Illustration: NASA

DTU on high-precision climate mission in space

Friday 18 May 18


Ole Baltazar Andersen
DTU Space
+45 45 25 97 54


John Leif Jørgensen
Professor and Head of Measurement and Instrumentation
DTU Space
+45 45 25 34 48

Two unique satellites

The GRACE-FO satellites are unique in that they will be the only satellites in space that can measure mass concentration on Earth and thus determine, for example, how much land ice is melting in a given location.

They will, over time, provide the most accurate picture to date of the climate changes that are taking place.

The satellites will orbit at a height of approx. 500 km above the Earth, and make 15 trips around the world in any 24-hour period. GRACE-FO will supply both longer time series and completely up-to-date data for ongoing use.

NASA’s Jet Propulsion Laboratory is behind the project, and is heading the mission in collaboration with the German Research Centre for Geosciences (GFZ) and a number of other partners. DTU Space is contributing to the project with both instruments and as a user of data from the satellites for climate research.

DTU is playing a major role in the Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) climate mission which NASA is scheduled to send into space from California within the next few days.

NASA will soon be launching the comprehensive climate mission GRACE Follow-On (GRACE-FO). The launch is scheduled for 21 May from Vandenberg Air Force Base in California with a SpaceX Falcon 9 rocket.

The mission comprises two satellites, which in the coming years and with great precision will examine the hydrological cycle and thus the Earth’s climate. Among other things, the satellites will be used to monitor the melting of the land ice in the Arctic and Antarctica, sea water levels and the distribution of groundwater. DTU Space has developed technology for the satellites, and is involved in the climate research, which is the purpose of the mission.

“The movement of water, whether it is rainwater, groundwater, seawater, or melted ice, is an important parameter in the climate system. With GRACE-FO, we can achieve long, continuous and very accurate measurements of the changes on our planet, and thus obtain a more accurate picture of the climate changes taking place and their effect,” says Professor and Head of Measurement and Instrumentation John Leif Jørgensen from DTU Space.

The measurements are made by the two satellites in partnership recording small changes in gravity. The drag the Earth exerts on the satellites as they orbit the planet depends on the local mass concentration. This concentration changes when ice melts, water levels increase in the oceans, or when groundwater reservoirs grow or decrease. In this way, it is possible to calculate to a high degree of accuracy how much ice is melting or how much global sea levels are rising.

Builds on unique concept
GRACE-FO builds on the unique work that began with the design and construction of the GRACE 1 and GRACE 2 satellites 20 years ago as part of a collaboration between Airbus, NASA’s Jet Propulsion Laboratory (JLP), and DTU Space. This successful mission was launched in 2001 and completed last year. Here, Danish, German, and US researchers cooperated on mapping with great precision the climate changes taking place.

Now take GRACE-FO is taking over where GRACE left off. But with even greater precision in measuring the condition of the Earth’s climate.

This type of satellite is unique, as they are the only satellites that can measure changes in mass concentration on Earth. The highly sensitive satellite system can actually measure so accurately that it can detect whether it has rained in central London, for example. Other climate satellites, such as the European Sentinel satellites, typically measure differences in altitude.

However, the mass concentration measurements conducted with the GRACE satellites have provided the same results as altitude measurements made with other types of systems. With the two independent types of measuring, today there is a relatively solid body of knowledge about the climate changes which are taking place at the moment. And it is this development and its consequences that we must continue to follow and research using data from the new and more precise GRACE-FO satellites.

Precision down to 1/10,000 of a hair’s breadth
The two satellites are flying at a distance of approx. 100 km between them. It is by measuring the variations in the distance between the two satellites which occur as a result of gravity differences beneath them that data on changes in the quantity of, for example, ice and groundwater, and therefore climate change, are obtained.

"With the new GRACE-FO satellites, we will obtain even better data to work with in future"
Ole Baltazar Andersen

The distance between the GRACE-FO satellites is measured, for example, by using laser light, whereas the previous GRACE mission used microwaves which is not quite as precise a technology.

Where the previous GRACE mission could measure variations in the distance between the two satellites down to 1/100 of a hair’s breadth, the new GRACE-FO satellites work by achieving a precision of 1/10,000 of a hair’s breadth. This is roughly the diameter of an atom. Thus, this will enable an unprecedented degree of precision in the task of mapping climate change from space.

DTU Space has developed the star trackers which the satellites use for positioning.

“It has been a big challenge, as it must be done with much higher precision than before,” says John Leif Jørgensen.

“The previous satellite missions have given us a clear picture of how the climate is changing. With the new GRACE-FO satellites, we will obtain even better data to work with in future in the form of much higher spatial resolution than from the original GRACE mission, so that you can see even smaller signals. Not least, GRACE-FO will extend the 15-year time series of data to climate studies first started by GRACE,” says Ole Baltazar Andersen, Senior Researcher at DTU Space.

Watch NASA’s two-minute video on YouTube showing how the GRACE-FO mission works. 

How the satellites measure changes on Earth

The measuring principle behind GRACE-FO is simple: When the two satellites are in space, one approx. 100 km in front of the other, the first satellite and then the second satellite are affected by the small anomalies in gravity which occur between wet and dry ground, or a thick or thinner icecap.

The front satellite is the first to be affected by the increased gravitational pull, and therefore accelerate slightly, thus increasing the distance between it and its twin. When, a short time afterwards, its twin passes the same point, it will be similarly affected and make up the distance to the leading satellite. In this way, the gravitational field can be measured under the satellites by measuring the distance between them. And this can be translated into changes in mass.

The distance between the satellites is measured to a level of precision equating to 1/10,000 of a hair’s breadth. It is done using laser light. This requires that the satellites are positioned extremely precisely, as the laser light from the rear satellite must strike an aperture the size of a five kroner coin or a 50 pence piece—approx. 30 mm in diameter. Moreover, the front satellite must be able to return the laser beam from a mirror to a similar hole in the sensor (interferometer) in the rear satellite.

This has been a major challenge. DTU Space has developed the star trackers which the satellites use for this positioning task, while Airbus has developed the mechanical structures and JLP has been responsible for the laser system. It has taken DTU Space and Airbus five years to solve their respective tasks.