  Magnetic flux trapped on the surface of superconducting rotors of the Gravity
Probe B (GP-B) experiment produces some signal in the SQUID readout. For the
needs of GP-B error analysis and simulation of data reduction, this signal is
calculated and analyzed in the paper. We first solve a magnetostatic problem
for a point source (fluxon) on the surface of a sphere, finding the closed form
elementary expression for the corresponding Green's function. Second, we
calculate the flux through the pick-up loop as a function of the fluxon
position. Next, the time dependence of a fluxon position, caused by rotor
motion according to a symmetric top model, and thus the time signature of the
flux are determined, and the spectrum of the trapped flux signal is analyzed.
Finally, a multi-purpose program of trapped flux signal generation based on the
above results is described, various examples of the signal obtained by means of
this program are given, and their features are discussed.
