Mass unbalance analysis for GP-B rotors

Using three different approaches, laboratory results were obtained for the mass unbalance of the NASA Gravity Probe B rotors designed to test Einstein's gravitational theory. In the first method, measurements of the precession rate of the spinning rotor are deduced from spin-frequency components of the trapped flux. The other approaches involve the measuring of niobium coating thickness using an electron back-scattering device and the measuring of the periodicities of a tumbling rotor. Assumptions of the methods are discussed, along with reasons for discrepancies among the results.