ESA's Studies of Next Generation Gravity Mission Concepts for Monitoring Mass Transport in the Earth System
Massotti, Luca; Siemes, Christian; Di Cara, Davina; del Amo, Jose Gonzalez; Girouart, Benedicte; Roger, Haagmans; Silvestrin, Pierluigi

The paper addresses the preparatory studies of future ESA mission concepts devoted to improve our understanding of the Earth’s mass transport phenomena causing temporal variations in the gravity field, at different temporal and spatial scales, due to ice sheet and glaciers melting trends, continental water cycles, ocean masses dynamics and solid-earth deformations. The ESA initiatives, started in 2003 with a study on observation techniques for solid Earth missions, continued recently through several system studies and technology developments, either for propulsion, e.g. tests on miniaturized ion thruster , or distance metrology, e.g. laser interferometry. These activities received precious inputs from the in-flight lesson learnt from ESA's Gravity field and steady-state Ocean Circulation Explorer (GOCE) mission and the American-German Gravity Recovery and Climate Experiment (GRACE) mission.

The "Assessment of a Next Generation Gravity Mission to Monitor the Variations of the Earth's Gravity Field" (concisely: NGGM) was performed in two parallel studies, one led by Thales Alenia Space (Turin, Italy) and another one by Astrium GmbH (Friedrichshafen, Germany). Both consortia included European universities and academic institutions for scientific support and requirement assessments. Several mission aspects were analyzed, leading to different mission concept features. The preferred mission concepts fitting the defined programmatic boundary conditions have been studied with prioritized science requirements and detailed system designs. In addition to the drivers due to tight propulsion requirements and accelerometer calibration issues, representing a dominant error source at large scales, the technical constraints on power and fuel generally dictate the choice of orbit. Thus, for each considered constellation type, the different interactions between drag-free and "loose" formation control have been analyzed together with the design of the relative attitude control that is necessary to ensure the inter-satellite laser link all along the mission length.

Since then, several complementary and TRP (Basic Technology Research Programme) studies were initiated and are currently running, notably:

  • Next Generation Gravity Mission: AOCS Solutions and Technologies, with the objective to define and evaluate the mission-critical Attitude and Orbit Control System (AOCS) solutions and technologies, to identify the critical technologies and to assess their feasibility and the design drivers. The preliminary definition and validation on a simulator will allow to identify the AOCS drivers and to define in detail the algorithms/hardware developments necessary. The final aim will be to provide a credible design that will be eventually able to fulfill the challenging requirements.

  • Miniaturized Gridded Ion Engine (GIE) Breadboarding and Testing: the latest results from the NGGM studies show that, in order to cope with the large variation of the drag forces encountered in a long duration mission, a miniaturised GIE with increased thrust level is needed, with thrust ranging from 50 μN to 2 mN. The ongoing activity aims at developing a miniaturised GIE for this application. An engineering model of the thruster will be designed, manufactured and tested.

  • Consolidation of the micro-PIM Field Emission Thruster design for NGGM: Previous work has demonstrated that a sharp, porous tungsten crown FEEP multi-emitter is capable of producing thrust from the μN-range to the mN-range. In particular the study aims to demonstrate that the mN-FEEP thruster is an excellent candidate for the lateral thrusting on NGGM.

  • High-Stability Laser with Fibre Amplifier and Laser Stabilisation Unit for Interferometric Earth Gravity Measurements: The objective of this development is the manufacturing of an elegant breadboard (EBB) of a high stability laser (HSL). The HSL consists of the laser head, including the master oscillator and the power amplifier, and the laser stabilisation unit (LSU), which in turn consists of an optical cavity and laser stabilisation electronics.

  • Assessment of Satellite Constellations for Monitoring the Variations in Earth's Gravity Field: aimed at the optimization of Bender-type constellations of two pairs of satellites for the retrieval of the time-variable gravity field for monitoring of mass distribution and transport, the study will pay special attention to the reduction of temporal and spatial aliasing, which pose a challenge for the gravity field retrieval, in the process of optimizing the constellation and in the exploration of more advanced methods for the time-variable gravity field retrieval.

    The latest results concerning the preferred satellite architectures and constellations, payload design and estimated science performance will be presented as well as remaining open issues for future concepts. Attention will also be given to the ongoing ESA-NASA inter-agency cooperation to align preparatory work on synergistic mission concepts beyond GRACE Follow-On.