Nearly 900 days of laser measurements in Earth orbit

Successful operation of the laser interferometer on board GRACE Follow-On began three years ago.

June 14, 2021

In mid-June 2018, it was “First Light!” for a novel laser instrument circling the Earth. The Laser Ranging Interferometer on both satellites of the German-US geodesy mission GRACE Follow-On was activated for the first time. At the first attempt, the instruments – 200 kilometers apart – were able to find each other 490 kilometers above the Earth's surface. Since then, the system has been running reliably and has been delivering high-precision distance measurements in scientific data taking mode. Now, researchers at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI) and Leibniz University Hannover look back the successful first three years and towards the future.

How does GRACE Follow-On observe ice and water on Earth?

The GRACE Follow-On satellite pair circles the Earth 490 kilometers above its surface. The satellites follow one another in a distance of 220 kilometers in a 90-minute orbit, which takes the satellites above the Earth’s poles. The inter-satellite distance varies over some hundreds of meters each orbit because of the flattening of the Earth. On top of these large changes are much smaller variations in the micrometer and nanometer range, caused by the local structure of Earth’s gravity field from mountain ranges, ice masses, bodies of ground water, and others.

Observing how these minuscule variations change over the course of months allows researchers to precisely detect the melting of ice sheets in Greenland and the Antarctic, rising sea levels, changing groundwater levels, droughts and floods, and to define the geoid, which is at the base of global height measurements. The novel Laser Ranging Interferometer technology could significantly increase the precision on future GRACE Follow-On like missions to help those missions provide more detailed measurements of Earth’s gravity field and its changes over time.

Why is GRACE Follow-On an important milestone on the path to LISA?

The GRACE Follow-On Laser Ranging Interferometer is the second space-borne laser interferometer with significant contributions from the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) and Leibniz Universität in Hannover, Germany. The first such instrument was a 40-centimeter instrument on board the LISA Pathfinder mission, which showed the key technologies for LISA, the future gravitational-wave observatory in space.

After its launch in 2034 LISA will use technologies very similar to those of GRACE Follow-On to measure tiny length changes over a distance of 2.5 million kilometers. LISA will detect low-frequency gravitational waves from millions of binary stars in our Milky Way, merging supermassive black holes in the entire Universe, and from other exotic objects.

Who is involved in GRACE Follow-On?

GRACE Follow-On is a joint project of NASA and German partners led by the GFZ German Research Centre for Geosciences. It is the improved successor to the successful GRACE mission which operated from 2002 to 2017. The project is funded by the Federal Ministry of Education and Research under the code 03F0654B.

Who is involved in the Laser Ranging Interferometer?

The LRI is a cooperation between NASA and German partners, with the German contribution led by the AEI. In Germany, the LRI concept, its prototypes and technical specifications were done at the AEI. AEI researchers have been heavily involved in developing and testing the flight hardware. The development of the LRI is based on a longstanding partnership between AEI and NASA’s Jet Propulsion Laboratory. Future missions can rely solely on the more precise laser interferometers instead of microwave interferometers.

The German contributions to the LRI include the entire optical system, consisting of a steering mirror, built by Hensoldt (previously Zeiss) in Oberkochen and the optical bench from SpaceTech GmbH in Immenstaad, opto-electronics from the German Aerospace Center (DLR) in Berlin-Adlershof, and electronic systems from Apcon AeroSpace & Defence in Neubiberg near Munich. SpaceTech GmbH is responsible for the industrial part of the entire German LRI contribution. The DLR Institute of Space Systems in Bremen developed and delivered calibration and testing instruments. Both satellites were built on behalf of NASA by Airbus in Friedrichshafen. Mission control for GRACE Follow-On is provided by the German Space Operations Centre (GSOC) in Oberpfaffenhofen near Munich under subcontract of GFZ.

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