Successful German-US environmental mission to be continued with GRACE-C

The next generation of the satellite pair will use a high-precision laser instrument with key contributions from the AEI Hannover to monitor indicators of climate change.

March 19, 2024

“Gravity Recovery and Climate Experiment - Continuity” (GRACE-C) will continue more than 20 years of space-based measurements of the Earth's gravity field. The observations, which enable studies of global groundwater levels and other indicators of climate change, are important for climate research. GRACE-C is a joint mission of the German Space Agency at the German Aerospace Center (DLR) and the US space agency NASA. The Max Planck Institute for Gravitational Physics (Albert Einstein Institute; AEI) in Hannover is involved in implementing the German contribution, the construction of the central laser-based measurement system. The launch is scheduled for 2028.

Looking beneath the surface

The droughts of recent years, falling groundwater levels and extreme weather events – in total, Germany has lost more than 15 billion tonnes of water over the past 20 years. To obtain such data and to use it to study the global water balance and regional groundwater levels, it is necessary to look beneath the Earth's surface – and that can only be done from space.

“Water balance, groundwater levels and their changes, as well as other indicators of climate change, can only be observed globally from Earth orbit. GRACE-C is expected to continue a series of measurements of this data, which began in 2002, from 2028. This is invaluable for climate research to identify long-term trends,” says Gerhard Heinzel, leader of the “Interferometry in Space” working group at AEI Hannover.

Like its predecessors GRACE (2002-2017) and GRACE Follow-On (2018-present), GRACE-C will map the Earth's gravity field every month. To do this, a pair of satellites will orbit the Earth at an altitude of about 490 kilometres above the surface, 220 kilometres apart. At the heart of the mission are precise measurements of distance changes between the satellites. These are caused by the varying gravitational pull of mountains, ice masses, local sea levels and groundwater levels on the satellites.

A reliable and precise laser instrument from gravitational-wave research

In addition to the main microwave instrument, a laser instrument was used for the first time on board GRACE Follow-On as a technology demonstrator. “With the laser instrument, a development from gravitational-wave research, we measure short-term distance changes between the GRACE Follow-On satellites with an accuracy of about 200 to 300 picometres – this is about the diameter of a single atom,” explains Vitali Müller, researcher in the “Interferometry in Space” working group at AEI Hannover. This is around 5,000 times more accurate than the previously favoured microwave technology used in parallel, and 400 times more accurate than specified in the mission requirements for the laser instrument.

“From the experience with our laser instrument on board GRACE Follow-On, we know that laser-based distance measurements between satellites in space work absolutely reliably and with high precision for years. For this reason, GRACE-C will measure the distance changes exclusively with a next-generation laser instrument,” says Gerhard Heinzel.

Construction of the central laser measurement system

AEI researchers are contributing their expertise to the design of the laser instrument and are working closely with industrial partners in Germany. A central part of this system called the “Laser Ranging Interferometer” (LRI) – the optical bench – comes from SpaceTech GmbH in Immenstaad an Bodensee (STI). The AEI in Hanover works closely with STI, funds components of the instrument, and provides technical advice to the German Space Agency at the German Aerospace Center.

“The Max Planck Society and the AEI are jointly funding the scientific contributions of our institute and the development and procurement of the flight hardware, namely the photoreceivers that receive the faint light from the distant satellite and convert it into electrical signals,” says Guido Müller, director at the AEI.

GRACE-C is scheduled for launch in 2028. The German Space Operations Center (GSOC) at DLR in Oberpfaffenhofen will be responsible for mission control. AEI scientists will monitor the technical functions of the LRI during the operational phase of the mission.

AEI scientists are also involved in designing the laser instruments for another satellite mission to study the Earth's gravitational field. The European Space Agency ESA is currently planning a “Next Generation Gravity Mission” (NGGM) which, like GRACE-C, will consist of a pair of satellites in an orbit lower than and inclined to that of GRACE-C. By combining the data from both missions in the “Mass-Change and Geosciences International Constellation” (MAGIC) project, the measurement accuracy of changes in the Earth's gravitational field – both in time and in space – will be significantly improved.

“It is great that the basic research instruments developed at our institute for observing gravitational waves in space are already finding practical applications in Earth observation and are important for future missions,” says Vitali Müller.

On the way to LISA

The AEI Hannover also plays a leading role in the development of key hardware components for the laser measurement system of the ESA mission “Laser Interferometer Space Antenna” (LISA), which was adopted by the European Space Agency earlier this year as the third of the large scientific missions in the “Cosmic Vision Programme” and has thus entered the construction phase.

“GRACE-C will continue the success of laser interferometry between two satellites and provide us with further valuable experience for the operation of these instruments in space on the way to LISA,” says Gerhard Heinzel.

“With LISA, we will operate the largest laser interferometer ever built in space. Three satellites will measure picometer distance changes over millions of kilometers to observe low-frequency gravitational waves,” adds Karsten Danzmann, director at AEI.


GRACE-C – a successful series of missions to observe Earth’s environment continues

GRACE-C is a joint mission of the US space agency NASA and the German Space Agency at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR). The German contribution is being realised with funds from the Federal Ministry for Economic Affairs and Climate Action (Bundesministerium für Wirtschaft und Klimaschutz; BMWK) and the Federal Ministry of Education and Research (Bundesministerium für Bildung und Forschung; BMBF). This is supported by contributions from the Helmholtz Association (HGF) and the Max Planck Society (MPG) on the German side. The GeoForschungsZentrum Potsdam (GFZ) will be responsible for the scientific evaluation of the mission data and the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) together with the company SpaceTech GmbH in Immenstaad for the construction of the laser system to measure the distance between the GRACE-C satellite pair. Like the previous GRACE and GRACE-FO missions, the GRACE-C ‘twins’ will be constructed by Airbus in Friedrichshafen on behalf of NASA. GRACE-C is scheduled for launch in 2028 on board a Falcon 9 rocket from the US company SpaceX.

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