The Gravity Probe B gyroscope

Abstract

The Gravity Probe B (GP-B) gyroscope, a unique cryogenically operated mechanical sensor, was used on-orbit to independently test two predictions of general relativity (GR). Here, we describe the development and performance of the GP-B gyroscope, its geometry and fabrication, spin-up and vacuum approach, magnetic considerations, and static charge management. The history of electrically suspended gyroscopes puts the current work in context. Fabrication and ground testing of the GP-B gyroscope are detailed, followed by a review of on-orbit initialization, calibration, operation, and performance. We find that the performance was degraded relative to the mission goals, but was still sufficient to provide excellent new tests of GR. The degradation is partially due to the existence of gyroscope torques due to an unanticipated interaction between patch potentials on the rotor and the housing. We discuss these patch potentials and describe the effect of related torques on gyro drift. It was essential to include models for the effects due to the patch potentials in the complete data analysis model to yield determinations of the two GR effects.

Effective decrease of photoelectric emission threshold from gold plated surfaces

“…However GP-B, UV-LED and LISAPF were validated through empirical testing 2,14,15 , and subsequent in-flight performance 2,16,17 . This study reports on a proposed 'working' model to describe the sub-threshold electron emission from gold (section II), an experimental measurement campaign to validate the model (section III) and the results from it (section IV).…”

Precision attitude control of the Gravity Probe B satellite

“…Use of the science gyroscopes for attitude reference is discussed in section 3.3 of this paper, although in practice it was not implemented on-orbit. A comprehensive description of the GP-B science gyroscopes can be found in [12]. The sensors used by the ATC system included both commercial components and custom instruments, like the science telescope, which were related to the GP-B science measurement.…”

Gravity Probe B gyroscope readout system

“…The rotor is fabricated from fused-quartz, ground and polished to form a nearly perfect 38 mm diameter sphere [9]. It is coated with 1.25 μm of niobium [10], deposited using an rf diode sputter system. The housing holds the rotor and is comprised of two halves: spinup and readout.…”

“…With the rotor delevitated, dc magnetic flux coupled into the pickup loop confirmed superconductivity. See Buchman [10], this issue, for a gyroscope overview.…”