Multisensor data integration in the NASA/Stanford gravity probe B relativity mission

Gravity Probe B (GP-B) is a gravitational experiment designed to measure two predicted by General Theory of Relativity precessions of a free-falling gyroscope placed in a polar orbit around the Earth. The frame-dragging effect (drift perpendicular to the orbital plane) has never been directly measured before, while the geodetic effect (drift in the orbital plane) will be measured with an unprecedented accuracy. GP-B Relativity Mission is an example of a system with multisensor data integration: it requires the optimal processing of information coming from at least nine physical sources (four science gyroscopes, the science telescope, the attitude control system of the spacecraft on-board GPS receivers, NASA/JPL Earth ephemerides, and the astronomical data on the proper motion of the reference star). This paper presents the structure of the GP-B data integration and the core filtering approach to the state-vector estimation. Two-step nonlinear filter (that may be applied for different problems with nonlinear measurements) is discussed and the specifics of its implementation for the GP-B data analysis is presented.