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(tn) with variances that are functions of off-boresight angles of detected stars. A method of the invention derives off-boresight angles theta from star-tracker signals of detected stars. These off-boresight angles theta are combined with variance coefficients alpha to generate off-boresight variances rob(tn) that are functions of the off-boresight angles theta and, in particular, correspond to star tracker color shift errors. A gain matrix K(tn) is then calculated with a weighted measurement-noise covariance matrix R(tn) that includes the off-boresight variances rob(tn). This gain matrix is used to generate an attitude estimate matrix X*(tn) for use in spacecraft attitude control. The variance coefficients alpha are preferably modified to include star tracker focal length shift errors which are also a function of off-boresight angles theta. It is noted that the variance coefficients alpha 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 rc(tn) that are used to modify the covariance matrix R(tn).