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What is claimed is: 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 ti for i=1, 2, . . . , N; converting the observed star positions in the first reference frame ST, for a plurality of stars observed at times ti for i=1, 2, . . . , N into observed star positions in a second reference frame, b, fixed with respect to a spacecraft body reference frame, f...
What is claimed is: 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 ti for i=1, 2, . . . , N; converting the observed star positions in the first reference frame ST, for a plurality of stars observed at times ti 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 ti for i=1, 2, . . . , N; determining an estimated spacecraft angular velocity, est{right arrow over (ω)}, at times ti for i=1, 2, . . . , N; determining, through identification of the plurality of stars as corresponding to entries in a star database, the star positions inertial{right arrow over (s)}i with respect to an inertial reference frame inertial, for i=1, 2, . . . , N; predicting star positions with respect to the second reference frame b, for the plurality of stars observed at times ti 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(ta), applicable at time ta; determining residuals in the second reference frame b for the plurality of stars observed at times ti 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 ti for i=1, 2, . . . , N as a function of the residuals in the second reference frame for the plurality of stars observed at times ti 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 ti for i=1, 2, . . . , N from the N equations. 2. The method of claim 1, wherein the step of 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 ti for i=1, 2, . . . , N as a function of the residuals in the second reference frame b for the plurality of stars observed at times ti for i=1, 2, . . . , N, and the refined spacecraft state estimate comprises the steps of: determining N intermediate equations for refined star positions in the second reference frame b at times ti for i=1, 2, . . . , N corresponding to the star positions inertial{right arrow over (s)}i with respect to an inertial reference frame inertial, for i=1, 2, . . . , N, as a function of the refined spacecraft state estimate, and the predicted star positions of the plurality of stars in the second reference frame b at times ti for i=1, 2, . . . , N; and determining the N equations from the N intermediate equations and the observed star positions in the second reference frame b. 3. The method of claim 1, wherein the inertial reference frame is an earth centered inertial reference frame. 4. The method of claim 1, wherein the observed star positions in the first reference frame are converted into observed star positions in a second reference frame fixed with respect to a spacecraft body reference frame for the plurality of stars at times ti for i=1, 2, . . . , N via a mapping based on one or more star sensor alignments. 5. The method of claim 1, wherein the state estimate comprises a spacecraft attitude estimate and the refined spacecraft state estimate comprises a refined attitude estimate. 6. The method of claim 5, wherein the refined attitude estimate is computed from the estimated spacecraft attitude est {right arrow over (q)}b--inertial (ta) adjusted by an attitude refinement. 7. The method of claim 1, wherein the state estimate comprises a spacecraft angular velocity estimate, and the refined spacecraft state estimate comprises a refined spacecraft angular velocity estimate. 8. The method of claim 7, wherein the refined spacecraft angular velocity estimate is computed from the estimated spacecraft angular velocity est{right arrow over (ω)} adjusted by an angular velocity refinement. 9. The method of claim 1, wherein a representation of the estimated spacecraft attitude est{right arrow over (q)}b --inertial(ta) comprises a quaternion. 10. The method of claim 1, wherein a representation of the estimated spacecraft attitude est{right arrow over (q)}b --inertial(ta) comprises a direction cosine matrix. 11. The method of claim 1, wherein the step 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 ti for i=1, 2, . . . , N comprises the step of measuring the observed star positions with at least one star tracker. 12. An apparatus for refining a spacecraft state estimate, comprising: a star sensor configured to determine for 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 ti for i=1, 2, . . . , N; a navigation subsystem configured to convert the observed star positions in the first reference frame ST, for a plurality of stars observed at times ti 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 ti for i=1, 2, . . . , N; determine an estimated spacecraft angular velocity, est{right arrow over (ω)}, at times ti for i=1, 2, . . . , N; and for determine through identification of the plurality of stars as corresponding to entries in a star database, the star positions inertial{right arrow over (s)}i with respect to an inertial reference frame inertial, for i=1, 2, . . . , N; a predictor module configured to predict star positions with respect to the second reference frame b, for the plurality of stars observed at times ti 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(ta), applicable at time ta; a differencer configured to determine residuals in the second reference frame b for the plurality of stars observed at times ti 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 an equation formulator configured to determine 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 ti for i=1, 2, . . . , N as a function of the residuals in the second reference frame for the plurality of stars observed at times ti for i=1, 2, . . . , N, and a refined spacecraft state estimate; and a solver configured to determine 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 ti for i=1, 2, . . . , N from the N equations. 13. The apparatus of claim 12, wherein the equation formulator determines N intermediate equations for refined star positions in the second reference frame b at times ti for i=1, 2, . . . , N corresponding to the star positions inertial{right arrow over (s)}i with respect to an inertial reference frame inertial, for i=1, 2, . . . , N, as a function of the refined spacecraft state estimate, and the predicted star positions of the plurality of stars in the second reference frame b at times ti for i=1, 2, . . . N, and determines the N equations from the N intermediate equations and the observed star positions in the second reference frame b. 14. The apparatus of claim 12, wherein the inertial reference frame is an earth centered inertial reference frame. 15. The apparatus of claim 12, wherein the observed star positions in the first reference frame are converted into observed star positions in a second reference frame fixed with respect to a spacecraft body reference frame for the plurality of stars at times ti for i=1, 2, . . . , N via a mapping based on one or more star sensor alignments. 16. The apparatus of claim 12, wherein the state estimate comprises a spacecraft attitude estimate and the refined spacecraft state estimate comprises a refined attitude estimate. 17. The apparatus of claim 16, wherein the refined attitude estimate is computed from the estimated spacecraft attitude est {right arrow over (q)}b--inertial (ta) adjusted by an attitude refinement. 18. The apparatus of claim 12, wherein the state estimate comprises a spacecraft angular velocity estimate, and the refined spacecraft state estimate comprises a refined spacecraft angular velocity estimate. 19. The apparatus of claim 18, wherein the refined spacecraft angular velocity estimate is computed from the estimated spacecraft angular velocity est{right arrow over (ω)} adjusted by an angular velocity refinement. 20. The apparatus of claim 12, wherein a representation of the estimated spacecraft attitude est{right arrow over (q)} b--inertial(ta) comprises a quaternion. 21. The apparatus of claim 12, wherein a representation of the estimated spacecraft attitude est{right arrow over (q)} b--inertial(ta) comprises a direction cosine matrix. 22. An apparatus for refining a spacecraft state estimate, comprising: means for 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 ti for i=1, 2, . . . , N; means for converting the observed star positions in the first reference frame ST, for a plurality of stars observed at times ti 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 ti for i=1, 2, . . . , N; means for determining an estimated spacecraft angular velocity, est{right arrow over (ω)}, at times ti for i=1, 2, . . . , N; means for determining, through identification of the plurality of stars as corresponding to entries in a star database, the star positions inertial{right arrow over (s)}i with respect to an inertial reference frame inertial, for i=1, 2, . . . , N; means for predicting star positions by with respect to the second reference frame b, for the plurality of stars observed at times ti for i=1, 2, . . . , N, from the star positions in the inertial reference frame inertial {right arrow over (s)}i, estimated spacecraft angular velocity est6 and an estimated spacecraft attitude est{right arrow over (q)}b--inertial(ta), applicable at time ta; means for determining residuals in the second reference frame b for the plurality of stars observed at times ti 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 means for 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 ti for i=1, 2, . . . , N as a function of the residuals in the second reference frame for the plurality of stars observed at times ti for i=1, 2, . . . , N, and a refined spacecraft state estimate; and means for 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 ti for i=1, 2, . . . , N from the N equations. 23. The apparatus of claim 22, wherein the means for 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 ti for i=1, 2, . . . , N as a function of the residuals in the second reference frame b for the plurality of stars observed at times to for i=1, 2, . . . , N, and the refined spacecraft state estimate comprises: means for determining N intermediate equations for refined star positions in the second reference frame b at times ti for i=1, 2, . . . , N corresponding to the star positions inertial{right arrow over (s)}i with respect to an inertial reference frame inertial, for i=1, 2, . . . , N. as a function of the refined spacecraft state estimate, and the predicted star positions of the plurality of stars in the second reference frame b at times ti for i=1, 2, . . . , N; and means for determining the N equations from the N intermediate equations and the observed star positions in the second reference frame b. 24. The apparatus of claim 22, wherein the inertial reference frame is an earth centered inertial reference frame. 25. The apparatus of claim 22, wherein the observed star positions in the first reference frame are converted into observed star positions in a second reference frame fixed with respect to a spacecraft body reference frame bobs{right arrow over (s)}(ti) for the plurality of stars at times ti for i=1, 2, . . . , N via a mapping based on one or more star sensor alignments. 26. The apparatus of claim 22, wherein the state estimate comprises a spacecraft attitude estimate and the refined spacecraft state estimate comprises a refined attitude estimate. 27. The apparatus of claim 26, wherein the refined attitude estimate is computed from the estimated spacecraft attitude est {right arrow over (q)}b--inertial (ta) adjusted by an attitude refinement. 28. The apparatus of claim 22, wherein the state estimate comprises a spacecraft angular velocity estimate, and the refined spacecraft state estimate comprises a refined spacecraft angular velocity estimate. 29. The apparatus of claim 28, wherein the refined spacecraft angular velocity estimate is computed from the estimated spacecraft angular velocity est{right arrow over (ω)} adjusted by an angular velocity refinement. 30. The apparatus of claim 22, wherein the representation of the estimated spacecraft attitude est{right arrow over (q)} b--inertial(ta) comprises a quaternion. 31. The apparatus of claim 22, wherein a representation of the estimated spacecraft attitude est{right arrow over (q)} b--inertial(ta) comprises a direction cosine matrix. 32. The apparatus of claim 22, wherein the step 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 ti for i=1, 2, . . . , N comprises the step of measuring the observed star positions with at least one star tracker.
