Tracking Gravity Probe B gyroscope polhode motion

The superconducting Gravity Probe B spacecraft is being developed to measure two untested predictions of Einstein's theory of general relativity by using orbiting gyroscopes; it possesses an intrinsic magnetic field which rotates with the rotor and is fixed with respect to the rotor body frame. In this paper, the path of the rotor spin axes is tracked using this trapped magnetic flux as a reference. Both the rotor motion and the magnetic field shape are estimated simultaneously, employing the higher order components of the magnetic field shape.