###### Abstract  
ESA's Next Generation Gravity Mission (NGGM) is a candidate Mission of Opportunity for ESA-NASA cooperation in the frame of the Mass Change and Geosciences International Constellation (MAGIC). The mission aims at enabling long-term monitoring of the temporal variations of Earth's gravity field at relatively high temporal (down to 3 days) and increased spatial resolutions (up to 100 km) at longer time intervals. This implies also that time series of GRACE and GRACE-FO can be extended towards a climate series. Such variations carry information about mass change induced by the water cycle and the related mass exchange among atmosphere, oceans, cryosphere, land and solid Earth and will complete our picture of global and climate change. The main observable is the variation of the distance between two satellites measured by a ranging instrument. This is complemented by accelerometers that measure the nongravitational accelerations, which need to be reduced from ranging measurements to obtain the gravity signal. The preferred satellite constellation comprises one satellite pair in a near-polar and another in an inclined circular orbit. The paper focuses on the orbit selection methods for optimizing the spatial sampling for multiple temporal resolutions and then on the methodology for deriving the engineering requirements for the space segment, together with a discussion on the main mission parameters. From 2002 to 2017, the GRACE satellites [4] provided measurements that were processed to obtain monthly estimates of Earth's global gravity field at scales of several hundreds of kilometers and larger. The time variations of the gravity field were used to determine changes in Earth's mass distribution [5], with applications ranging from measurement of continental water storage (e.g., seasonal changes in large river basins and groundwater depletion) [6; 7; 8; 9; 10], to ice and snow accumulation and depletion in the polar regions and large glaciers [11; 12; 13], to the monitoring of global mean barystatic sea-level variations and oceans [14; 15; 16; 17; 18; 19; 20]. The two GRACE satellites were identical and flew in near-circular, polar (89\({}^{\circ}\) inclination) orbits, initially at 500 km altitude, at along-track distance varying around a mean value of 220 km. The instantaneous distance variation measured by a dual-band microwave ranging instrument (24 GHz, 32 GHz) was the main observable, supplemented by GPS positions and nongravitational acceleration measured by high-precision accelerometers. The satellite altitude decayed naturally under atmospheric drag down to about 320 km at the end of the extended mission lifetime, with the consequence that the ground track pattern was changing continuously, resulting in variable quality of monthly solutions. The GRACE Follow-On (GRACE-FO) satellites [21] were launched on 22 May 2018 and are meant to continue the GRACE time series for at least five years. The satellite design is fully inherited from GRACE but includes a laser ranging interferometer (LRI) as a technology demonstration of a more precise ranging capability. Acceleration measurement errors (e.g., temperature-induced bias drifts), the relatively high and variable altitude and the one-dimensional North-South sampling are known to affect the GRACE gravity model quality. Improvements to the spacecraft design (thermal control, attitude measurement and control) can help to reduce systematic errors. Beyond that, however, aliasing mainly due to monthly temporal sampling dominates due to unavoidable errors in the aliasing reduction modeling of high-frequency ocean and atmospheric mass variations: even a substantially improved instrument such as the LRI cannot be fully exploited [22]. A single pair of satellites cannot meet operational and global user community needs. This would result only in partial information, and it would not be possible to support key applications, e.g., ground water and aquifer monitoring for improved water management, at the required spatiotemporal resolution.