Superconducting Angular Accelerometers for the Superconducting Gravity Gradiometer Experiment

Abstract

A test of the University of Maryland’s Superconducting Gravity Gradiometer1 (SGG) on the Space Shutde is currendy being studied. The SGG can measure the spatial gradient of Earth’s gravity to very high precision, having a sensitivity of 10-3 E Hz-1/2 (1 E≡10-9 m/s2 per m) in each axis. For successful application of this instrument in an orbital platform, a number of difficulties must be overcome. One is that the measured gravity signal is strongly dependent on the orientation of the instrument with respect to Earth. Another is that centrifugal acceleration and gravity gradients are indistinguishable in a rotating reference frame. Also, angular accelerations can couple to the gradient output through imperfections in the instrument. It is therefore necessary to measure the angular position, velocity and acceleration of the gradiometer in order to recover the true gravity signal. Superconducting angular accelerometers are described which can meet the sensitivity requirements for all three quantities. The acceleration signal will be integrated once to obtain angular velocity and then again for position. Angular position measurement is most critical, and sets a requirement of measuring accelerations to 10-10 rad/s2 Hz-1/2. The sensors must be compatible with a cryogenic environment since they will be mounted onto the SGG. Two designs are being pursued, differing significantly only in how the proof mass is confined to rotations about a single axis. Both use niobium proof masses, superconducting position sensing circuits, and SQUIDs for signal readout as does the SGG. Design parameters and anticipated performance are described.