitting CCD's all pixel data for ground-based calibration. This paper presents the design and test results of the star tracker flight model, with a particular emphasis on its new designs.