Patent ID: 7062363
Filing Date: 2006-06-13
Classification: B64G,G01C,G01S

Abstract:
1. A method of refining a spacecraft state estimate, comprising the steps of: determining observed star positions in a first reference frame, ST, fixed with respect to a star sensor reference frame, for a plurality of stars observed at times t i for i=1, 2, . . . , N; converting the observed star positions in the first reference frame ST, for a plurality of stars observed at times t i for i=1, 2, . . . , N into observed star positions in a second reference frame, b, fixed with respect to a spacecraft body reference frame, for the plurality of stars observed at times t i for i=1, 2, . . . , N; determining an estimated spacecraft angular velocity, determining, through identification of the plurality of stars as corresponding to entries in a star database, the star positions predicting star positions with respect to the second reference frame b, for the plurality of stars observed at times t i for i=1, 2, . . . , N, from the star positions in the inertial reference frame inertial {right arrow over (s)} i , estimated spacecraft angular velocity est {right arrow over (ω)} and an estimated spacecraft attitude est {right arrow over (q)} b inertial (t a ), applicable at time t a ; determining residuals in the second reference frame b for the plurality of stars observed at times t i for i=1, 2, . . . , N, from a difference between the predicted star positions in the second reference frame and the observed star positions in the second reference frame determining N equations for differences between the refined star positions in the second reference frame b and observed star positions in the second reference frame b at times t i for i=1, 2, . . . , N as a function of the residuals in the second reference frame for the plurality of stars observed at times t i for i=1, 2, . . . , N, and a refined spacecraft state estimate; and determining the refined spacecraft state estimate to minimize the differences between the refined star positions in the second reference frame and observed star positions in the second reference frame at times t i for i=1, 2, . . . , N from the N equations.