Time variations in the Earth's gravity field detectable with geopotential research mission intersatellite tracking

Abstract

The Geopotential Research Mission (GRM) is a proposed system of two artificial satellites in tandem, drag‐free polar orbits of low altitude (∼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 (∼ 1 μ/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. In particular, many nonzonal as well as zonal components affected by postglacial rebound should be detected in a 6‐month GRM mission. Repeat passes over the fault zones of earthquakes similar to Alaska (1964), Chile (1960), or Sumbawa (1977) should reveal signal changes due to their earth movements. The effect of ocean tides on the GRM signal should be readily detectable over a broad band of frequencies even if all but 10% of the tides are correctly modeled. Continued geophysical interest in these now largely unobserved phenomena warrants extension of the GRM mission to somewhat higher altitudes.