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Instrument: EGG : Electrostatic Gravity Gradiometer
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[Source: ESA's Gravity Mission GOCE Brochure, ]

The principle of operation of the gradiometer relies on measuring the forces that maintain a ‘proof mass’ at the centre of a specially engineered ‘cage’. Servo-controlled electrostatic suspension provides control of the ‘proof mass’ in terms of linear and rotational motion. Three pairs of identical accelerometers, which form three ‘gradiometer arms’, are mounted on the ultra-stable structure. The difference between accelerations measured by each of two accelerometers (which are about 50 cm apart), in the direction joining them, is the basic gradiometric datum. The average of the two accelerations is proportional to the externally induced drag acceleration (common mode measurement). The three arms are mounted orthogonal to one another: one aligned with the satellite’s trajectory, one perpendicular to the trajectory, and one pointing approximately towards the centre of the Earth. By combining the differential accelerations, it is possible to derive the gravity gradient components as well as the perturbing angular accelerations.

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Instrument Start Date: 2009-03-20
Instrument Owner: ESA