GP-B Post-Flight Analysis—Final Report - Gravity Probe B - Stanford ...

Table 12-2. Measured trapped field compared to the London moment equivalent field in each of the gyrorotors.GyroNo.Trapped field (μG) (Uniformfield equivalent and asaligned late in mission)12.10.2 AC ShieldingLondon moment equiv.field (μG)The second function of the magnetic shielding system is to prevent time-varying fields from coupling to thereadout pickup loops and contaminating the science signal. In this regard, the shields built into the dewar aresupplemented by superconducting cylindrical local shields around each gyro and by the diamagnetic effect ofthe superconducting rotors themselves. As it turns out, the limits on the AC magnetic shielding derived fromthe data while the gyros were caged early in the mission were not as tight as the limits obtained from the groundverification, which were 2x10 -12 (-234 db) attenuation. In any case, no evidence of a signal with the signature ofa leakage field was observed above the noise. In addition, the fact that the ambient dc field is nominal is goodprima facie evidence that the AC shielding has not been degraded from the ground test result.12.11 Probe Vacuum PerformanceRatio of trapped field toLondon moment1 3.1 57 0.0552 1.3 44 0.0293 0.859 59 0.0144 0.2 47 0.005Vacuum in the Probe is important during science data acquisition because of the damping effect residualhelium has on the gyro rotors. Of particular concern is differential damping wherein the component of spinalong the roll axis decays at a different rate than the component transverse to the roll axis. This can cause aprecession if the spin and roll axes are not exactly aligned. In order to keep this effect to an acceptable level,there is a requirement that the Probe pressure be less than or equal to 2x10 -10 torr after the low temperaturebakeout. In the absence of other damping torques, residual pressure can be determined by measurement of thespin-down decay time. In actuality, the spin-down decay time varied between 7,000 years for gyro 3 and 25,700years for gyro 4, however. This variability, along with other evidence, supports the contention that the spindownrates were dominated by some other mechanism (possibly patch effect) rather than by gas damping. Byusing the longest decay time, however, an upper limit of ~1.5x10 -11 torr (2 nPa) can be placed on the residualgas pressure—a result that is an order of magnitude below the requirement, demonstrating ample margin.Gravity Probe B — Post Flight Analysis • Final Report March 2007 349