Gradiometer calibration and performance verification: GOCE approach

The GOCE project aims at measuring the gravity gradient of the Earth with a noise spectral density of 1 to 5 mE/Hz1/2, depending on the scenario, in a bandwidth going from 0.005 to 0.1Hz, and at an altitude of 250 km. An Electrostatic Gravity Gradiometer (EGG) is studied at ONERA, and requires (i) ultra-sensitive space accelerometers with a noise of around 4 x 10-13 ms-2/Hz1/2, (ii) a very demanding calibration, and (iii) a very quiet environment inside the satellite (thermal and geometrical stability, fine drag compensation...). These performances cannot all be verified directly, on the ground, with the required level. The main reasons are the presence of 1g, and seismic noise. As a consequence, the EGG calibration, with its performance verification plan, demand a trade-off between the on-ground partial verifications, the in-flight ones, the pure calculations, and the combination of complementary approaches. This approach has been developed at ONERA, in parallel with experimental verifications, under ESA contract.