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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 resolution in time (down to 3 days) and space (100 km). Such variations carry information about mass transport 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; accelerometers measure the non-gravitational accelerations to be separated from the gravity signal in the data processing. The optimal satellite system comprises two pairs of satellites on low (around 340 km) circular orbits, at 100 km mutual distance, one pair near-polar and the other around 65° inclination. The technique of satellite-to-satellite tracking for detecting the temporal variations of gravity was established by GRACE, which reached 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 (in the upper MBW) which however cannot be fully exploited due to other system limitations. At 150 km spatial resolution, mass change would become observable in 80% of all significant river basins, up from 10% achieved with GRACE. High temporal resolution will reveal large-scale daily mass variations, with applications in water management and operational prediction. Currently, the NGGM is a candidate Mission of Opportunity for ESA-NASA cooperation. Over the last decade, numerous system and technology activities have advanced the maturity of the system and the key subsystems, and the mission can now be proposed for launch around 2028. The paper focusses on the latest design and test achievements, with a discussion on alternative drag compensation scenarios.
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