Gyroscopes and charge control for the Relativity Mission Gravity Probe B Original Research Article

The most demanding goal of the Gravity Probe B Relativity Mission (GP-B) is the measurement of the parametrized post-Newtonian parameter γ to one part in 105. This goal requires a total experimental accuracy of ≤ 0.044 marcsec/yr. Analysis of and results from 100,000 hours of gyroscope operation on the ground show that the residual Newtonian drift will be < 0.17 marcsec/yr for a supported gyroscope in 10−9 m/s2, and < 0.020 marcsec/yr for an unsupported gyroscope in a fully inertial orbit. The expected error due to gyroscope drift is thus consistent with the measurement goal. The main gyroscope disturbance caused by cosmic radiation is charging of the rotor. A force modulation technique allows measurement of the charge of the gyroscope rotor to about 5 pC, while bipolar charge control to 10 pC is achieved using electrons generated by UV photoemission.