The GRACE Follow-On twin-satellites, a partnership between NASA (US) and GFZ (Germany), were launched on May-22, 2018 to continue the data record of mass transport in the Earth system that was initiated by the GRACE mission in 2002, and which ended in mid-2017 after more than 15 years of successful operations and science discoveries. The combined GRACE & GRACE-FO measurements now span 20 years, and provide a unique data record of monthly to decadal observations of global mass changes and transport in the Earth system from observations of temporal variations in the Earth’s gravity field. These observations have become indispensable for climate-related studies and provide critical measurements of Earth's time varying mass distribution that enable process understanding of the evolving global water cycle, including ocean dynamics, polar ice mass changes, and global ground water changes.

In this presentation, we will provide updates on essential mass change climate variables such as changes of ice sheet mass, sea level and ocean heat content, as well as land water storage trends from GRACE through GRACE-FO. GRACE Follow-On is also demonstrating improved inter-satellite ranging observations from a Laser-Ranging Interferometer (LRI) for the next-generation of mass change missions. We will discuss (1) key data processing and calibration approaches on GRACE-FO, and (2) a high-level analysis and comparison of three years of laser vs microwave ranging measurements on GRACE-FO. We will conclude with an outlook towards achieving continuity with future mass change missions.

NASA´s Earth Science Decadal Survey Report highlights mass transport monitoring as one of top priorities in Earth Observation for the next decade. To realize such a Mass Change Mission, NASA is seeking international partnership. Based on the large success of the GRACE and GRACE FO missions and their contributions to climate change research, there is a large interest in Germany to continue mass change measurements.

In March 2021 a 10-months Phase-0 study was kicked-off by the German Space Agency at DLR to investigate a “GRACE-I” mission based on a GRACE-like concept combined with an optional ICARUS (International Cooperation for Animal Research Using Space) payload. The study was closely discussed with JPL/NASA as a future continuation of the very successful US-German GRACE/GRACE-FO technological and scientific partnership. GRACE-I will be a single satellite pair based on a fully redundant Laser Ranging Interferometer on a polar orbit at 490 or optionally at 420 km (with drag compensation) altitude. Launch shall be not later than 2027 to guarantee data continuity w.r.t. GRACE-FO. Besides ICARUS it was also investigated if a single-axis Quantum Gravity Gradiometer could be added as a technology demonstrator for future gravity missions. GFZ has supported Phase-0 with full scale simulations investigating the baseline single pair concept, a three-satellite configuration by combing GRACE-I with a pendulum satellite connected to GRACE-I by chronometric laser ranging and double pair constellations by adding an inclined pair to GRACE-I to improve spatial and temporal resolution.

GRACE-I could be a first component of a hybrid Bender constellation if combined with a first (inclined) MAGIC pair. The realization of this Mass-change And Geoscience International Constellation is currently discussed between ESA and NASA.

At the time of writing this abstract the main focus was on the next steps to refine the technical design and to select the final payload configuration for a US/German GRACE-I mission. We will present the proposed mission architecture and simulation results. These are based on realistic assumptions of background model errors as well as instrument noise characteristics which are conform with MAGIC’s mission requirements in order to be comparable with results obtained during parallel ESA simulation studies. Finally, we will discuss further steps towards realization of GRACE-I.

Projects on which this publication is based were carried out on behalf of the Federal Ministry of Economic Affairs and Energy (FKZ: 50EE2004; 50EE2019).

In November 2020 it was decided at ESA’s Ministerial Conference to investigate a European next-generation gravity mission (NGGM) in Phase A as first Mission of Opportunity in the FutureEO Programme. The Mass-change And Geoscience International Constellation (acronym: MAGIC) is a joint investigation with NASA’s MCDO study resulting in a jointly accorded Mission Requirements Document (MRD) responding to global user community needs. On NASA side, a pre-Phase A study to address these needs is expected to start in summer 2021. On ESA side, the MAGIC concept is currently being investigated in two parallel industry Phase A studies, complemented by a science support study.

In the frame of this science study, several potential mission constellations are investigated and numerically simulated in great depth. This includes Bender-type double pair mission concepts and single/multiple pendulum configurations, with realistic error assumptions regarding the key payload products, in close interaction with the parallel industry studies. Methodological improvements of processing strategies, for example the co-estimation of short-term gravity field models with various resolution, and the optimum treatment of long-term signals and tailored post-processing techniques, will be investigated. Further aspects such as the benefit of including DORIS for improving satellite orbits to support accelerometer calibration and contributing to gravity retrieval, and advanced methods for accelerometer calibration, shall be studied. The results of these studies will be evaluated by an associated science expert panel, leading to potential modifications of the MRD.

In this overview contribution, we will outline the motivation and set-up of this study, present results of potential candidate mission scenarios and evaluate their performance against the science requirements defined in the MRD. Furthermore, we will present and discuss recent developments in processing methodology to be applied to current and next-generation mission concepts. Additionally, we will give an overview on the potential of DORIS for improved precise-orbit determination and associated accelerometer calibration.

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. Studies on scientific analysis of possible mission architectures for QSG are also at the stage of being initiated.
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.

The GRACE and GRACE-FO missions have demonstrated the importance of continuous gravimetric observations of mass transfers within the Earth system for Earth sciences. A new generation of gravity missions is brewing. In this context, and following the recommendations of the 2019 CNES Scientific Prospective Seminar, a pre-Phase-A mission concept study, MARVEL, was launched in January 2020 by CNES.
The implementation framework of MAGIC for ESA is the Next Generation Gravity Mission (NGGM) as a Mission of Opportunity and for NASA the Mass Change Designated Observable (MCDO). MARVEL's scientific objectives are in the same line, in order to continue observing the temporal variations of the Earth's gravity field with increased precision compared to the pioneer GRACE and GRACE-FO missions. Combined with altimetry, these observations allow for a wealth of advances in many fields of Earth sciences: climate, cryosphere, oceanography, hydrology, geodesy, geophysics and also the development of services in management of natural resources and disaster prevention.
One of the key progress realized has been to solve the problem of the North-South striations which complicate the interpretation of the gravity results obtained by GRACE and GRACE-FO missions. This progress can only be obtained through an improvement of the geometry of the observations, which are practically North-South in the case of GRACE and GRACE-FO because of the quasi-polar orbit of these satellites and the in-line configuration of the pairs of satellites.
MARVEL concept, named "Pendular", is based on a single pair of satellites separated by ~200 km, slightly offset in mean anomaly and ascending node so that the measurements between the spacecraft are alternatively oriented to the right and to the left of the track, up to +/- 45°. The succession of ascending and descending tracks allows a quasi-isotropic observation of the Earth's gravity field. MARVEL is therefore an alternative concept to the NGGM/MAGIC "Bender" concept, based on a double pair of satellites: a quasi-polar pair and a pair inclined at ~ 70° providing the East-West observability.
Here, we present results obtained during the MARVEL pre-Phase-A study, mainly related to:
- assessment of the scientific performance of the concept compared to the current GRACE and GRACE-FO results and to the "Bender" configuration. This study concluded that, at equal altitude, the pendulum concept has a comparable performance in accuracy to the "Bender" concept, that the two configurations bring an improvement in the accuracy of the gravity solutions by a factor of 4 to 5 compared to GRACE-FO and solve totally the problem of striping;
- development of an inter-satellite telemetry instrument agile enough to perform a ranging measurement to better than 1 micrometer over an azimuthal scan of +/- 45°, with an extremely light mobile assembly so as not to disturb the carrier satellite. The instrument design is based on existing laser telecom technologies and the expected ranging accuracy is 0.5 micrometer @ 5 s sampling rate.

In the context of an increased public interest in climate-relevant processes, a number of studies on Next Generation Gravity Missions (NGGMs) have been commissioned to better map mass transport processes in the Earth system. On the basis of the successfully completed gravity field missions CHAMP, GOCE and GRACE as well as the current satellite mission GRACE-FO, different concepts were examined for their feasibility and economic efficiency. The focus is on increasing the spatiotemporal resolution while simultaneously reducing the known error effects such as the aliasing of temporal gravity fields due to under-sampling of signals and uncertainties in geophysical background models. An additional inclined pair to a GRACE-like satellite pair (so-called Bender constellation) is the most promising solution. Since the costs for a realization of the Bender constellation are very high, this contribution focuses on alternative concepts in the form of different constellations and formations of small satellites. The latter includes both satellite pairs and chains consisting of trailing satellites. The aim is to provide a cost-effective alternative to the previous gravity field satellites while simultaneously increasing the spatiotemporal resolution and minimizing the above-mentioned error effects.
In numerical closed-loop simulations, the impact of different satellite formations and constellations will be investigated for the retrieval of monthly gravity fields. The configurations differ in the orbital setup including the number of orbital planes and key orbit parameters like altitude and inclination. The ground track coverage of the selected orbits will be analysed since an improved spatial sampling with specific sub-cycles is beneficial for estimating short-temporal gravity fields which will be co-parametrized in the overall solution approach. Due to the large number of observations, it is possible to retrieve sub-daily gravity fields down to quarter-day resolution, which exceeds the capabilities of the existing gravity mission like GRACE or GRACE-FO by far. These (sub-)daily gravity field solutions can also improve the overall monthly gravity product, which will be proven for several satellite constellations and formations. All in all, the opportunities and limits of multiple satellites pairs and chains of trailing satellites for achieving the highest possible spatial and temporal resolution shall be analysed in detail.