Global gravity survey by an orbiting gravity gradiometer



Three dedicated space missions proposed for the 1990s promise to provide data for recovering the Earth's gravity anomaly with unprecedented accuracy and resolution: the Geopotential Research Mission (GRM), the Aristoteles Mission, and the Superconducting Gravity Gradiometer Mission (SGGM). SGGM, the most ambitious of the three, aims at recovering the global gravity field to a precision of 2–3 mGal with a resolution of 50 km. Such accuracy and resolution are required to answer many key questions in geophysics. The SGGM instrument package is a three-axis superconducting gravity gradiometer which is integrated with a six-axis superconducting accelerometer and a six-axis shaker for active platform control. The intrinsic sensitivity of the gradiometer is 10−4 E Hz−½ (where E represents the unit of gravity gradient (1 E = 1 Eötvös = 10−9 s−2)) and that of the accelerometer is 10−13 gE Hz −½ in linear acceleration and 10−11 rad s−2 Hz−½ in angular acceleration. While precise attitude control of the Experiment Module is essential to mission success and is also technically the most challenging, pointing accuracy and disturbance isolation requirements of SGGM are less stringent compared to that of other missions such as the Hubble Space Telescope (HST) and Gravity Probe-B (GP-B). Thus they are within the reach of technologies of the 1990s. In the recently completed Phase A study, the SGGM Study Team addressed the problem of scientific requirements and mission feasibility. Marshall Space Flight Center is continuing with mission and spacecraft design, while mission simulation is being done at Goddard Space Flight Center, Greenbelt, Md. At the University of Maryland, prototypes of the three-axis gradiometer and the six-axis accelerometer are being fabricated, improved, and tested. A Shuttle flight test of the Experiment Module with the actual instrument package is suggested for 1994–1995. The full science mission hopefully will take place before the year 2000.