# Drag and Attitude Control for the Next Generation Gravity Mission
## 1 Introduction
Preferred mission concepts fitting the defined programmatic boundary conditions were identified in the \"Assessment of a Next Generation Gravity Mission to Monitor the Variations of the Earth's Gravity Field\" and studied with prioritized science requirements and detailed system designs. These activities received precious inputs from the in-flight lessons learnt from the GOCE and GRACE missions [9]. The scientific studies aimed at complementing the technology studies, consolidating the science requirements, and focusing on the optimization of the satellite constellation, and advancing the gravity field retrieval algorithms. Notably, the science studies [10; 11] produced a detailed assessment of the impact on the gravity field solutions of ocean tide aliasing, which was found to cause larger errors than non-tidal signals, and developed ocean tide mitigation strategies to be applied in post-processing. To prepare the technology required for the NGGM concept, several complementary studies were initiated within the former Technology Research Programme (TRP), covering attitude and drag-free control aspects [12], electric propulsion elements (miniaturized radiofrequency ion thrusters [13] and indium-fed micro-thrusters [14]), breadboarding of a high-stability laser [15] with fiber amplifier, and a frequency stabilization unit for the interferometric measurements of the inter-satellite distance variation. The focus of the ESA's architectural (pre-Phase A) studies was the optimal combination of the orbit and the mission lifetime, being a design driver for the satellite system. As for all gravity missions, the orbit shall be as low as possible, while the mission lifetime was intended to cover a full solar cycle, i.e., 11 years. The technical solution to this set of requirements was found in developing a sophisticated attitude and orbit control system that is based on electric propulsion, where eight smaller ion thrusters (baselined given the higher gridded ion engine maturity), supported by three magnetic torquers, control the attitude of the satellites, and one larger ion thruster compensates for the effect of drag in the along-track direction [16]. Concerning the laser ranging instrument, two options were investigated in [PERSON] et al. [17]: an optical transponder and a retroreflector scheme. The transponder scheme is very similar to the laser ranging instrument (LRI) of the GFO mission, where a laser beam, generated on one satellite, is received by the other satellite, which regenerates the laser with its own laser source and retransmits the laser beam back to the first satellite. In contrast, in the retroreflector scheme, a heterodyne laser interferometer transmits a laser beam from one satellite, which is passively reflected by the other satellite. At the time of writing, the transponder scheme is baselined in the feasibility studies of the NGGM Phase A studies. On the basis of the objectives and requirements expressed by the International Union of Geodesy and Geophysics (IUGG) [1], the NASA/ESA Interagency Gravity Science Working Group (IGSWG) [18], the US Decadal Survey for Earth Science and Applications from Space (DS) [19], ESA, and NASA are jointly coordinating the Phase A studies of the Mass Change Designed Observable (MCDO) and the Next Generation Gravity Mission (NGGM) within the Mass Change And Geosciences International Constellation (MAGIC). The paper addresses first the NGGM objectives, the measurement technique of Earth's gravity field, the fundamental observables, and their measurement requirements by highlighting their role in the achievement of the mission performance. Then, it explains the relation between the measurement of the non-gravitation acceleration and the control of the drag forces and of the spacecraft attitude; from the characteristics of the reference accelerometer and its coupling factors, the requirements on drag and attitude control are derived and compared with the NGGM orbital environment.