Patent ID: 7227114

Claim:
A method of determining the attitude (φ, θ, Ψ) of a spacecraft, the method comprising: a) sensing the projections (xmi, ymi) of a plurality of stars on an optical star sensor mounted on the spacecraft; b) providing a reference focal length (f ref ) for said sensor, said focal length serving to convert the projections of the stars (xmi, ymi) as picked up on the sensor into sensor-star directions (wi) in a frame of reference associated with the sensor; and c) determining the angular values of the attitude of the spacecraft relative to the stars by solving a system of equations which has as its input data in particular the projections of the stars (xmi, ymi) on the sensor and the reference focal length of the sensor; the method being characterized by introducing, as unknowns in the system of equations, at least one parameter (D 1 , . . . , D 7 ) defining a series of focal length errors, each of which errors has an influence on converting the projection (xmi, ymi) of a respective star into the corresponding sensor-star direction (W i ); and in step c), by determining said parameter(s) (D 1 , . . . , D 7 ) simultaneously with determining the values for the attitude angles (φ, θ, Ψ) of the spacecraft; in such a manner that taking said focal length error parameter(s) into account as unknowns in the system reduces the uncertainty introduced by incomplete knowledge of these errors on the results for the values of the attitude angles (φ, θ, Ψ) of the spacecraft.