Development of Superconducting Gravity Gradiometers

Precise gravity measurements are required to study the fundamental nature of gravitation. Measurements of gravity can also provide a better understanding of the Earth and the planets, help find natural resources, and improve inertial navigation and surveying. To distinguish gravity from platform accelerations, the Equivalence Principle requires a differential measurement. A gravity gradiometer detects a spatial derivative of the gravitational field and ideally is immune to the vibrations of the platform.

Several versions of the superconducting gravity gradiometer (SGG) have been developed. A three-axis in-line component SGG with a baseline of 19 cm, developed with NASA support, reached a performance level of 2 ´ 10^-11 s^-2 Hz^-1/2 in the laboratory, which is three orders of magnitude more sensitive than the demonstrated sensitivities of atom gravity gradiometers to date. A short movie is available showing the gravity gradient signal from a 1.45-kg lead block and a 0.35-kg aluminum block. The blocks are mounted on a 50-cm diameter turntable located next to the cryostat, which houses the three-axis SGG. 

The achieved common-mode rejection of 10⁷ is sufficient for a spacecraft environment. However, for terrestrial moving-base applications, the linear acceleration rejection must be improved to 10⁹ or higher. A cross-component device can be designed to be inherently insensitive to linear accelerations by employing pivoted moment arms whose mass moments are precisely balanced prior to assembly. We have developed such a cross-component SGG and demonstrated sensitivity better than 10^-9 s^-2 over a bandwidth of 0.001 to 2 Hz.

The existing SGGs have mechanically suspended test masses. Magnetic levitation gives a number of advantages. The resulting magnetic spring is much more compliant and gives two degrees of freedom to each test mass. Hence a tensor gradiometer can be constructed with only six test masses, and sensitivity better than 10^-12 s^-2 Hz^-1/2 can be achieved with a device miniaturized by an order of magnitude in volume and mass over the existing devices. Such a tensor SGG is under development with NASA support for Earth and planetary applications.