The GOCE gradiometer consists of three pairs of identical ultra-sensitive accelerometers, mounted on three mutually orthogonal "arms" (see upper part of the image). It also contains other elements related to the gradiometer control and read-out electronics, to the structure and to the thermal control elements. The three arms are mounted orthogonal to one another: one aligned with the satellite’s trajectory, one pointing towards the centre of the Earth, and the third is perpendicular to the other two.
The principle of operation of each ultra-sensitive accelerometer relies on measuring the electric field required to 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. The difference between accelerations measured by each pair of accelerometers (which are 50 cm apart) is the basis for the gravity gradients.