The EGG (Electrostatic Gravity Gradiometer) on board GOCE was a three-axis diagonal gradiometer, based on three pairs of electrostatic servo-controlled accelerometers and had a double role on the GOCE mission. It provided the gravity gradient measurements, mapping Earth's geoid to a highly accurate level. This opened a window into Earth's interior structure, as well as the currents circulating within the depths of its oceans. It was also used as the main sensor in the DFACS (Drag Free Attitude Control System), so if the common mode acceleration in flight direction was not zero, the DFACS responded by either increasing or decreasing the ion engine thrust to maintain the spacecraft in near-freefall conditions.
The EGG instrument was designed and developed at ONERA (Office National d'Etudes et de Recherches Aérospatiales) and manufactured at Thales Alenia Space, France.
The GOCE mission was launched on 17 March 2009 from the Plesetsk Cosmodrome, Russia and came to an end in late 2013.