Principles of gradiometry

Housed at the centre of the GOCE (Gravity Field and Steady-State Ocean Circulation Explorer) satellite, the gradiometer contains three pairs of proof masses positioned at the outer ends of three 50 cm long orthogonal axes. All experience the gravitational acceleration of the Earth slightly differently because of their different position in the gravitational field. The common acceleration of each pair of accelerometers is proportional to non-gravitational forces such as air-drag acting on the satellite. The difference in acceleration for each pair, scaled with the length of the connecting arm, called the gravity gradient, is used for gravity field analysis.