Stellar attitude-control systems and methods with weighted measurement-noise covariance matrices

摘要

Stellar attitude-control systems and methods are provided with enhanced accuracy because they recognize that important star tracker errors exhibit a boresight symmetry and that these errors can be accurately defined by weighting a measurement-noise covariance matrix R(t_n) with variances that are functions of off-boresight angles of detected stars. A method of the invention derives off-boresight angles from star-tracker signals of detected stars. These off-boresight angles are combined with variance coefficients to generate off-boresight variances r_ob(t_n) that are functions of the off-boresight angles and, in particular, correspond to star tracker color shift errors. A gain matrix K(t_n) is then calculated with a weighted measurement-noise covariance matrix R(t_n) that includes the off-boresight variances r_ob(t_n). This gain matrix is used to generate an attitude estimate matrix X*(t_n) for use in spacecraft attitude control. The variance coefficients are preferably modified to include star tracker focal length shift errors which are also a function of off-boresight angles . It is noted that the variance coefficients can be reduced by identifying the spectral classes of detected and identified stars. The invention recognizes that Other star tracker errors that are not functions of off-boresight angles are modeled with appropriate constant variances r_c(t_n) that are used to modify the covariance matrix R(t_n).