GOCE: Gravitational gradiometry in a satellite

Abstract

Spring 2009 the satellite Gravity and steady-state Ocean Circulation Explorer (GOCE), equipped with a gravitational gradiometer, was 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 threedimensionalGPS tracking of GOCE. The gravitational tensor consists of the nine second-derivatives of the Earth's gravitational potential. Due to its symmetry only six of them are independent. These six 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 six components are measured with high precision (10-11 s-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 mission ended in November 2013. Until August 2012 in total 2.3 years of data were collected. They entered into ESA's fourth release of GOCE gravity models. After August 2012 the orbit altitude was lowered in several steps by altogether 31km in order to test the enhanced gravitational sensitivity at lower orbit heights. The fields of application range from solid earth physics, via geodesy and oceanography to atmospheric physics. For example, several studies are concerned with the state of isostatic mass compensation in regions such as South America, Africa, Himalaya, and Antarctica. GOCE will help to unify height systems worldwide and enable the direct conversion of GPS-based ellipsoidal heights to accurate and globally consistent heights above the geoid. For the first time, it became possible to derive mean dynamic ocean topography and geostrophic ocean velocities with high spatial resolution and accuracy directly from space, combining the altimetric mean sea surface and the GOCE geoid. Assimilation into numerical ocean circulation models will help to improve estimates of ocean mass and heat transport. Commonmode accelerations as measured by GOCE lead to improved atmospheric density and wind estimates at GOCE altitudes.