Next October 5 the GOCE (Gravity field and steady-state Ocean Circulation Explorer) mission satellite will be launched. The goal of this first of four ESA missions dubbed Earth Explorer, which officially got off the ground in 1999, is to identify and map the Earth's gravity field with an accuracy heretofore never reached. To do so, the 45 European partners of the mission headed by Thales Alenia Space had to accomplish several technological feats for the satellite and the onboard gradiometer. ONERA DMPH (Department of Physical Measurements) teams designed the ultra-sensitive electrostatic accelerometers of the instrument and are now working on developing the accelerometers for MICROSCOPE, another extreme space mission scheduled for launching in 2011-2012. DMPH Director Pierre Touboul believes that with GOCE a new stage has been reached in the field. Interview by Jean-François Desessard.
BE France - ONERA can rightly claim to be world leader in the field of ultra-sensitive accelerometers. The results of earlier missions such as CHAMP or GRACE, for which DMPH teams designed very efficient and still operational accelerometers, bears witness to its unique skills and expertise. Nevertheless, wasn't GOCE development different from its predecessors?
Pierre Touboul - GOCE goal is far-reaching since expected efficiency should be 50 times higher than the efficiency reached during the GRACE mission where resolution (which has since been confirmed in orbit) was already extremely high, to the order of 10-10 m/s2. However, we knew that to reach an efficiency of this order, we had to go beyond our limits in every fields, from electronics to mechanical engineering, at every stage, from instrument design to construction.
Until now, the design and building of ultra-sensitive accelerometers was similar to piecework by a craftsman, albeit very high level piecework. Furthermore, ONERA teams carried out a very large share of the work. However, for GOCE we had to work in an industrial type environment. This entailed greater interaction between the different mission partners, and close interactions because the "satellite instrument" system is more complex. GOCE enabled us to reach another stage in the development of these very special instruments called ultra-sensitive electrostatic accelerometers.
BE France - When are the first results expected?
Pierre Touboul - After launching from the Plesetsk launch base next October 5, the one-ton satellite will be lofted into a low 260-km orbit. The satellite will have to orbit close enough to Earth to collect the required data on gravity while remaining free of the disruptions due to the Earth's atmosphere at this altitude. That is why the 5-meter long aerodynamic satellite is equipped with low power ionic thrusters to compensate for atmospheric drag.
In the upcoming weeks, after the requisite routine satellite checks, the six accelerometers in the GOCE gradiometer will be turned on. To take a 3-D measurement of the gravity gradient, the satellite is carrying a three-axis gradiometer aboard. Each axis is bearing a pair of electrostatic accelerometers. Now, by measuring the gaps in the acceleration on the two test masses, each differential pair will supply a component of the gravity gradient.
First, the proper working order of the instruments will be checked. Several stages will have to be completed, as for example gradiometer calibration in orbit. We should have the first results before the year is out. Whether we will successfully reach the expected resolution of 10-12 m/s2 will depend on related data processing, among others. Then the real data collection will begin, lasting some twenty months, during two measurement periods separated by a period when the device will be on standby during an eclipse phase.
BE France - What will mission results be used for?
Pierre Touboul - Numerous scientists in different fields are eagerly waiting for these results. They will help geodesists fine-tune the knowledge of what we call the geoid, i.e., the particular equipotential surface of the Earth's gravity, a surface which depends on inner components that geophysicists will characterize. Oceanographers will be able to deduct ocean circulation models that climatologists can use to optimize their forecasts. Last, the Earth's gravitational signature identified from the results collected by the GOCE gradiometer will supply more accurate data on the Earth's subsoil.
