Achievement of the Magnetic Environment Requirements for Gravity Probe B

The proper function of the Gravity Probe B gyroscope readout system necessitated the most stringent magnetic environment requirements of any NASA flight program. We describe the generation of an ultra-low magnetic field of $<$ 10$^{-11}$ Tesla in the flight dewar, non-magnetic materials, fabrication, and assembly regimen to minimize remanent fields in the vicinity of the science gyros, and a magnetic shield system that attenuated external magnetic field variation by a factor of 10$^{12}$.~ Techniques for requirement verification, including the development of specialized magnetic measurement facilities, will be discussed.~ On orbit gyro trapped flux and readout data that confirm the achievement of the fundamental magnetic requirements will be presented.