GRACE-C – German-US-American environmental mission has been extended
March 19, 2024 | New pair of satellites will detect the consequences of climate change by measuring mass changes
GRACE-C – German-US-American environmental mission has been extended
The GRACE principle
The idea behind the GRACE principle is quite simple: GRACE is used to ‘weigh’ ice sheets and continents to see how their mass decrease or increases from month to month. To do this, the two satellites fly one behind the other at an average distance of only around 220 kilometres, recording the masses of ice sheets and continents solely based on their gravitational effect. The stronger the gravitational force, the more the leading satellite is attracted by the mass as it flies over it. This causes the first satellite to accelerate and move away from the one behind. The weaker this force is, the less the leading satellite is accelerated. The first satellite then gets closer to the satellite behind it again. Their relative distance variations and speed will be constantly and precisely measured using lasers, achieving an accuracy of 200 to 300 picometres – roughly the size of an atom.
Artist's impression of the GRACE satellites orbiting Earth
For 20 years, the German-US twin GRACE satellites documented changes in Earth's gravitational field, providing valuable data to better understand the Earth system as well as the specific effects of climate change. From 2028, GRACE-C is expected to continue this data series.
GRACE’s observations are so different from other types of hydrology data that researchers had to develop innovative techniques before they could use the new data set and combine it with other observations and models. Within a year or two of its launch, however, the GRACE dataset was providing new insights into how the hydrological cycle is changing, especially in places that are difficult to monitor from Earth’s surface. Even today, researchers continue to develop new techniques to process and analyse GRACE’s data to obtain information that is not measured by other sensors. This map shows where Earth's gravity field deviates from the calculated model. Red and yellow colours indicate the areas where the field is stronger, dark blue colours indicate the areas where it is weaker than the simulation. The GRACE-C satellites are expected to continue these measurements from 2028 onwards.
The German Space Agency at the German Aerospace Center (DLR) and the US space agency NASA are continuing their gravity field measurements from space with the 'Gravity Recovery and Climate Experiment - Continuity' (GRACE-C) mission, providing unique observations of Earth's changing water cycle.
The German contribution is being implemented with funding from the Federal Ministry for Economic Affairs and Climate Action (BMWK) and the Federal Ministry of Education and Research (BMBF), with the participation of the GeoForschungsZentrum (GFZ) in Potsdam and the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Hanover.
The data sets from the missions GRACE (2002-2017) and GRACE-FO (2018 until now) are now one of the foundations for the reports created by the Intergovernmental Panel on Climate Change (IPCC).
Focus: Spaceflight, climate change, global change
Europe's Mediterranean region has been drying out for years. In some regions of Spain, such as the city of Barcelona, there is a state of alarm because the groundwater level is falling by three metres per year in some places. It has also been consistently low across the continent since the record drought year of 2018, even though recent extreme weather events with flooding have given a different impression. Germany has lost more than 15 billion tonnes of water over the past 20 years. In order to obtain such data and use it to gain an accurate picture of groundwater levels and the global water balance, it is necessary to 'look' beneath Earth’s surface from space. Together with other measurement methods, the data from a very special pair of satellites has been helping with this for over two decades. On 17 March 2002, 'Tom' and 'Jerry', the first two satellites in the 'Gravity Recovery and Climate Experiment' (GRACE) mission were launched by the US space agency NASA and the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR).
Twenty-two years later, the German Space Agency at DLR and NASA have extended this highly successful mission for the second time with GRACE-C, which succeeds GRACE Follow-On (GRACE-FO). The 'C' stands for 'Continuity', which recognises the consistency of the measurement series of these environmental missions. The German scientific partners are the GeoForschungsZentrum (GFZ) in Potsdam and the Max Planck Institute for Gravitational Physics (Albert Einstein Institute; AEI) in Hanover. The satellites will be constructed at Airbus in Friedrichshafen. Important parts of the instrumentation will come from SpaceTech GmbH in Immenstaad. The launch of the new GRACE-C satellite pair is scheduled for 2028, on board a Falcon 9 rocket from the US company SpaceX. The German Space Operations Center (GSOC) at DLR in Oberpfaffenhofen near Munich will then take over mission control.
"Without water, life would not exist. That is why water, alongside clean air, is by far the most important resource on Earth. But groundwater levels around the world are constantly changing. This is not a trivial matter. With the GRACE satellites, we have been recording every change in these mass transports globally for more than 20 years with such precision that researchers have been able to measure Earth’s water balance, for example, with previously unattainable accuracy and consistency. The GRACE-C mission will continue this invaluable data collection, which is one of the foundations for the reports created by the Intergovernmental Panel on Climate Change," explains Walther Pelzer, a member of the DLR Executive Board and Director General of the German Space Agency at DLR in Bonn. "Together with NASA, we are now continuing along the GRACE route in Earth observation, thereby strengthening our international cooperation in space-based research. The USA and Germany have been working closely together for a long time on climate and environmental research from space. The trust that our US partners are placing in German space expertise for these missions by commissioning the satellite construction and the delivery of important parts of the GRACE-C instrumentation and mission control is also a sign of Germany’s capabilities as a prime location for spaceflight," emphasises Pelzer.
"GRACE-C represents an international and collaborative effort to observe and study one of our planet’s most precious resources," said Nicola Fox, Associate Administrator for the Science Mission Directorate at NASA Headquarters in Washington. She adds, "From our coastlines to our kitchen tables, there is no aspect of our planet that is not impacted by changes in the water cycle. The partnership between NASA and the German Aerospace Center will serve a critical role in preparing for the challenges we face today and tomorrow."
GRACE-C – NASA relies on German space expertise
The two satellites will be constructed at Airbus in Friedrichshafen on behalf of NASA’s Jet Propulsion Laboratory (JPL). The centrepiece of the GRACE-C mission is the precise measurement of very small distance variations between the two satellites as they orbit Earth. For GRACE-C, this distance is determined using laser interferometry. An important part of this Laser Ranging Interferometer (LRI) system – the optical bench – is being manufactured by SpaceTech GmbH in Immenstaad on Lake Constance. Its engineers are being supported by the Max Planck Institute for Gravitational Physics (Albert Einstein Institute; AEI) in Hannover. The AEI is providing technical advice and funding for the procurement of LRI components and test equipment, which in turn will be commissioned by SpaceTech. The AEI will also monitor the technical functionality of the LRI during the operational phase.
GRACE-C – bodies of water and continents will be weighed from space
How do the satellites with this special laser system actually measure the movement of the masses? The idea behind the GRACE principle is actually quite simple. The pair of satellites measure the masses solely on the basis of their gravitational effect. To do this, the two satellites will fly one behind the other at an average distance of only approximately 220 kilometres. Their relative distance variations and speed will be constantly and precisely measured using lasers. An accuracy of 200 to 300 picometres can be achieved, which corresponds to around roughly to the size of an atom.
"Rock and water – whether in solid or liquid form – influence the trajectory of the satellites in space with their masses. The stronger this force is, the more the leading satellite is attracted by it as it flies over. This causes it to accelerate and move away from the other satellite. The weaker this force is, the less the leading satellite is accelerated. It then approaches the trailing satellite. This minute change in the mutual distance is measured continuously over each orbit around Earth. In a figurative sense, we use GRACE to weigh how ice sheets and continents decrease or increase in mass from month to month," explains Sebastian Fischer, GRACE-C Programme Manager for the German Space Agency at DLR. However, weighing does not only take place in space; the tiny relative movements of the satellites in Earth orbit are only translated into gravity field values using complex computational procedures on the ground. GFZ in Potsdam will play an important role here; it will be responsible for setting up the Science Data System (SDS) on the German side. During the operational phase, GFZ will be responsible for the scientific operations of GRACE-C.
GRACE-C – German-US mission under DLR control
Following the launch of the two GRACE-C satellites on board a Falcon 9 rocket from the US company SpaceX, which is expected to take place in 2028, they will be deployed at an altitude of approximately 500 kilometres. The first contact with a ground station will take place approximately one minute later. As with GRACE and GRACE-FO, the two GRACE-C satellites will be controlled by the German Space Operations Center (GSOC) at DLR in Oberpfaffenhofen near Munich after launch.
GRACE – a successful series of missions to observe Earth’s environment
GRACE was a joint mission of NASA and the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR), which was operated until 2017 and thus lasted three times longer than originally planned. The scientific data analysis was carried out by the University of Texas and the GeoForschungsZentrum Potsdam (GFZ). Operations were the responsibility of the German Space Operations Center at DLR in Oberpfaffenhofen and were financed by DLR (currently the German Space Agency at DLR) with funds from the Federal Ministry for Economic Affairs and Climate Action (Bundesministerium für Wirtschaft und Klimaschutz; BMWK) and the GFZ. JPL managed the mission on behalf of the NASA Science Mission Directorate in Washington. The GRACE ‘twins’ were built by Airbus in Friedrichshafen on behalf of NASA. Their successors, for the GRACE-FO mission, which have been continuing the gravitational measurements since their launch on 22 May 2018, were also built there, again financed by NASA. The GRACE-C mission spacecraft, which are due to be launched in 2028, will also be constructed in Friedrichshafen. The German contribution is being realised by the German Space Agency at DLR with funding from the 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.
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Martin Fleischmann
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