Superconducting techniques for gravity survey and inertial navigation

Major improvements in sensitivity and drift can be made in inertial instruments by utilizing benign properties of materials available at liquid helium temperatures. We are developing a three-axis gravity gradiometer in which magnetic fields produced by persistent currents are modulated by motions of superconducting proof masses. Signals arising from both differential and common mode motions of the proof masses are detected by SQUIDs. The present design parameters give an intrinsic gravity gradient noise of 2 × 10^-12s^-2Hz^-1/2, subject to improvements with new superconducting techniques under tests. A demonstration of such high sensitivity requires a very strict control of all error sources. The performance of our prototype gravity gradiometer when tested in a noisy terrestrial environment with its sensitive axis tilted at an angle of with respect to Earth\´s gravity is degraded to an error level of 7× 10^-10s^-2Hz^-1/2. In order to compensate for common-acceleration induced errors, we are also developing a six-axis superconducting, accelerometer, based on a similar principle, which can detect the linear and angular acceleration vectors of the gradiometer platform simultaneously. Motion of a single, magnetically levitated, proof mass is monitored to resolve linear acceleration components to 4× 10^-12m s^-2Hz^-1/2and angular acceleration components to 3 × 10^-11rad s^-2Hz^-1/2. This accelerometer, combined with the gradiometer, represents a gradiometer-aided inertial navigation/survey system.