Effects Of Image Sensor Noise On Position Accuracy Of Star

The star sensor (or star tracker) is a kind of absolutely independent instrument used to provide the absolute 3-axis attitude of a spacecraft and it is one of the important components which give the high accuracy position information of the satellites,space shuttle,and space station. The image sensor noise limits the improvement of the attitude accuracy measured by the star sensor. It's necessary to study the effect mechanism about the photoelectric noise of star sensor in order to manufacture a star sensor with better performance. The study in this paper is based on two methods producing noise and the main works are presented as follows.1. The mechanism of the noise was analyzed. The model of the CCD (Charge Coupled Devices) noise and the model of the SNR of the detection system were established. On the base of the model of the star point centroid extraction, the mechanism of the photoelectric noise effects on the position accuracy of the star sensor was studied. The results show that it's effective to improve the poison accuracy of the star point by choosing suitable windows used for the centroid extraction and the energy distribution of the star point,prolonging the integral time,reducing the magnitude of the target star and the work temperature of the CCD chip. The relationship between the limited star magnitude and the Signal to Noise Ratio(SNR) was gained according to the signal detection theory. The limited star magnitude reached to 9.5m when the aperture integral was 1(cm2.s) for the system used CCD 47-20.2. The star motion is inevitable for the oppositely motion between the target star and the star sensor. The effect of noise was studied based on the simulating of the noise and the energy distribution of star point when the star motion was considered. The results indicated that the effects of CCD noise on the star position accuracy increased with the star motion, and the position accuracy decreased by 0.0036 pixel when the star motion was 3 pixels and the magnitude of the object star was 6m.3. The effect of CCD noise on the star point position accuracy was studied through experiment. The results showed that, the star point position of 0.004 pixel is achieved when the comparative magnitude of the object star is 2m, and the changes of the Gauss width and the chip's temperature affected the star point position accuracy. The accuracy of the measured angle for different lighteness star pairs was also studied. It indicated that the identification accuracy is improved by choosing suitable target stars.4. The experiment of centroid extraction of the star point image with different spectrum was accomplished, which overcomed the limitation of lack of spectrum information during current research. The results of the experiment indicated that, the image position of the red star point deviates from blue star point by 0.7644 pixel, since the effect of chromatic aberration. Moreover, there wasn't obvious relationship between the effect of CCD noise on the position accuracy of star point with the spectrum component of colorized star.