4. GOCE: Gravitational Gradiometry in a Satellite

Spring 2009 the satellite Gravity and steady-state Ocean Circulation Explorer (GOCE), equipped with a gravitational gradiometer, has been launched by European Space Agency (ESA). Its purpose is the detailed determination of the spatial variations of the Earth’s gravitational field, with applications in oceanography, geophysics, geodesy, glaciology, and climatology. Gravitational gradients are derived from the differences between the measurements of an ensemble of three orthogonal pairs of accelerometers, located around the center of mass of the spacecraft. Gravitational gradiometry is complemented by gravity analysis from orbit perturbations. The orbits are thereby derived from uninterrupted and three-dimensional GPS tracking of GOCE.The gravitational tensor consists of the nine second-derivatives of the Earth’s gravitational potential. These nine components can also be interpreted in terms of the local curvature of the field or in terms of components of the tidal field generated by the Earth inside the spacecraft. Four of the nine components are measured with high precision (10^− 11s^− 2 per square-root of Hz), the others are less precise. Several strategies exist for the determination of the gravity field at the Earth’s surface from the measured tensor components at altitude. The analysis can be based on one, several or all components; the measurements may be regarded as time series or as spatial data set; recovery may take place in terms of spherical harmonics or other types of global or local base functions. After calibration GOCE entered into its first measurement phase in fall 2009. First results are expected to become available in summer 2010.