*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 10^{7} is
sufficient for a spacecraft environment. However, for terrestrial moving-base
applications, the linear acceleration rejection must be improved to 10^{9}
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.