Next generation gravity mission design activities within the mass change and geoscience international constellation

The objective of ESA's Next Generation Gravity Mission (NGGM) is long-term monitoring of the temporal variations of Earth's gravity field at high temporal (down to 3 days) and spatial (100 km) resolution. Such variations carry information about mass change induced by the water cycle and the related mass exchange among atmosphere, oceans, cryosphere and land, and will complete our picture of Global Change with otherwise unavailable data. The observable is the variation of the distance between two satellites measured by a laser interferometer; ultra-precise accelerometers measure the non-gravitational accelerations to correct the gravity signal in the data processing. The optimal satellite system comprises two pairs of satellites on low (between 396 and 488 km) circular orbits, at 220 km separation, one pair quasi-polar and the other around 65 degrees-70 degrees inclination. The satellite-to-satellite tracking technique for detecting the temporal variations of gravity was established by GRACE (300-400 km spatial resolution at monthly intervals) using tracking in the microwave band. Today, GRACE is being continued by GRACE-Follow-On, with similar objectives, where the laser interferometry has improved the measurement resolution by a factor of 100 (upper MBW). At 150 km spatial resolution, mass change would become observable in 80% of all significant river basins, against 10% achieved with GRACE. High temporal resolution will reveal large-scale sub-weekly mass variations, with applications in water and emergency management. NGGM is a candidate Mission of Opportunity for ESA-NASA cooperation in the framework of MAGIC. The paper focusses on the on-going Phase A system design and technology pre-development activities.