Between 2002 and 2017, the US/German GRACE satellite mission provided monthly time series of gravity field models describing mass transport in the Earth system. These models are used by a broad scientific community to observe and analyze seasonal and sub-seasonal variations in the continental hydrological cycle, ice mass loss in Antarctica or Greenland, or surface and deep ocean currents. Since 2018, this time series has been extended by the GRACE Follow-on (GRACE-FO) mission and now spans more than 20 years. GRACE-FO was jointly realized and now operated in NASA/GFZ partnership.  

There is a broad consensus in the scientific user community that these time series need to be a) extended over several decades to improve the plausibility of the climate-related trends and variations derived so far and b) that at the same time the spatial and temporal resolution as well as the measurement accuracy need to be further improved.

For this reason, the GFZ, together with the German Aerospace Center (DLR) and German industry (Airbus Defense and Space and SpaceTech GmbH), and in close coordination with JPL, has conducted various studies between 2020 and 2022 for the realization of a GRACE-FO successor mission with the primary goal of ensuring data continuity after launch in 2028. This mission shall again be realized in cooperation with NASA and is currently still running under the name "GRACE-I or MCM (Mass Change Mission)". The funding of the German contributions of the mission could be secured in October 2022. Unfortunately, the ICARUS component (see Phase-A study) could not be funded.

To increase spatial and temporal resolution, ESA, together with NASA, is planning a Mass change And Geosciences International Constellation (MAGIC). MAGIC will consist of a double pair, with the polar-flying pair being GRACE-I/MCM and ESA, together with NASA, will contribute another inclined-flying pair in 2031. This Next Generation Gravity Mission (NGGM) will also fly deeper than GRACE-I/MCM and will have a more precise accelerometer for observing non-gravitational perturbation forces.

Section 1.2 was and is still involved in several national and ESA funded studies, all of which had or still have the goal to realize data continuity and increased resolution. Some short details about these studies can be found in the following list.

• 2021-2023: “Third Party Mission Science Study” (ESA)
• 2021-2023: “NGGM/MAGIC Science Support Study” (ESA)
• 2021-2023: “NGGM/MAGIC Phase A System Study” (ESA)
• 2022: “Phase-A Study GRACE-I” (BMBF)
• 2018: ESA EE10 Call “MOBILE“
• 2016: ESA EE9 Call "E2.motion"
• 2013-2014: “NGGM-D” (DLR)
• 2009-2012: “Future Gravity Field Satellite Mission" (BMBF)
• 2009-2011: ESA ITT “NG2"
• 2010: ESA EE8 “E.motion”
• 2008-2010: Various GFZ-funded Research and Development Studies

GFZ Section 1.2, together with Section 1.3 and contractors Max Planck Institute for Gravitational Physics (Albert-Einstein-Institute) and HafenCity University Hamburg, have preformed between 2021 and 2023 a 24 months ESA funded study to

1. Investigate options to continue a two-satellite GRACE-FO mission after failure of the redundant Instrument Processing Unit (IPU) on GRACE-FO-2 (substituting the failed primary IPU since already October 2018) and consequent non-availability of microwave tracking data (GPS, K-band ranging (KBR)) based on Laser Ranging Interferometer (LRI) and Satellite Laser Ranging Data.
2. Characterize the LRI data to maximize the scientific return for MAGIC (Mass change And Geosciences International Constellation). The main goal is to investigate and assess GRACE-FO LRI data quantitatively and qualitatively for global gravity field recovery and evaluate its potential benefits against the nominal KBR measurements. Since the LRI is currently successfully operated as a technology demonstrator on GRACE-FO with significantly lower noise compared to KBR, the key Satellte-to-Satellite Tracking (SST) instrument of the MAGIC constellation will be based on LRI technology. During this study, valuable insights and lessons learnt from LRI real data assessment should serve as input for MAGIC’s mission system design in order to contribute to an improved LRI data product serving as the main observable of the MAGIC mission.
3. Perform full-scale end-to-end simulations of selected scenarios for in-line pairs of two satellites and four satellites in a Bender-type double-pair constellation, as well as 3-pair satellite formation consisting of an in-line pair and a third satellite in pendulum configuration (i.e. Advanced Pendulum). The main goal is to identify optimized Advanced Pendulum and MAGIC constellations regarding space-time sampling and maximizing sensitivity to temporal Earth´s gravity changes that are caused mainly by mass transport processes in continental hydrology hydrosphere, oceans, atmosphere, cryosphere, and solid Earth.
4. Demonstrate the value of GRACE/GRACE-FO Level-3 products for applications in Climate Sciences and Hydrometeorology.

The final meeting of the study is planned for summer 2023.

GFZ contributed to following topics 1 (Section 1.2) and 4 (Section 4.4) within a TU Munich led study to

1. Perform full-fledged end-to-end simulations of selected NGGM/MAGIC scenarios as provided by ESA. The main goal is to identify optimized NGGM constellations regarding space-time sampling and maximizing sensitivity to temporal Earth’s gravity changes that are caused mainly by mass transport processes in continental hydrosphere, oceans, atmosphere, cryosphere, and solid Earth. This involves mainly single-pair and double-pair inter-satellite ranging concepts in low-low mode, including in-line and pendulum pairs. Specifically, the provided orbit configurations will be used as building blocks to investigate and assess against each other single-pair, in-line double pair, single-pair pendulum and double-pair pendulum (polar and/or inclined) constellations. In a later step, these orbit scenarios will be optimized especially regarding capabilities for short-term retrievals, taking high frequency atmosphere and ocean non-tidal and ocean tide signals into consideration.
2. Analyze the impact of DORIS as additional observation component onto the achievable performance of orbits and derived gravity field estimates. For this, the methodology has to be set up, and implemented into a simulation chain in order to propagate the impact on temporal gravity field estimates. This will be done in parallel studies by the sub-contractors CNES and TU Delft.
3. Analyze improved methods for accelerometer calibration, especially regarding additional performance gains by inclusion of star tracker, LRI and DORIS observations. Additionally, the effect of the re-orientation of the accelerometer for improved calibration capabilities in all three accelerometer axis is to be analyzed and quantified. This will be done in parallel studies by the sub-contractors CNES and TU Delft.
4. Perform a science impact analysis of the simulation results. In the first step, simulation results for all investigated scenarios are to be matched against ESA provided science and mission requirements. These results shall be fed back to the design of scenarios. The evaluation of simulation results will be transformed to science and service related parameters, and in case of mismatches with the requirements specific science questions have to be rephrased. For this task, the project team will be supported by user consultancy by an expert panel covering all main fields of application on NGGMs.

The final meeting of the study is planned for fall 2023.

Section 1.2 supported Airbus Defense and Space (Airbus D&S) within their “Next Generation Gravity Mission (NGGM) Phase A System Study” in the identification and documentation of the NGGM Payload Data and Ground Segment (PDGS) functions and relevant characteristics.

Section 1.2 led a Phase-A study in the period April-September 2022 to realize the GRACE-I satellite mission. Phase-A followed Phase-0 (April-September 2021, funded by the German Space Agency (DLR)) and was strongly supported by DLR and NASA's Jet Propulsion Laboratory (JPL). GRACE-I is intended to provide data continuity for observation of the global water cycle after GRACE (2002-2017) and GRACE Follow-on (since 2018). Optionally, GRACE-I shall contribute to biodiversity research using the ICARUS (International Cooperation for Animal Research Using Space) payload realized as a tech demo on the ISS between 2020 and 2022. Primary observable are high-precision (nanometer accuracy) LRI (Laser Ranging Interferometer) distance measurements between the twin satellites from which variations in the Earth's gravity field or maps of the Essential Climate Variables (ECV) Terrestrial Water Storage (TWS) are derived on a monthly basis. The LRI is a very successful technology demonstrator on GRACE-FO. Biodiversity research shall be conducted by observing animals and other relevant biodiversity variables using miniaturized terrestrial sensors that transmit their data to a receiver onboard the GRACE-I satellite. GRACE-I, like its two predecessors, shall gain be realized jointly with JPL. Launch is planned for beginning of 2027; the operations Phase E shall last for (at least) 5 years.

To accomplish these goals, a contract was awarded by GFZ to Airbus Defense and Space to (1) investigate in depth the various mission options and payload configurations from Phase 0, (2) compare the options at the system level, (3) develop the detailed design of required technical improvements w.r.t. GRACE-FO, and (4) provide a detailed schedule and cost estimate. The primary outcome of the project should be the preparation of a revised Phase 0 Customer Technical Requirements Specification (CTRS) and a technically and scientifically feasible GRACE-I mission scenario.

Further details can be found in the GRACE/GRACE-FO Science Team Meeting 2022 presentation

Frank Flechtner, Christoph Dahle, Markus Hauk, Josefine Wilms, Michael Murböck, Michael Nyenhuis, and Peter Schaadt (2022): GRACE-I: A joint US-German mission for continued mass transport monitoring and enabling global biodiversity monitoring, online available here

A comprehensive team of European scientists proposed a next-generation gravity field mission “MOBILE” in response to the European Space Agency (ESA) call for a Core Mission in the frame of Earth Explorer 10 (EE10). MOBILE is based on the innovative observational concept of a high-low tracking formation with micrometer ranging accuracy, complemented by new instrument concepts. Since a high-low tracking mission primarily observes the radial component of gravity-induced orbit perturbations, the error structure is close to isotropic. This geometry significantly reduces artefacts of previous along-track ranging low-low formations (GRACE, GRACE-Follow-On) such as the typical striping patterns.

Further details can be found in the paper

Pail, R., Bamber, J., Biancale, R., Bingham, R., Braitenberg, C., Flechtner, F., Gruber, T., Güntner, A., Savenije, H., & More Authors (2019). Mass variation observing system by high low inter-satellite links (MOBILE) - A new concept for sustained observation of mass transport from space. Journal of Geodetic Science, 9(1), 48-58. https://doi.org/10.1515/jogs-2019-0006

In contrast to E.motion the E².motion mission concept was based on a dual GRACE-like pair where the second pair should have been flown on an inclined orbit (so called “Bender configuration”). The proposal was not accepted due to too high implementation cost but can be seen as a milestone towards realization of MAGIC. The final report is available here.

Starting in June 2013 DLR has funded a 12 months R&D study to derive a mission concept for the long-term, highly precise and homogeneous derivation of the time variations of the Earth´s gravity field with much higher sensitivity and spatial resolution as available nowadays with GRACE and starting in 2018 with GRACE-FO.

Following partners participated:

• Institute for Astronomical and Physical Geodesy of the Technical University of Munich (IAPG, coordination)
• Geodetic Institute of the University Stuttgart (GIS)
• Institute for Physical Geodesy of the Leibniz University Hannover (IFE)
• Institute for Gravitational Physics (Albert Einstein Institute) of the Leibniz University Hannover (AEI)
• Institute for Geodesy and Geoinformation of the University Bonn (IGG)
• TransMIT GmbH Gießen (IQM)
• SpaceTech GmbH Immenstaad (STI)
• Astrium GmbH (ASG)
• Department 1 Geodesy and Remote Sensing, Helmholtz Center Potsdam, German Research Center for Geosciences (GFZ)

The results of the study are described in

NGGM-D Team (ed.) with contributions by Baldesarra, M., Brieden, P., Danzmann, K., Daras, I., Doll, B., Feili D., Flechtner, F., Flury, J., Gruber, T., Heinzel, G., Iran Pour, S., Kusche, J., Langemann, M., Löcher, A., Müller, J., Müller, V., Murböck, M., Naeimi, M., Pail, R., Raimondo, J. C., Reiche, J., Reubelt, T., Sheard, B., Sneeuw, N., Wang, X. (2014): e2.motion – Earth System Mass Transport Mission (Square) – Concept for a Next Generation Gravity Field Mission – Final Report of Project “Satellite Gravimetry of the Next Generation (NGGM-D)”,  DGK Reihe B, Nr. 318, München 2014; ISBN 978-3-7696-8597-8; 200 S.

and are available here.

The BMBF Geotechnologien project “Future Gravity Field Satellite Mission” was a joint project between GFZ, German universities and partners from industry and was funded between 2009 till 2012. The overall target was the development of innovative concepts for future satellite based gravity missions to improve the current GRACE spatial and temporal resolution of the time variable components of the Earth’s gravity field. This can only be achieved by a strong cross-link between geodesy, measurement technology and system design. Therefore, the increased scientific requirements have to be conciliated with improvements in the observation techniques and satellite bus design. Consequently, some key questions of this project were

• Which gain do we get from different satellite constellations with equal instruments?

• Do we need a Laser instead a MW SST link?

• Do we have to realize the optical link by satellite or instrument (mirrors) pointing?

• Do we need a drag-free design or improved accelerometers?

• How can we adopt experiences e.g. from LISA Pathfinder project to a gravity mission?

GFZ was one of the scientific partners and supported the project with experience derived from previous GFZ R&D studies and was responsible for the WP120 “Comprehensive numerical simulation of mission concepts”.

The results have been presented in:

Flechtner, F., Sneeuw, N., Schuh, W.-D. (Eds.)(2014):Observation of the System Earth from Space – CHAMP, GRACE, GOCE and future missions, (GEOTECHNOLOGIEN Science Report ; 20) (Advanced Technologies in Earth Sciences), Berlin [u.a.] : Springer, XV, 230 p., doi.org/10.1007/978-3-642-32135-1

Beginning of 2009 ESA offered an Invitation to Tender (ITT) on a study for a Next Generation Gravity Mission (NG2). Two proposals have been accepted and had kick-off in October 2009. The first is led by Thales Alenia (Italy) and was performed together with TU Munich, University of Stuttgart and TU Delft. The second was led by Astrium (Germany) with scientific partners from University Bonn and GFZ. Major tasks were a) to develop a gravity simulator, consisting of a Forward, Backward and Satellite Simulation Module, capable to generate and analyze various instrument data and errors (e.g. SST, accelerometer, star camera data) based on ESA provided time variable background models as well as b) to use the simulator to investigate various mission options (e.g. alternative orbits such as Pendulum or Cartwheel or the use of Laser instead of Microwave links) to derive increased resolution and accuracy. The Final Presentation was in February 2011 at ESTEC.

Based on major GFZ/STI R&D GRACE-FO 2008-2010 study findings a broad science and industry team from various European countries with lead in Germany and France has submitted end of June 2010 a proposal called E.motion (Earth System Mass Transport Mission) in response to the ESA Explorer Opportunity Mission Call EE8. Proposal secretaries were Thomas Gruber (TU Munich) and Isabelle Panet (IGN Paris), the PI was Johnny Johannessen (NERSC). The E.motion proposal was targeting on a GRACE-like mission to be launched in 2018 with a Laser Doppler Interferometer as prime satellite-to-satellite tracking instrument, a pendulum orbit at a lower altitude (ca. 410 km) and improved spatial resolution (ca. 200 km). The proposal was ranked high by ESA, but unfortunately not accepted due to too high implementation cost.

Further reading:

Gruber, T. and the e.motion team: e.motion - A Proposal for a Future Satellite Mission for the Determination of the Time-Variable Earth Gravity Field; GRACE Science Team Meeting, Potsdam, 11.11.2010, download 

Starting in 2008 GFZ has initiated three R&D studies together with SpaceTech GmbH (STI).The first was called “GRAF” (GRACE Follow-on, August 2008 – March 2009) and was focusing on a mission with improvements in terms of accuracy and resolution based on lessons learnt from GRACE and domestic technology. As one of the outcomes of the GRAF study was that a microwave (MW) link with sufficient accuracy is not available in Germany, the second study “3M4C” (Mass Motion Monitoring for Climate, May 2009 – August 2009) investigated the substitution of the MW link by a Laser Doppler Interferometer (LDI) as well as alternative orbit scenarios, in particular a pendulum orbit at lower altitude. The third study “3M4C-FPS” (3M4C Fine Pointing Study, January 2010 – September 2010) investigated the various options for the LDI pointing (spacecraft or instrument pointing) which has much higher requirements than a MW SST link. A suggestion for a GRAF satellite bus incl. a LDI demonstrator (implemented in parallel to the prime MW link) is shown in the figure below.