# Drag and Attitude Control for the Next Generation Gravity Mission
## 1 Introduction  
All geophysical phenomena involving mass displacements (such as hydrological cycles, floods, drying of water reserves, glaciers melting, tectonic movements) can be \"observed\" through the variation they induce in Earth's gravity field. Monitoring these phenomena on a global scale and over a long period provides essential information to study climate change evolution, manage natural resources, preserve ecosystems, and prevent and contain natural hazards [1], and space missions are the most appropriate means to perform this task. The use of satellites for Earth's gravity field determination dates back to the launch of the first Sputnik in 1957, while the idea of using tracking between spacecraft for gravimetry was spelled out already in 1960 [2], becoming more concrete in 1969 [3]. From 2002 to 2017, the Gravity Recovery And Climate Experiment (GRACE) Mission [4] produced monthly maps of Earth's gravity field; a service that since 2018 is carried out by the GRACE-Follow On (GFO) Mission [5]. In both missions, a pair of satellites flying at a mean relative distance of 220 km on a circular low-Earth orbit (initially at 500 km altitude) with near-polar inclination \"observes\" the gravitational potential through its effect on the variation of the satellite-to-satellite distance (measured by a dual-band microwave 24-23 GHz ranging instrument), \"cleaned\" from the contribution of the non-gravitational forces (measured by very sensitive accelerometers). GRACE satellites were mostly left to decay under the action of the atmospheric drag down to about 320 km at the end of the extended mission's lifetime, with the consequence that the ground track pattern was changing continuously. The GFO satellites [5] were launched in May 2018 and are meant to continue the GRACE time series for at least five years. The satellite design is inherited from GRACE but includes a laser ranging interferometer (LRI) as a technology demonstration of a more precise ranging capability. In parallel to the GRACE Mission, the European Space Agency launched its first Earth Explorer satellite for gravimetry in 2009: the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) [6]. Unlike GRACE and GFO (aimed at the long-term monitoring of Earth's gravity variations at coarse spatial, from 500 to 300 km, and temporal, 1 month, resolution), the GOCE Mission objectives were to measure Earth's static gravity field and model the geoid with unprecedented accuracy and spatial resolution. The GOCE satellite was orbiting from 2009 to 2013 at a mean altitude of 255 km (nominal mission) and 225 km (extended mission) in the drag-free mode. The measurement instrument was a gravity gradiometer, consisting of six ultra-precise accelerometers, complemented by a dual-frequency GPS receiver. The measurements of these instruments were used to derive gravity gradients and precise orbits, which were transformed into a gravity map of Earth with a mean global accuracy of 2 cm on geoid heights and 0.5 mGal on gravity anomalies, at 100 km spatial resolution [7]. The low controlled altitude, the drag compensation control (so-called \"drag-free\"), and the accurate angular accelerations measured as a by-product of the gradiometer payload were all instrumental in GOCE's outstanding result. The GOCE Mission's concept and technology have been enabling for the ultra-sensitive payload for the Microscope Mission, and have paved the way to the future NGGM [8], as explained later on. Soon after the selection of GOCE as its first Earth Explorer (1999), the ESA started preparatory studies towards the NGGM. Since the mid-2000s, these studies have focused on a mission concept devoted to the sustained observation and quantification of mass transport processes in the Earth system, encompassing AOHIS (atmosphere, ocean, hydrology, ice, solid-earth), over a decade-long time span.