Measuring the Earth's gravity field

Measuring the Earth's gravity is useful for many applications, from geophysics and geodesy to navigation, fundamental physics and metrology. In geophysics [R 2 345][C 224] , the analysis and modelling of spatial and temporal variations in the gravity field provides access to the density structure of the Earth and any variations therein. Applications range from earth physics to civil engineering, volcanology, natural resources studies, oceanography and hydrology. In geodesy [C 5 010] , knowledge of gravity anomalies enables us to determine the altitude of the geoid (equipotential surface of the Earth's gravity field, which merges with mean sea level) relative to a reference ellipsoid. It is therefore a fundamental measure of the Earth's shape. Knowing the altitude of the geoid relative to a reference ellipsoid is also essential for comparing the results of levelling measurements using space techniques (GPS: Global Positioning System) with those of conventional measurements [1] .

In recent years, our knowledge of the Earth's gravitational field has progressed enormously, thanks in part to developments in measuring instruments and in part to the contribution of space technology. Satellites have made it possible to carry out complementary measurements such as orbitography, satellite altimetry [E 4 140] , positioning methods (GPS) [TE 6 715] , knowledge of topography and direct measurements of the Earth's gravity from dedicated missions.

We now...