Research on Novel Approach for Gyroless Angular Rate Measurement Based on Star Tracker
|School||National University of Defense Science and Technology|
|Keywords||Star tracker (Star sensor) Angular rate measurement On-orbitautonomous calibration Star spot location Inter-star-angle Adaptive filter Defocus adjustment|
With the development of autonomous navigation for deep space and smallspacecraft, there is often a desire to do away with gyros and use other means todetermine the angular rate. This dissertation focuses on proposing the novel approachfor gyroless angular rate measurement using a star tracker; and its key techniques, e.g.,angular rate estimation, star spot location and camera parameters calibration, have beendeveloped. The main contents and contribution of this dissertation are as follows:1. Gyrosless angular rate determination from measurement of star trackerTwo approaches (i.e., the difference method and star tracking filter method,respectively) have been put forward to determine the angular rate, while the attitude andstar reference vectors are not required to be known. In the difference method, theangular rate is determined by the backward difference of the vector measurements,which are obtained directly from the star tracker. In the star tracking filter method, theangular rate is determined according to the star spot location and its velocity, which areachieved using the tracking filter of the Singer model. Therefore, the needs fordifference and previous vector measurements of star tracker are eliminated.Adaptive Kalman filters are designed for the difference method and star trackingfilter method respectively. The Kalman filter states are updated on different guide starsin a same star image frame. Thus, the best estimates of angular rate can be achievedwithout the information of dynamic model. The simulations based on the orbit data ofnearly Earth-pointing satellite and experimental test with night sky observation areperformed. Both results indicate that the accuracies of the estimated angular rate are inthe order of10-6rad/s magnitude, when the refresh rate is10Hz.2. Star spot location estimationTwo approaches for star spot location estimation with the Kalman filter (i.e.,attitude-dependent and attitude-independent, respectively) have been proposed, whichconsist of three steps. The approximate locations of the star spots in successive framesare predicted by the Kalman prediction equation firstly; then the measurement locationsare acquired by defining a series of small m×m pixels windows around each predictivelocation. Finally, the star spot location is updated using the designed Kalman filter. Theremarkable advantage of the Kalman filter over other methods for star spot location isthat it combines the predictive location to give more accurate estimates than that byusing the achieved centroids alone. Furthermore, as the threshold scan is only performedin small windows, it takes rather less time and can avoid false star spots effectively.In the attitude-dependent approach, the prediction equation of the Kalman filter isconstructed according to the attitude matrix and angular rate. However, in theattitude-independent approach, the prediction equation is constructed according to the famous Singer model, which can estimate star spot location and velocity directly. It laysthe foundation of the star tracking filter method for angular rate determinion.3. Laboratory calibration for star tracker cameraA simple and available calibration approach for star tracker camera has been putforward. The calibration procedure consists of two steps:(1) the principal point isestimated using autocollimation adjustment;(2) the focal length and distortions areestimated via least-squares iteration, taking into account the extrinsic parameters.Compared with traditional method, in which the calibration data are acquired viarotation of the star tracker installed on a two-axes rotating stage, the star data ofdifferent field angles are acquired via rotation of the adjustable plane mirror in theproposed approach. It could decrease the setting error during the rotation of the startracker and has the advantages of simplicity, low cost, and high accuracy. Thetheoretical analysis and experimental results both indicate that the uncertainties of themeasured star direction vectors are less than4.0×105rad after calibration, which can befurther improved.Besides, an approach for defocus adjustment has been proposed for star tracker.This approach can measure the intensity distribution and spot size of pointsource imagebased on the calibration setup and Gaussian surface fitting method.4. On-orbit autonomous calibration for star tracker cameraThe calibration methods, i.e. attitude-dependent method and inter-star-anglemethod, are introduced to estimate parameters of on-orbit star tracker camera. And thenight sky observation is adopted to test their performance. In the attitude-dependentcalibration method, the camera internal parameters are combined with attitude matrices(or external parameters) determination. The error of attitude estimate will influence theerror of camera parameter estimate and vice-versa. Thus, the calibration accuracy isaffected by the number of stars identified as the control points. In the inter-star-anglecalibration method, a modified version of the least-squares iteration algorithmcombining Kalman filter is put forward on the assumption that the angle between twovectors is invariant. Compared with the traditional EKF (Extended Kalman Filter)method, the estimates are converged even for the poor initial guesses with nonlinearcamera distortions. The experimental results indicate that the deviation of the measuredinter-star angle is in the order of10-5rad magnitude after calibrated.The research method and conclusion would have significance in improving theperformance of gyroless angular rate measurement using star tracker.