GRACE Follow-On observes critical indicators of climate change through changes in Earth's gravitational field with LISA technology. The mission uses a laser interferometer between two satellites with nanometre-level accuracy.
GRACE (Gravity Recovery And Climate Experiment, 2002-2017), was a joint US-German satellite mission that has provided new and unexpected insights into the natural processes of the Earth. In the GRACE mission, the distance between two spacecraft was measured using a microwave ranging system. Temporal estimates of the Earth's gravity field are inferred from changes in this distance. The changes in Earth's gravity field in turn can be used to measure indicators of climate change – like polar ice melt or changes in ground water level.
The GRACE Follow-On mission (GRACE FO, 2018-today) reflies the identical GRACE spacecraft and instruments, but supplements the micrometre-level accuracy microwave measurement with a laser interferometer with nanometre-level accuracy.
The laser interferometer demonstrator on GRACE Follow-On is a partnership between NASA, which provides the laser, cavity assembly, and ranging processor, the German Research Center for Geosciences (GFZ), and the Albert Einstein Institute, which is responsible for and overseeing the laser ranging instrument and in particular the measurement optics and steering mirror assembly along with instrument integration and testing.
The mission was successfully launched into Earth orbit on May 22, 2018. First observational data were received in the summer of 2018. Since then, the system has been running reliably and has been delivering high-precision distance measurements in scientific data taking mode.
LISA technology in an Earth orbit
For monitoring the Earth gravitational field LISA Pathfinder technology is employed: The LISA phasemeter was adapted to the needs of the gravity field mapping mission.
Under this project a consortium of U.S. and German institutions produced hardware for the laser ranging interferometer flying on the GRACE Follow-On mission. This system improved the inter-satellite ranging sensitivity by a factor of 200 over the initial GRACE mission.
Preparing the next geodesy satellites
In the future, two orbiting satellite missions could measure the Earth's gravitational field and how it changes even more precisely.
GRACE-I is a joint project of the Helmholtz Association, NASA, DLR, and the Max Planck Society. The pair of identical satellites, will fly in an Earth orbit significantly lower than GRACE Follow-On. This will allow it to monitor changes in the Earth's gravitational field and the global water cycle with unprecedented accuracy.
The European Space Agency (ESA) is currently planning a “Next Generation Gravity Mission” (NGGM), which could consist of two pairs of satellites in low Earth orbits (one over the Earth's poles and one inclined). This should significantly increase the measurement accuracy for changes in the Earth's gravity field – in terms of both temporal and spatial resolution.
Because of their expertise, AEI researchers are involved in the design of the laser interferometers used in both projects.