1. A method of refining a spacecraft state estimate, comprising the steps of: determining observed star positions ST ⁢ s obs ⇀ ⁡ ( t i ) 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 ST ⁢ s obs ⇀ ⁡ ( t i ) 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 b ⁢ s obs ⇀ ⁡ ( t i ) 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, est {right arrow over (ω)}, at times t i 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 b ⁢ s ⇀ pred ⁡ ( t i ) , 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 b ⁢ s ⇀ res ⁡ ( t i ) 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 b ⁢ s ⇀ pred ⁡ ( t i ) and the observed star positions in the second reference frame b ⁢ s ⇀ obs ⁡ ( t i ) ; determining N equations for differences between the refined star positions b ⁢ s ⇀ refined ⁡ ( t i ) in the second reference frame b and observed star positions b ⁢ s ⇀ obs ⁡ ( t i ) 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 b ⁢ s ⇀ res ⁡ ( t i ) 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 b ⁢ s ⇀ refined ⁡ ( t i ) and observed star positions in the second reference frame b ⁢ s ⇀ obs ⁡ ( t i ) at times t i for i=1, 2,. .. , N from the N equations.