Time variations in the earth's gravity field detectable with Geopotential Research Mission intersatellite tracking

The Geopotential Research Mission (GRM) is a proposed system of two artificial satellites in tandem, drag-free polar orbits of low altitude (about 160 km) designed for a detailed global mapping of the earth's gravitational and magnetic fields. The mission should provide a large amount of new information about the gravity field, particularly over continents, resolving spatial variations in the gravity field as short as 100 km (half wavelength). However, GRM should also perform a much more difficult task, the detection of time variations in the gravity field. Among the effects which produce changes in the field that are large enough to be detected by the intersatellite millimeter wavelength (about 1 micron/s precision) GRM sensor are the following: (1) rebound from ancient deglaciation, (2) very large dip-slip earthquakes, (3) detailed ocean tides, and (4) the seasonal growth and decline of continental glacial fields.