Between 2002 and 2017 the US/German GRACE satellite mission delivereds monthly time series of mass transport in the system Earth. These models are used by a broad science community to monitor seasonal and sub-seasonal variations in the continental hydrological cycle, ice mass loss in Antarctica or Greenland as well as surface and deep ocean currents.
On the other side, there is a broad consensus in the user community that these time series a) have to be extended over several decades to strengthen the so far derived values of climate related trends and variations and b) that the spatial and temporal resolution as well as the measurement accuracy shall be increased in parallel. However, a GRACE-2 mission, likely solely based on a Laser Doppler Interferometer, is not scheduled at NASA before about 2025 and a GOCE follow-on mission based on Satellite-to-Satellite Tracking SST is presently also not planned at ESA.
As a gap in the GRACE time series is unacceptable for the national and international user community, GFZ has initiated already in 2008 several R&D studies together with German industry partners to realize a GRACE Follow-on mission with notable improvements based on today’s technology and lessons learnt from GRACE. As a result of these studies the GRACE Follow-on (GRACE-FO) mission is currently realized in a NASA/GFZ partnership with target launch date in spring 2018.
Additionally Section 1.2 was and is involved in various national and ESA studies dealing with the realization of a Next Generation Gravity Mission (NGGM):
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.
Based on major GFZ/STI R&D GRACE-FO 2008/09 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. 410km) and improved spatial resolution (ca. 200km). The proposal was ranked high by ESA, but unfortunately not accepted due to too high implementation cost.
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
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.
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