Abstract

The next generation of drag-free spacecraft will a) open up a new window to our universe through the currently illusive gravitational wave spectrum, b) more accurately map the time variations in the Earth’s mass distribution due to our changing global climate, and c) aid in our understanding of the fundamental force of gravity. These exciting missions require more accurate determination of the position of the free floating test mass contained within the drag-free spacecraft, as well as drastic reduction of the disturbances applied to the test mass. A sphere has the advantage of being orientation invariant. This removes all requirements for test mass orientation control with respect to the spacecraft, which ultimately reduces disturbances applied to the test mass. However, the free motion of the sphere coupled with its residual irregularities (geometric, electric, magnetic, etc.) can complicate the determination of the mass center position or the angular momentum orientation. This talk will present techniques for modeling and estimating the rigid body motion of a nearly torque-free sphere in order to separate the effects of the irregularities from the desired measurement. These techniques are applied to the flight data from Gravity Probe B (GP-B) in order to estimate the spin and polhode motion of the GP-B gyroscopes to 1 degree over the course of a year, as well as the readout scale factor to 10^-4. Then we will examine an advanced Gravitational Reference Sensor capable of measuring the mass center location of its spherical test mass to 10^-11 m, which will enable for future gravitational wave observatories, geodesy missions and fundamental physics tests in space. Laboratory demonstrations of key technologies for this sensor are described, focusing on a novel method for determining the mass center of a spherical test mass to about 100 nm, a factor of 10 improvement over any previous method.

Please see the Faculty Candidate Seminar Announcement for more details.