On 17 March 2009, the Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite was lofted into a near-Sun-synchronous, low Earth orbit.

GOCE was selected in 1999 as the first Earth Explorer Core Mission under ESA’s Living Planet Programme. For 24 months, GOCE will collect three-dimensional gravity data all over the globe. The raw data will be processed on the ground to produce the most accurate map of the Earth’s gravitational field to date and to refine the geoid: the actual reference shape of our planet. Precise knowledge of the geoid, which can be considered as the surface of an ideal global ocean at rest, will play a very important role in further study of our planet, its oceans and atmosphere. It will serve as the reference model for our measurement and modelling of sea-level change, ocean circulation and polar ice cap dynamics.

A unique payload onboard a unique spacecraft

The main payload instrument is a state-of-the-art Electrostatic Gravity Gradiometer incorporating six highly sensitive accelerometers, mounted in pairs along three perpendicular axes on an ultra-stable carbon-carbon structure. The mission will measure not gravity itself but the tiny differences in gravity between the accelerometer pairs 50 cm apart.

The data collected by GOCE will yield accuracy of 1 to 2 cm in the geoid altitude (the surface of equal gravitational potential of a hypothetical ocean at rest) and 1 mGal for the detection of gravity-field anomalies (mountains, for instance, usually cause local gravitational variations ranging from tens of milligals to approximately one hundred). The spatial resolution will be improved from several hundreds or thousands of kilometres on previous missions to 100 km with GOCE.

In order to get the maximum performance from the Gradiometer, GOCE is designed to provide a highly stable and undisturbed environment, despite its low-altitude orbit (285 kilometers above the Earth's surface) which forces the spacecraft to endure slight but significant drag from the uppermost layers of the atmosphere. This is the main reason for its slender 5 metre-long arrowhead aerodynamic shape design.

The spacecraft also incorporates two low-power xenon ion engines, one primary and one backup, each able to deliver 1 to 20 milli-Newtons of thrust (the force equivalent to our exhaling). These thrusters will be used to make real-time compensation for atmospheric drag, based on the mean acceleration detected by the two accelerometers mounted along the velocity axis.

The spacecraft’s structure and design were also optimised to filter out all kinds of disturbance, by using ultra-stable materials to limit thermal cycling effects, without any deployable or moving parts.

Source: http://www.esa.int/SPECIALS/GOCE/index.html