Gravity Missions Studies

Since 2002 the US/German GRACE satellite mission delivers 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. Unfortunately, the mission lifetime will likely end 2014/15. Regularly updated details on the GRACE mission status are provided at the University of Texas Center for Space Research (UTCSR).

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 2022-25 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 summer 2017.  

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):

 

GFZ funded studies

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.

GRACE follow-on design inclusive a Laser Doppler Interferometer Demonstrator

E.motion

Based on major GFZ/STI R&D GRACE-FO 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.

ESA ITT NG2

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 lead by Thales Alenia (Italy) and was performed together with TU Munich, University of Stuttgart and TU Delft. The second was lead 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.

BMBF Geotechnologien

The BMBF Geotechnologien project “Future Gravity Field Satellite Mission” was a joint project between 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. The major goal was the improvement of the current 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 GFZ R&D studies (see above) and is responsible for the WP120 “Comprehensive numerical simulation of mission concepts”.

NGGM-D

Starting in June 2013 DLR has funded a 12 month 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 2017 with GRACE-FO.

Following partners participate:

  • 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)

Contact

Prof. Dr. Frank Flechtner

Head of Section 1.2:
Global Geomonitoring and Gravity Field

Telegrafenberg
building A17 room 00.12

tel. +49 331 288-1130

c/o DLR Oberpfaffenhofen
Münchner Str. 20
D-82234 Wessling
building 126 room 113

tel. +49 8153 90832 10