Olivier Carraz

In the past twenty years, gravimetry missions have demonstrated a unique capability to monitor not only major climate-related changes of the Earth directly from space - quantifying the melt of large glaciers and ice sheets, global sea level rise, continental draught, major flooding events, and also effects of large earthquakes and tsunamis. Adding to fundamental knowledge of the Earth, a quantum gravimetry mission will provide essential climate variables (ECV) of unprecedented quality for ground water, mass balance of ice sheets and glaciers, heat and mass transport,.. as demonstrated – within limits of past technology – by successful missions like GOCE and GRACE (FO). In order to respond to the increasing demand of the user community for sustained mass change observations at higher spatial and temporal resolution, ESA and NASA are at the moment coordinating their activities and are harmonizing their cooperation scenarios in an implementation framework, called MAGIC (MAss change and Geosciences International Constellation). In future post -MAGIC mission, a combination of classical sensors with CAI, or at a later stage a full quantum sensor will bring up the Quantum Missions for Climate to sensitivity that will open to many applications and user needs with respect to water management and hazard prevention among others [1] [2]. Special note must be taken also on the adoption of Quantum Technology (QT) for Earth Observation by the European Commission (COM), notably in the Horizon Europe programme, under the thrust of Commissioner T. Breton, and of the inclusion of QT in ESA Agenda 2025.COM and ESA are setting up a process that would realize a Pathfinder Mission to demonstrate the scientific and technical maturity of quantum gravimetry in space with a view to implement a ground-breaking Quantum Mission for Climate and other applications in the next decade.Several studies related to these new sensor concepts were initiated at ESA, mainly focusing on technology development for different instrument configurations (gravity gradiometers and satellite-to-satellite ranging systems) and including validation activities, e.g. two successful airborne surveys with a CAI gravimeter. A new study has been initiated in 2022, the Quantum Space Gravimetry for Earth Mass Transport (QSG4EMT) with the focus on QSG mission architectures that monitor Earth's mass transport processes and development of QSG user requirements.A technology roadmap will also be outlined for potential implementation of a Quantum Space Gravimetry Pathfinder mission before the end of this decade, aimed at improving state of the art accelerometers in the low frequency band and pave the way to developing a Quantum Mission for Climate in continuity and enhancement of MAGIC. [1] ESA-EC User Requirements workshop for Space Gravimetry Mission, Nov 2021.[2] Towards a sustained observing system for mass transport to understand global change and to benefit society, NASA/ESA Interagency Gravity Science Working Group (IGSWG), TUD-IGSWG-2016-01.

ESA and NASA are currently intensifying their long-term efforts on a collaborative implementation of a next generation mass change and gravity monitoring satellite mission under the umbrella of the NASA-ESA Joint Programme Planning Group sub-group 1. MAss-change and Geosciences International Constellation (MAGIC) is the joint NASA/ESA constellation concept based on NASA’s MCDO and ESA’s NGGM studies. The main objective of MAGIC is to extend the mass transport time series from previous gravity missions such as GRACE and GRACE-Follow on with significantly enhanced accuracy, spatial and temporal resolutions and to demonstrate the operational capabilities of MAGIC. The concept is based on a joint ESA/NASA Mission Requirements Document (MRD) which summarizes the goal requirements of the global scientific community (including requirements from the IGWSG report, IUGG, MCDO, etc.). The first pair of the MAGIC Constellation will be implemented via a NASA/DLR fast-paced cooperation to ensure continuity of observations of GRACE-Follow On, with some potential ESA in-kind contributions for a new generation of accelerometers. The second pair will be implemented by ESA with some potential NASA in-kind contributions for the Laser Tracking Instrument.This paper will address the novel science and applications enabled by MAGIC for the fields of hydrology, cryosphere, oceanography and solid Earth and demonstrate the significant added value of MAGIC constellation for unravelling and understanding mass transport and mass change processes in the Earth system.

The objective of ESA’s Next Generation Gravity Mission (NGGM) is long-term monitoring of the temporal variations of Earth’s gravity field at high temporal (down to 3 days) and spatial (100 km) resolution. Such variations carry information about mass change induced by the water cycle and the related mass exchange among atmosphere, oceans, cryosphere and land, and will complete our picture of Global Change with otherwise unavailable data. The observable is the variation of the distance between two satellites measured by a laser interferometer; ultra-precise accelerometers measure the non-gravitational accelerations to correct the gravity signal in the data processing. The optimal satellite system comprises two pairs of satellites on low (between 396 and 488 km) circular orbits, at 220 km separation, one pair quasi-polar and the other around 65°-70° inclination. The satellite-to-satellite tracking technique for detecting the temporal variations of gravity was established by GRACE (300-400 km spatial resolution at monthly intervals) using tracking in the microwave band. Today, GRACE is being continued by GRACE-Follow-On, with similar objectives, where the laser interferometry has improved the measurement resolution by a factor of 100 (upper MBW). At 150 km spatial resolution, mass change would become observable in 80% of all significant river basins, against 10% achieved with GRACE. High temporal resolution will reveal large-scale sub-weekly mass variations, with applications in water and emergency management. NGGM is a candidate Mission of Opportunity for ESA-NASA cooperation in the framework of MAGIC (MAss Change and Geosciences International Constellation). The MAGIC constellation will build upon the heritage from the GOCE, GRACE and GRACE-FO missions, the ESA NGGM Phase 0 System studies and past technology pre-developments on laser ranging interferometry, and other key technologies, developed over the years in preparation for NGGM and for the LISA mission. MAGIC will be composed of two pairs of satellites. The first Pair (P1) is to be implemented via a DE-USA fast-paced cooperation programme to ensure continuity of observations with GRACE-FO, with some potential ESA in-kind contributions. The second pair (P2) is to be implemented via a Europe-USA cooperation programme with some potential NASA in-kind contributions with a target launch date compatible to maintain at least 4 years of combined operations.The presentation focusses on the on-going Phase A system design, giving an overview of the activities at system and technology level for NGGM, as currently running at the European Space Agency.

Long-term observation of Earth’s temporal gravity field with enhanced temporal and spatial resolution is a major objective for future satellite gravity missions. Improving the performance of the accelerometers present in such missions is one of the main paths to explore. In this context, we propose to study an original concept of a hybrid accelerometer associating a state-of-the-art electrostatic accelerometer (EA) and a promising quantum sensor based on cold atom interferometry. To assess the performance potential of such an instrument, numerical simulations were performed to determine its impact in terms of gravity field retrieval. Taking advantage of the long-term stability of the cold atom interferometer (CAI), it is shown that the reduced drift of the hybrid sensor could lead to improved gravity field retrieval. Nevertheless, this gain vanishes once temporal variations of the gravity field and related aliasing effects are taken into account. Improved de-aliasing models or some specific satellite constellations are then required to maximize the impact of the accelerometer performance gain. To evaluate the achievable acceleration performance in-orbit, a numerical simulator of the hybrid accelerometer was developed and preliminary results are given. The instrument simulator was in part validated by reproducing the performance achieved with a hybrid lab prototype operating on the ground. The problem of satellite rotation impact on the CAI was also investigated both with instrument performance simulations and experimental demonstrations. It is shown that the proposed configuration, where the EA’s proof-mass acts as the reference mirror for the CAI, seems a promising approach to allow the mitigation of satellite rotation. To evaluate the feasibility of such an instrument for space applications, a preliminary design is elaborated along with a preliminary error, mass, volume, and electrical power consumption budget.

The scope of the paper is to propose different concepts for future space gravity missions using Cold Atom Interferometers (CAI) for measuring the diagonal elements of the gravity gradient tensor, the spacecraft angular velocity and the spacecraft acceleration. The aim is to achieve better performance than previous space gravity missions due to a very low white noise spectral behaviour of the CAI instrument and a very high common mode rejection, with the ultimate goals of determining the fine structures of the gravity field with higher accuracy than GOCE and detecting time-variable signals in the gravity field.

We propose a concept for future space gravity missions using cold atom interferometers for measuring the diagonal elements of the gravity gradient tensor and the spacecraft angular velocity. The aim is to achieve better performance than previous space gravity missions due to a very low white noise spectral behavior and a very high common mode rejection, with the ultimate goals of determining the fine structures of the gravity field with higher accuracy than GOCE and detecting time-variable signals in the gravity field better than GRACE.