Essential and unique: the GRACE missions

Since 2002, the German-American satellite missions GRACE and GRACE-FO have been providing us with data from which we generate high-resolution models of the Earth's gravity field. In this way, mass changes can be recorded on a monthly basis, for instance the melting of the ice masses in the Arctic and Antarctic, changes in the global water cycle or the contribution of meltwater to sea level rise. The years of drought in Europe and even the catastrophic floods in the Ahr valley and the neighbouring regions are clearly visible in the gravity field maps.

So as to prevent an interruption in the globally unique data series, we are now planning the third generation of the successful satellite tandems. In the recent report of the intergovernmental panel of experts on climate change IPCC (IPCC Assessment Report 6 WG-1), GRACE and GRACE-FO are among the three most frequently mentioned satellite missions. German aerospace travel and the scientific community producing and using the gravity field data have, as a result, gained considerable visibility.

Read more about GRACE (2002–2017)
Read more about GRACE-Follow-On (2018)

How data collection works

Due to the continuous, monthly measurement of variations in the Earth's gravitational field, the GRACE satellites provide important information on the impacts of climate change. Two identical satellites circle the Earth at an altitude of about 490 km in an orbit taking them over the poles. Their distance to each other varies depending on the mass beneath them.

With the help of microwaves, the satellites constantly determine this distance with ultra-precisioncontinuously and ultra-precisely: they measure with the an accuracy of a one tenth of a hair's breadth over a distance of around 220 kilometres. On GRACE-FO, a technology demonstrator, measuring instrument developed largely in Hanover  for future gravity field missions, even makes it possible to measure this distance about thirty times more precisely with the help of laser signals.

Please see this video | 4:10min for further information on the measurement method.

View below the earth's surface from space

The measuring principle attained with GRACE is the only method that can observe variations in the water cycle even far below the surface. This includes groundwater fluctuations, which can, thus, be measured via satellites for the first time. The "Terrestrial Water Storage Data" are, therefore, an important contribution to observing climate change and to making decisions on adaptation measures. The data also contributes to improving the management of increasingly stressed water resources, as we are currently observing in global droughts or in the depletion of groundwater.

Based on the on-going variation in distance between the two satellites, the GFZ generates monthly maps of the gravity field and derives the changes in the global water cycle. The data and maps are available to researchers worldwide free of charge and without access restrictions, for example via the GFZ information portal "Gravity Information Service (GravIS)".

To the Gravity Information Service (GravIS)
To Global Groundwater Product

Grace-FO data: Floods in the Ahr valley

In July 2021, an area of heavy rain moved over the west of Germany and neighbouring states and led to disastrous flooding. North Rhine-Westphalia, Rhineland-Palatinate and the neighbouring countries of Holland and Belgium were particularly affected. Using GRACE-FO data, the three maps show the deviations of the complete water reservoir in the months of June, July and August 2021. It can be seen that in June the soil is too dry, in July the soil is oversaturated due to the heavy rain. But already in August the soil moisture decreased drastically again. These data from GRACE-FO underline that during heavy rainfall events a lot of water runs off the surface with hardly any water being actually stored in the soil.


GRACE-FO-Data: Drought in Europe

Since 2018, most summers in Europe have been far too dry. This leads to sometimes dramatic groundwater deficits. The maps show the regions where the total water storage (Terrestrial Water Storage; TWS) showed a deficit in May of each of the last three years. The situation in Ukraine is drastic, but large parts of Germany have also been affected by drought and water shortages for years. TWS is a so-called essential climate parameter of the Global Climate Observing System (GCOS) and only the GRACE missions are in a position to determine this value globally.

Questions and Answers about Grace

The GRACE (Gravity Recovery and Climate Experiment; 2002-2017) and GRACE-FO (GRACE Follow-on, since 2018) satellite pairs measure regional changes in the Earth's gravitational field. They "see" mass shifts, for example the winter increase and summer melting of the ice sheet on Greenland. In short, Greenland alone loses about 280 billion tons of ice every year. But large-scale groundwater changes can also be recorded from space. The results are important, for example, to quantify the contribution of melting ice to the rise in sea level and the thermal expansion of the warming water.


From the constantly observed variation in distance between the satellites and from other measurement data, researchers at the GFZ generate monthly maps of the gravitational field and derive the resulting mass transports. The data and the maps are available to the scientific community worldwide and are free of charge. Currently, almost 2,700 scientific publications are based on GRACE and GRACE-FO data.

GRACE was launched in March 2002 and ended in December 2017. The successor mission GRACE-FO was launched in May 2018 and has so far been flying trouble-free. Its nominal mission duration ends in May 2023. All in all, a unique long-term series of monthly gravity field maps has been created over the past 20 years.


Determining the Earth's global gravity field with satellite methods requires the continuous measurement of the position and velocity of (propulsion-free) test masses in low-Earth orbit. Satellites in polar orbits at an altitude of 500 km are only visible from a particular ground station for a short time, so measurements from very many different observatories would be required for accurate gravity field data. Currently, the most accurate maps of the Earth's gravity field are provided by the continuous measurement of the relative variation in distance between two GRACE satellites, whose entire design, including the complementary sensors (e.g. accelerometers and star sensors), has been optimised for this surveying task of the very highest accuracy.

The data gained from the missions so far document large-scale mass changes in the Earth system that cannot be detected by other missions. The effects of the diverse and complex feedbacks of human activity on the global water cycle and on the climate system must be permanently observed. Only then can this be reliably mapped in models, which are indispensable for forecasts of future developments.

The goal of the follow-up mission is to map the data series from 2027 onwards for at least a further 7 years in a way comparable to the method used to date, in order to then have data available for a complete 30-year climate period. In addition, prototypes for operational services (e.g. global groundwater monitoring) are currently being developed, which depend on the future availability of mass transport data. Continuously available TWS data are, therefore, essential to better assess climate change on regional and global scales and to help decide on necessary management measures. An interruption or even the end of the time series must, therefore, be avoided at all costs.

To ensure comparability of the data, GRACE-I is largely identical in construction to GRACE-FO and GRACE. However, the mission will boast the Laser Ranging Interferometer —tested very successfully on GRACE-FO— as the sole distance measuring instrument on board.. In addition, as with GRACE-FO, a technology demonstrator for future gravity field missions will be on board: an accelerometer for the more precise observation of non-gravitational disturbance accelerations such as solar radiation pressure or the residual atmosphere at flight altitude.


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