###### Abstract
A future gravity mission dedicated to mass change in the Earth system, as studied in the context of a Next Generation Gravity Mission [23], will require improvements in the instrumentation, the spacecraft (disturbing accelerations) and the mission design (sampling). In addition, a constellation of two pairs of satellites in an optimized orbit configuration and a strategy for reducing potential remaining aliasing errors are required. A number of authors have studied future gravity field mission concepts based on precise ranging between two low-flying satellites forming a pair. Most of them considered either a single pair flying in formation or two satellite pairs flying in a so-called Bender constellation [24], where one pair is in a polar orbit and the other pair in an orbit with an inclination of 63\({}^{\circ}\) (see, e.g., [25] and the references therein). Other satellite formations, such as the Cartwheel, Pendulum and Helix formations, impose excessive attitude and orbit control and, consequently, power demands on the satellite system and were therefore abandoned. The favored option seems to be the Bender constellation with two satellite pairs flying in an in-line formation like the GRACE and GRACE-FO satellites. The orbit optimization of the Bender constellation is a complex problem. For individual satellite pairs, we could use the Nyquist-type rule introduced in [26] or its revised version presented in [27]. For a Bender constellation, [28] proposed to use a genetic algorithm. All these approaches have the drawback that they optimize the orbits for a single temporal resolution, whereas multiple temporal resolutions are required to serve the needs of all users [28; 29]. For this reason, a new orbit selection approach that aims at optimizing the spatial sampling of the Bender constellation for multiple temporal resolutions [30] was developed. The approach was successfully used for generating the orbits in a number of ESA-funded simulation studies [31; 32; 33]. NGGM can be understood as one of the Bender pairs of the MAGIC constellation, providing either global coverage via the polar pair or enhanced coverage of the mid-latitudes through the inclined pair. In this case, the combination of NGGM with a second pair is under study together with NASA to arrive at the MAGIC constellation. The global user community requirements were identified by the Joint NASA/ESA Ad-hoc Science Study Team (AJSST) composed of US and European representatives of the global scientific community, for a univocal consolidation of the threshold and target user requirements and initial mission requirements for observation systems orbiting at different altitudes. The Mission Requirements Document (MRD) includes user and application needs in user community reports from the IUGG [29], the NASA/ESA Interagency Gravity Science Working Group [34], the US Decadal Survey for Earth Science and Applications from Space [35] and other recent work cited in the MRD. A full science traceability matrix can be found in Appendix A of the MRD [36]. The spatiotemporal mapping shall be such that atmospheric, ocean and ocean tide (AO + OT) signals and/or errors can be decoupled from signals from other Earth system constituents (ice, hydrology, oceans and solid Earth), taking into account possible aliasing periods. Notably, a double satellite pair mission concept has the intrinsic potential to retrieve the full atmosphere, ocean, hydrology, ice and solid Earth (AOHIS) signal in contrast to a single-pair mission, where tailored postprocessing further reduces the signal and is not able to achieve the same resolution and performance as a double-pair mission without postprocessing [31].