ESA GNC Conference Papers Repository

Star Tracker (STT) has come into wide use as main attitude sensor for its accurate and autonomous characteristics in various space missions. Especially in scientific and Earth observation missions, a requirement for attitude determination accuracy is becoming higher and higher than before. In order to achieve such requirements, it is important that error budgets of STT should be defined clearly and each error factor should be reduced. Although the reduction of random errors might be possible when attitude determination sensor is used with precision gyros, errors such as bias errors, thermal distortion errors and low frequency spacial errors still remain. Besides, high frequency spacial errors could be residual if the attitude rate is low. So the reduction of these errors is essential for realization of high accurate STT. We developed an autonomous star tracker called NSTT with high accuracy and versatility. With regard to the accuracy, following attitude determination accuracy was expected; bias error : < 6 arcsec (from -25 to +55 ?) random error : < 4 arcsec (3s) During the development process, however, a problem with thermal distortion that the thermal distortional error against the change in temperature was larger than expected occurred. Finally, bias error requirement was satisfied in mitigated condition, +20 to +55 ?. As the result of the cause investigation against this problem, we found the lens mounting method affected the increase of the thermal distortion error, and the refinement of the head of NSTT successfully solved this problem. By applying the methods of reduction errors of STT to NSTT, it could be possible to improve its accuracy without hardware change that both bias error and random error are less than 2 arcsec. With the new hardware design, it is conceivable that very high accurate model can be realized such as both errors are much less than 1 arcsec. For the purpose of future requirements, we also study the methods of downsizing and lightening of STT while maintaining high accuracy. In this paper, methods of improving accuracy of STT, the result of applying the methods, and the prospect of high accurate STT are reported.