How attractive is our planet?

Geodesy is concerned with the determination of the figure of the Earth, the actual physical shape as well as the gravimetric one. Nowadays space techniques enable us to measure the shape - and more importantly - the changes of the shape of land, ice and ocean surfaces with extremely high precision and resolution. The gravimetric figure of the Earth is the geoid. Its determination is more of a challenge. However, new concepts of satellite gravimetry with missions such as GRACE and GOCE have dramatically improved the situation. While GRACE is designed to measure temporal gravity changes caused by large scale mass redistribution in the Earth system, the emphasis of GOCE is on the determination of gravity and geoid with maximum spatial detail. GOCE was launched in March 2009 as part of an ESA mission. Its core sensor is a gravity gradiometer, the first of its kind in space. It measures gravity differences along three orthogonal axes using three pairs of highly sensitive accelerometers. The gradiometer delivers the components Vxx, Vyy, Vzz and Vxz with high precision in the instrument frame. In order to separate the gravitational signals from both the non-gravitational accelerations and the effect of the satellite's angular motion, the instrument is embedded in a sensor system comprised of a GPS receiver, three star trackers, ion thrusters and magnetic torquers. All sensors are performing well. In order to enhance the gravitational sensitivity the satellite is kept drag free at an altitude of only 255km. The end of the mission is projected for the second half of 2013. Currently plans are being discussed to choose an even lower orbit altitude for the remaining part of the mission. Several geoid and gravity field models have been released. The ultimate geoid accuracy will be 3cm with a spatial resolution somewhere between 100km and 80km. The main areas of application are oceanography, solid Earth physics and geodesy. In ocean areas the actual mean sea surface almost coincides with the geoid. The small difference is the mean dynamic ocean topography. It is the signature of global ocean circulation and the starting point for the detailed determination of geostrophic velocities and of heat and mass transport in the oceans. In solid Earth physics the geoid is primarily a measure of mass imbalance, i.e. of the deviation of topographic masses from isostatic equilibrium. Comparisons with EGM2008 show that particularly in parts of South America, Africa, the Himalaya region and South-East Asia major improvements will result from GOCE. This is also true for Antarctica, where it will help to better understand crustal composition and tectonic processes. The GOCE geoid model is also the basis of a global unification of national and regional height systems. From validation experiments we know that off-sets exist of up to one metre. A globally consistent height reference is needed for sea level studies, geo-information systems and the conversion of GPS-heights to physically meaningful heights above the geoid.