High-tech for surveying the world: funds provided
Climate satellite GRACE Follow-on to be launched into space in 2017 with new technology
A high-precision measurement technology developed for gravitational-wave research is now also being used in Earth system and climate research: Starting in 2017, the so-called Laser Ranging Interferometer (LRI) will make it possible to record mass flows in the Earth system - including melting ice sheets or changes in the continental water cycle, for example - with unprecedented precision. The technology will be an important component of the new German-US mission GRACE Follow-on (GRACE-FO). GRACE-FO will succeed the successful GRACE (Gravity Recovery and Climate Experiment) mission of NASA and the German Aerospace Center (DLR) which operated from 2002 to 2017.
The new technology was developed under the leadership of the Albert Einstein Institute Hannover (AEI, Max Planck Institute for Gravitational Physics and Institute for Gravitational Physics of Leibniz Universität Hannover) within the framework of the cluster of excellence “Centre for Quantum Engineering and Space-Time Research (QUEST)”.
The Federal Ministry of Education and Research, the Federal Ministry of Economics and Technology and the Helmholtz Association are funding the satellite project with a total of 49.2 million euros. In addition, DLR is providing parts of the LRI as well as laser retro-reflectors for both satellites. The mission operation is funded by the German Research Centre for Geosciences (GFZ).
“We are very happy that our method developed for gravitational-wave research can now also contribute to climate research. Changes in the Earth's gravitational field will be detected with unprecedented precision,” says Prof. Dr. Karsten Danzmann, Director at the AEI and Professor at Leibniz Universität Hannover.
“The gravity fields that we have been deriving monthly since 2002 from the distance measurements with the previous GRACE microwave instrument will be significantly improved with the LRI on GRACE-FO. This will lead to more reliable statements on climate change. With its expertise in gravity and experimental methods, the AEI is an ideal partner for the GFZ in this project,” says Prof. Dr. Frank Flechtner of the GFZ in Potsdam, who leads the German contribution to GRACE-FO.
GRACE-FO will succeed the GRACE project at the latest in 2017, and will record the spatial and temporal changes in the Earth's gravitational field with a significantly better resolution and measurement accuracy. The corresponding data are in great demand in international Earth system and climate research in order to better understand current climate events and to better predict climate changes over long periods of time. The German contributions to GRACE-FO are realised under the leadership of the GFZ. In addition, the GFZ will evaluate the scientific data of the mission. The AEI is responsible for the design and management of the LRI instrument, which was developed together with NASA's Jet Propulsion Laboratory (JPL). Other partners in Germany are SpaceTech GmbH in Immenstaad, Astrium GmbH and the German Aerospace Center (DLR).
The Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI), with branches in Hanover and Potsdam-Golm, is the only research institute in the world that researches the full range of gravitational physics: From abstract theory to experimental application. The long-standing intensive cooperation between AEI, the Jet Propulsion Laboratory and NASA's Goddard Space Flight Center in the field of laser interferometry for LISA (Laser Interferometer Space Antenna), a gravitational-wave observatory in space, also underlines the ideal partnership in this project.
Since its foundation, the German Research Centre for Geosciences GFZ has been working intensively on the determination of the Earth's gravity field. For this purpose, the first passive satellite GFZ-1 was launched in 1995. It carried 60 laser retroreflectors on its spherical surface, which were targeted with lasers from the ground. Its successor in 2000 was the CHAMP (CHAllenging Minisatellite Payload) satellite developed at GFZ, which, like GRACE, housed an onboard GPS receiver. For the first time, CHAMP carried an accelerometer with which the non-gravitational forces due to the residual atmosphere or solar radiation pressure could be measured directly. For the first time it was possible to determine the gravitational field from the data of a single satellite alone. GRACE is based on CHAMP, a concept in which two “CHAMP-like” satellites were connected by a high-precision inter-satellite link. Prof. Dr. Frank Flechtner from GFZ is Co-PI for GRACE, the GFZ is part of the scientific processing system. Based on the great success of GRACE, the GFZ planned and realized the follow-up mission GRACE-FO together with US colleagues and currently manages the German mission shares.
Hydrologists, glaciologists and oceanographers have for a long time been asking for more precise measuring methods with higher temporal and spatial resolution and improved accuracy for intersatellite distance measurements on the one hand, and long-term measurements of mass fluxes in the Earth system beyond the mission duration of GRACE on the other hand. GRACE-FO meets both requirements: The mission will allow to continue the measurements which started with GRACE - which are expected come to an end in 2014/15 for technical reasons. At the same time, an additional high-precision measurement system will be introduced with the LRI. It will operate in parallel with the existing, less accurate microwave measurement system and will also provide very accurate data on the orientation of the two satellites relative to each other.
What is measured
The measuring systems on GRACE and GRACE-FO register even the smallest distance changes between the satellites. The corresponding data allows to draw conclusions about changes in the Earth's gravitational field and the causes of these changes. This information is an essential basis for research into global climate change. For example, it is observed how the ice mass balance of Greenland and Antarctica changes or how regional water reservoirs in the Earth's large river basins and in continental aquifers develop. In addition, this is also about how ocean masses and sea levels change, what causes these changes and what conclusions can be drawn from the data on climate-relevant deep ocean currents, for example.
About three years ago, American (NASA/Jet Propulsion Laboratory) and German (GFZ/AEI) institutions started initial studies on the planning and implementation of a GRACE follow-up mission (GRACE-FO). The aim was to bring a cost-effective follow-up mission into a GRACE-like orbit as soon as possible. Therefore, a replica of the GRACE system based on the current technology status was planned, which will be additionally equipped with an experimental Laser Ranging Interferometer (LRI).
Like GRACE, GRACE-FO will consist of two identical satellites, which will fly behind each other at a distance of about 220 kilometres in the same polar orbit at an altitude of about 490 km. Each of the two satellites is equipped with a GNSS (Global Navigation Satellite System) receiver to determine its position, an accelerometer to correct disturbing accelerations caused by the residual atmosphere and solar radiation, and three star sensors to determine the satellite's position in space. At the heart of the instrumentation is an ultra-precise microwave distance measuring system, which can measure the distance between the satellites with an accuracy of a few thousandths of a millimetre, and the new Laser Ranging Interferometer (LRI), which measures up to 50 times more accurately. When this technology proves successful, the distance measurement between the two next-generation satellites (after 2020) will be based entirely on an LRI system. The German companies currently building the LRI instrument will benefit from this. They possess key qualifications that will be of decisive importance for future interferometry missions.
The Laser Ranging Instrument (LRI)
The LRI is the essential innovation in GRACE-FO compared to GRACE. It will operate in parallel to the existing microwave instrument and is primarily intended to provide the same measurand - the distance between the satellites - but with significantly improved accuracy. In addition, it provides very accurate data about the orientation of the satellites relative to each other. This data will allow a mutual calibration of the two distance measurements and an improved correction of the interference caused by the fluctuating orientation of the satellites relative to each other. The LRI is designed as an experimental add-on instrument, thus it can be built cost-effectively with lower requirements for lifetime and reliability. When it works as expected, it will significantly improve the data of GRACE-FO compared to GRACE; if it fails, GRACE-FO will provide data of the same high quality as GRACE.
The LRI on GRACE-FO will be the first laser interferometer between satellites ever. It will provide crucial experience on design, construction, operation and behaviour in orbit that cannot be gained in any other way. The associated “Preliminary Design Review” was successfully passed in the USA at the end of May.
The design of the LRI is largely based on the many years of experience gained at the AEI in theoretical and experimental preparations for the LISA and LISA Pathfinder missions for detecting gravitational waves in space. A successful operation would thus also provide decisive support for the eLISA mission and further strengthen the German leadership in the field of laser interferometry between satellites worldwide.
Laser interferometry at the AEI
Since the 1970s, Max Planck scientists have been among the pioneers of laser interferometry in gravitational-wave research. In this context, the Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) develops high-precision methods and technologies for Earth-bound and space-borne gravitational-wave detectors. Since 2006, the AEI has also been investigating how these technologies can be applied in satellite gravity field missions. Since 2009, scientists at the AEI have been working specifically on the LRI of GRACE-FO, developing the basic design, and conducting extensive optical simulations and first experiments.
In the laboratory, numerous critical components were tested, selecting those that allow length measurements in orbit with picometer stability (10-12 meters) - that is one quadrillionth of a meter. In addition, AEI scientists demonstrated the frequency stabilization of lasers required for LISA in the laboratory as early as 1998.
AEI scientists have been investigating the effects of fluctuations in the satellite's orientation and how they can be measured in detail for years, experimentally and with numerical simulations, so that today the problems and their solutions are well understood.
For LISA, more advanced techniques have been developed that are not necessary or applicable to GRACE-FO. These include, for example, the transmission of data with the laser beam, the multi-stage stabilization of the laser frequency with the interferometer arms as reference, or the in-house development of a phase meter.