Predicted performance of the superconducting gravity gradiometer on the Space Shuttle

Abstract

A high-resolution global gravity map would provide tremendous benefits to the Earth sciences. Mapping the lateral fluctuations of a planet's gravity field is one of the few means of probing its interior. For the Earth, a high-resolution gravity map would also reveal dynamic ocean current information presently hidden in existing sea surface height data. The use of gradiometers for gravity mapping has inherent advantages over other techniques and has the potential of providing the highest spatial resolution for global surveys from orbit. Extremely sensitive superconducting gravity gradiometers (SGGs) have been developed for space applications under NASA support. The present laboratory model has demonstrated a sensitivity almost two orders of magnitude better than any other gradiometer (< 20 mE Hz −1 2 , where 1 mE ≡ 10 −12 m s −2 m −1 ≈ 10 −13 g E m −1). In preparation for a dedicated gravity mapping mission, we have proposed a flight test of a spaceworthy version of the instrument in the SHOOT (Superfluid Helium On Orbit Transfer) dewars, which flew on the Shuttle in 1993. We have examined potential error sources for the instrument operating in the Shuttle environment. Of the largest error sources, most are motion-related. We obtained the motion of the Orbiter in six degrees of freedom (d.o.f.) by examining the SAMS (shuttle acceleration measurement system) and the CAS (calibrated ancillary system) data sets. Using this information and the measured error coupling coefficients of the laboratory SGG, we can predict the resolution with which the Earth's gravity gradient signal can be recovered on this Shuttle test flight. Although the Orbiter provides a much worse environment for the SGG than a dedicated satellite, our analysis shows that during quiet periods on the Orbiter, the ability to recover gravity data will approach the limit imposed by the instrument thermal noise.