Spin-Axis Attitude Determination and Covariances in Local Sun-Earth Frame

This paper presents practical analytical models for spin-axis attitude determination using measurements generated by sun and Earth sensors. In particular, explicit expressions for the least-squares estimates and their associated covariance matrices are established. The adoption of the local reference frame defined by the instantaneous sun and Earth vectors leads to compact analytical results. These results provide a more immediate understanding of the attitude error covariance matrix in terms of its dependence on the sun–Earth geometry. The application of the covariance results is illustrated by using the actual geometrical in-orbit conditions of two satellites.