ESA GNC Conference Papers Repository
The next generation gravity mission: challenges and consolidation of the system and AOCS concepts
Following the success of GRACE and GOCE, the scientific communities and the space agencies have started to focus their attention towards the preparation of a future gravity mission. In particular, since 2003 the European Space Agency (ESA) has promoted studies to establish the scientific requirements, to identify the most appropriate measurement techniques, to start the associated technology developments, and to define the system scenarios for a 'Next Generation Gravity Mission' (NGGM). The NGGM mission objective aims at measuring the temporal variations of the Earth gravity field over a long time span (namely a full solar cycle, 11 years) with an unprecedented level of accuracy, both in spatial (comparable to that provided by GOCE) and temporal (weekly or better) resolution. The distance variation between the satellites and the non-gravitational accelerations of the satellites, measured respectively by a laser interferometer (with nanometric resolution) and by ultra-sensitive accelerometers (like those installed on GOCE), are the fundamental observables from which the Earth's gravity field anomalies are retrieved. The satellite formation needed to achieve the mission objectives consists of two pairs of satellites in 'Bender' configuration with inter-satellite distance around ~100km. Circular orbits, with altitudes around ~340 km, and polar (one pair) and medium (second pair) inclination are suitable for the NGGM, providing all-latitude coverage, short repeat cycles/sub-cycles and excellent gravity signal retrieval compatible with a long lifetime. The products of such a mission will allow investigating with high detail geophysical phenomena involving re-distribution and transportation of mass in the atmosphere, continental hydrosphere, oceans, cryosphere, and lithosphere, enabling new science in geodesy, geophysics, hydrology, ocean circulation and many other disciplines. To accomplish its objectives, the NGGM mission concept requires, with respect to GOCE and GRACE, a number of innovations, both at technological level and on the control side, involving the satellite-to-satellite tracking technology, new technologies for the payload as well as different spacecraft and GNC design drivers. Each satellite shall be designed for long lifetime in low-Earth-orbit, large variation of the solar illumination (due to the non-sun-synchronous orbit), minimum disturbance of the payload and shall be endowed with a complex control system capable of carrying out several tasks in close coordination: orbit maintenance, formation keeping, attitude stabilisation, drag compensation and laser beam pointing at micro-radian level. This paper focuses on the NGGM mission challenges and on the relevant system and AOCS technological innovations. In particular, an overview is given of the main results of the on-going 'Consolidation of the system concept for the Next Generation Gravity Mission' study, carried out for ESA by Thales Alenia Space - Italia (TAS-I).