| The work presented in this thesis aims at modeling a perturbation in the science signal of the Gravity Probe B (GP-B) experiment.; The GP-B satellite was launched in 2004. It contains four spherical gyroscopes used to test predictions of general relativity (OR). According to OR, the spin axis of the gyroscopes should drift in two perpendicular directions and the goal of the experiment is to measure the rates of these drifts.; As the gyroscopes are superconductive, they expel a magnetic field, the London moment, which is parallel to their spin axis. The orientation of each gyroscope is then measured using a low-noise SQUID magnetometer. Because of the relativistic drift of the gyroscopes, a slow drift is expected in the SQUID signal: the main science information is thus found in the low frequency part of this signal.; There is however another component to the field measured by the SQUID: as on type II superconductive bodies, point sources of magnetic field called fluxons are frozen on the gyroscopes' surface. These fluxons therefore spin with the body and create high frequency modulations of the SQUID signal. However, the resultant field created by the fluxons also has a spin-averaged component aligned with the spin axis. This component shows up in the low frequency of the SQUID signal: it is then a perturbation to the London moment measurement.; The goal of this work is to estimate the magnetic potential distribution created by the fluxons using the high frequency of the SQUID signal in order to model their low frequency contribution. We show that this requires a precise knowledge of the gyroscope's motion. The second chapter of this thesis is thus devoted to the measurement of the gyroscopes' spin and precession frequencies. The precession rate was found to vary with time, in contradiction with predictions made before the launch of the satellite, and the third chapter thus focuses on interpreting this phenomenon. The fourth finally chapter describes the procedure designed and implemented to estimate the magnetic potential distribution around the gyroscopes and the fluxons' contribution to the low frequency SQUID signal. |
