The scientific objective of GOCE is to map the gravity field of the Earth at high resolution (100 Km over the globe, or a gravity field model developed up to degree 200) with an error smaller than 2-3 cm at this scale. An improvement of the accuracy of about one order of magnitude of the gravitiy field model is expected thanks to the combination of efficient instruments and a low altitude (the sensitivity of a satellite to the gravity field decreases rapidly with the altitude).
GOCE, unlike GRACE, measures the static, or mean, part of the gravity field, and will provide the reference surface, called geoid, necessary to oceanographers to deduce mean sea level and currents. The gravity field model, and the gravity gradients, will also be very useful to geophysical studies and to geodesy.
The launch, from the site of Plesetsk, took place on March 17th, 2009.
Originally due to complete its mission of mapping Earth's gravity in April 2011, approval has been given to extend the life of the GOCE mission to the end of 2012.
To reach the scientific objective, the payload is constituted of the following instruments:
An orthogonal 3-axes gradiometer (radial, along the track, normal to the orbit plane), constituted of three pairs of accelerometers (3-axes, precision 10-12 ms-2Hz-0.5) per axis separated by a distance of 50 cm. The gravity gradients are measured with a very high accuracy of 3 mE (milli-Eötvös ; 10-12s-2) within the measurement bandwidth of 0.005 to 0.1 Hz. The accelerometers are built by ONERA (Office National d'Etudes et de Recherches Aerospatiales);
A Lagrange GPS receiver for an accurate orbit computation;
A laser retroreflector for an accurate orbit computation;
Star sensors to measure the satellite's inertial attitude.
It is the first scientific drag-free mission, a requirement necessary for the good operation of the gradiometer, but also for the survival of the satellite: the very low altitude of the mission, 265 Km, will lead to its loss in the atmosphere in just a few months without drag compensation. The atmospheric drag compensation in the direction of motion is possible thanks to ion thrusters with highly variable thrusts.
The CNES's Space Geodesy team is a member of the EGGC consortium (European Gravity Gradient Consortium) selected by ESA for the mission's data processing, and particularly for the gravity field modelling by means of the direct numerical method. The following level 2 products will be available about 6 months after the end of the mission:
Gravity field model up to degree 200 (minimum) and its variance-covariance matrix;
The precise orbits (by kinematic and reduced-dynamic methods) ;
The calibrated and corrected gravity gradients.
A national bureau of coordination of the users, FROG (French Resources Organization for GOCE), was also created.