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Future Satellite Gravity Missions - Activities in Germany

Jürgen Müller (Univ. Hannover), Nico Sneeuw (Univ. Stuttgart), Frank Flechtner (GFZ, Potsdam)

Activities related to recent satellite gravity missions

German institutions (e.g., GFZ Potsdam and University of Bonn) continued to determine various releases of (monthly and even weekly) global gravity field models based on GRACE and CHAMP mission data, applying refined background models and improved processing strategies (e.g., EIGEN-GRACE05S, ITG-Grace03). A combination with complementary gravity field information derived from terrestrial and air-borne data, satellite altimetry, and satellite laser ranging led to the generation of high-resolution combined gravity field models, such as EIGEN-5C (degree 360).

The most recent high resolution European quasigeoid model EGG2008 was computed by IfE (University of Hannover) within the framework of the European Gravity and Geoid Project (EGGP).

For GOCE, the High-Level Processing Facility (HPF) has been developed, responsible for the generation of GOCE final orbit and gravity field products. This task has been performed by a consortium of 10 university and research facilities in Europe with strong German contribution. At national level the GOCE-HPF is supported by the project RealGOCE, funded by the German Geotechnologien-Program of the BMBF (Federal Ministry of Education and Research). This project, chaired by WD Schuh, Bonn, combines the expertise of several German research groups involved in GOCE.

A further joint German research project, mainly motivated by the successful performance of GRACE and its overwhelming input for Earth system research resulted in a so-called DFG priority programme “Mass transport and mass distributions in the system Earth” in 2006. This programme will run six years (www.massentransporte.de). Moreover, within the aforementioned Geotechnologien-Program, F. Flechtner from GFZ Potsdam chairs the project TOBACO-CHAMP/GRACE dealing with the consistent reprocessing of CHAMP and GRACE data.

Future gravity field missions

Two initiatives with German contribution have been run to evaluate the conditions for realising a short-term GRACE Follow-on mission (launch until 2012/13):

• NASA (lead M. Watkins, JPL) investigated a kind of a copy of GRACE, using again K-band ranging as basic component but with refined instrumentation. Here, a close cooperation exists with EADS/Astrium (manufacturer of the CHAMP and GRACE spacecraft).
• GFZ (lead F. Flechtner) investigated a GRACE-like SST-tracking concept but with an optical link between the two satellites (“GRAF”) and mainly using German technology. This study has been carried out in collaboration with the company STI. There, different inter-satellite separations, orbit altitudes and performances of the laser link have been studied.

In October 2009, representatives of both study groups, GRACE PI’s and a few colleagues from the German science community will meet at GFZ to discuss the conditions for combining both concepts including financial constraints and defining concrete steps towards the realization of a quick GRACE follow-on mission.

On the long-term perspective, a large group of industrial and university partners has successfully applied for funding of studies on future gravity field missions (also considering novel concepts and technology) within the framework of the German Geotechnologien-Program of the BMBF. Start of the three-years program, which is chaired by N. Sneeuw, Stuttgart, was in August 2009.

The German Aerospace center DLR released a call for the study of novel Earth observation concepts in the first half of 2009, where three proposals dedicated to gravity field missions of the next generation have been submitted. Unfortunately, none of them has been selected.

Within the ESA study “Monitoring and Modelling Individual Sources of Mass Distribution and Transport in the Earth system by Means of Satellites” a complete multi-year forward simulation of mass variations in the Earth system has been carried out, potential mission scenarios have been identified, and their performance was investigated by numerical gravity field retrievals.

Further calls have been released by ESA (e.g., ITT “Assessment of a Next Generation Gravity Mission to monitor the variations of Earth’s gravity field”) where also two proposals with German contributions have been submitted and selected. In one of them, German Universities cooperate with Alenia to work on a new simulator system for satellite gravimetry including requirements from Earth system research. In the second, GFZ, University of Bonn and EADS/Astrium further develop their tools for investigating gravity field missions.

Further activities related to future gravity field missions are conducted at various universities at various levels. At the University of Hannover, e.g., a special task group within the center of excellence QUEST (Quantum engineering and space-time research) integrates competences from geodesy and physics for developing advanced satellite gravimetry concepts. There, also a new professorship (Precision Geodesy on Earth and in Space) has been established with a major focus on future gravity field missions.