Although the GOCE (Gravity Field and Steady-State Ocean Circulation Explorer) gradiometer is highly accurate, it is not possible to map the complete gravity field at all spatial scales with the same quality. To overcome this limitation the position of the GOCE satellite is tracked by GPS relative to GPS satellites at an altitude of 20 000 km – this procedure is known as satellite-to-satellite tracking. The gradiometer is used to measure small scales of the gravity field whilst GPS is used to obtain large-scale data.