Forming the heart of GOCE (Gravity Field and Steady-State Ocean Circulation Explorer), the gravity gradiometer consists of three pairs of identical ultra-sensitive accelerometers, mounted on three mutually orthogonal 'arms'. One of the arms is aligned with the satellite’s trajectory, one pointing towards the centre of the Earth, and the third is perpendicular to the other two. This allows the simultaneous measurement of six independent but complementary components of the gravity field.