At first glance, masses appear rigid and immobile. However, this is deceptive because they are constantly in motion. Liquid rock shifts in Earth's interior, water is redistributed in large quantities across the oceans and on the continents, and air masses are constantly in flux. This uneven distribution means that Earth's gravitational field is not constant across the globe. In locations with more mass, the gravitational force is slightly stronger than elsewhere.
Since their launch, the satellites of the GRACE satellites on 17 March 2002 and those of the successor mission 'GRACE Follow-On' (GRACE-FO) have been recording these slow movements with great precision. "The GRACE satellites have made it possible to globally survey the mass transports in the Earth system for the first time. The GRACE and GRACE-FO missions have recorded every small change in these mass flows so precisely that researchers have been able to measure Earth's water balance, among other things, with previously unattained accuracy and consistency. These measurements enable us to better understand the consequences of climate change, because, for example, the melting of the ice masses and the changes in the sea and groundwater levels can be determined more precisely,” explains Walther Pelzer, DLR Executive Board Member and Head of the German Space Agency at DLR.
4.7 trillion tonnes of ice have melted in Greenland
The Greenland ice sheet is melting – much faster than expected. Every year, this ice sheet loses 277 gigatonnes of mass. One gigatonne corresponds to a cube with edges that are one kilometre long. "Over the last 20 years, the Greenland ice sheet has lost approximately 4.7 trillion tonnes of ice. Danish researchers have obtained these findings from the long GRACE data series, which is also regularly included in the reports of the Intergovernmental Panel on Climate Change (IPCC). In the latest report, GRACE is the third most cited satellite mission. This underlines the importance of this mission for climate research," emphasises Pelzer. However, ice melt is not the only important climate indicator. Dry periods lasting several months in the Amazon and drastic decreases in the groundwater level in northern India could be observed just as reliably with the GRACE data as the rise in sea level over two decades. In addition, the Potsdam gravity globe was created using GRACE data. This 'gravitational globe' shows that the gravitational field is not evenly distributed. Thanks to this three-dimensional geoid, the deviations in Earth’s gravity become clearly visible. According to this, the influence of gravity is particularly strong over the Himalayas and the North Atlantic but rather weak over the Indian Ocean and the Lesser Antilles.
Determining the weight of water masses and continents from space
How exactly can satellites measure these mass transports? GRACE and GRACE-FO measure masses solely based on their gravitational effect. The two satellites fly with an average separation of approximately 220 kilometres between them. The relative distance and speed of the two satellites are constantly measured with great precision using microwaves and a laser. The accuracy of one to two micrometres corresponds to about one hundredth of the thickness of a sheet of printer paper. "Rock and water – whether in solid or liquid form – exert a gravitational force with their masses. If it is stronger, the satellite in front is attracted by it as it flies over. As a result, it speeds up and moves away from the other satellite. This minimal change in the mutual distance is measured continuously over each orbit around Earth. Lower masses accelerate the leading satellite less and bring it closer again. In a figurative sense, we can use the satellites to weigh ice sheets and the continents and determine a decrease or increase in weight from month to month," explains Peter Schaadt, GRACE-FO Programme Manager for the German Space Agency at DLR. The determination of the weight is not done in space but by using complicated computational procedures on the ground, whereby the very small relative movements of the satellites in Earth orbit are translated into gravity field values.
'Tom' and 'Jerry' in safe hands at the German Space Operations Center at DLR
During the launch of the two GRACE satellites on 17 March 2002, 'Tom' and 'Jerry' were released at an altitude of approximately 500 kilometres. The first contact was made with the DLR ground station in Weilheim approximately 70 seconds later. The first data from the satellites were sent directly and without delay to the German Space Operations Center (GSOC) in Oberpfaffenhofen via a communications network. The 'acquisition phase' of the satellites then began there. This early phase following the launch was also carried out by the control centre in 2018 for the GRACE-FO mission, as Sebastian Löw, Project Manager of the GRACE-FO ground segment from DLR Space Operations and Astronaut Training, explains: "We determined the orbital parameters, analysed the status of GRACE, tested the functional readiness of all systems as well as their configuration for their actual scientific tasks. During the acquisition phase, we also commissioned the autonomous on-board systems for determining position and attitude, as well as the on-board star sensors and the attitude control system." The operational tasks were identical for both missions, GRACE and GRACE-FO. During the first two weeks following each launch, the operations team then also commanded several orbital manoeuvres to adjust the relative distance between the satellites to 220 kilometres. The control of this formation flight requires regular correction manoeuvres at intervals of several weeks, commanded by GSOC. For this purpose and for satellite control during the mission, they use two of the antennas at DLR's Weilheim ground station – supported for the first 30 days by NASA's Polar Network. Following a two-week test phase, the control and operation of the two GRACE satellites lay solely in the hands of GSOC. All received data were processed, archived and distributed to the scientific evaluation centres at the German Remote Sensing Data Center (Deutsches Fernerkundungsdatenzentrum; DFD) in Neustrelitz.
20 years of successful German American cooperation – a successful model with potential for the future
The GRACE and GRACE-FO time series measurements are essential for climate research. This is why NASA, DLR and GFZ are working together with Max Planck Institutes to prepare the next mission that will continue the observations following GRACE-FO. "The cooperation with NASA in Earth observation clearly represents the common goals that the USA and Germany want to pursue in climate policy," says Pelzer. "NASA's commissioning of the satellite construction also demonstrates the capabilities of the German space industry."
GRACE and GRACE-FO – two successful missions
GRACE was a joint mission of NASA and DLR that operated until 2017, three times longer than originally envisaged. The scientific data evaluation was carried out by the University of Texas and German Research Centre for Geosciences in Potsdam (Deutsches GeoForschungsZentrum; GFZ). GSOC was responsible for operations and received funding from DLR, the German Space Agency at DLR – with funding from the Federal Ministry for Economic Affairs and Climate Action (Bundesministerium für Wirtschaft und Klimaschutz; BMWK) – and GFZ. The Jet Propulsion Laboratory (JPL) was responsible for managing the mission on behalf of the NASA Science Mission Directorate in Washington. The GRACE 'twins' were built by Airbus (formerly Astrium) in Friedrichshafen on behalf of NASA. The successors for the GRACE-FO mission were also built there, again funded by NASA, and have been conducting gravitational measurements since their launch on 22 May 2018.