On-board computer for star tracker

Abstract

On-board computer for Star tracker is designed for providing high accuracy attitude information to improve the location accuracy of the high resolution imagery of the Indian Remote Sensing Satellite (IRS-1C) scheduled to be launched by the middle of 1995. The on-board computer is based on a 16 bit microprocessor CMOS 80C86, which has an address capability of 1 MByte, operating at 3 MHz. The processor operates in minimum mode of configuration without numeric data processor (NDP). The on-board computer controls dual star sensors based on area array charge coupled device (CCD). The on-board computer searches for star in the field of view (FOV) by dynamic exposure sequencing and auto- thresholding. A background estimation is also done to avoid stray light in the sky. Once a star is searched and is within the FOV of the sensor, the star is continuously tracked until it leaves the FOV of the sensor, while taking care of the new star entry. The processor dynamically relocates the track window as the spacecraft moves in the orbit. The processing consists of centroid computation of star image by means of interpolation technique, valid star identification, attitude determination, data formatting for telemetry interface etc. The design details of the on-board computer is presented in this paper.