A Legacy of Discoveries

GRACE-FO will extend the GRACE data record an additional five years and expand its legacy of scientific achievements. GRACE chronicled the ongoing loss of mass from the Greenland and Antarctic ice sheets and mountain glaciers. That wealth of data shed light on the key processes, short-term variability, and long-term trends that impact sea level rise, helping to improve sea level projections. The estimates of total water storage on land derived from GRACE data, from groundwater changes in deep aquifers to changes in soil moisture and surface water, are giving water managers new tools to measure the impact of droughts and monitor and forecast floods.

GRACE data also have been used to infer changes in deep ocean currents, a driving force in Earth’s climate. Its atmospheric temperature profile data, derived from measurements of how signals from the constellation of GPS satellites were bent as they traveled through the atmosphere and received by antennas on the GRACE satellites, have contributed to U.S. and European weather forecast products. GRACE data have even been used to measure changes within the solid Earth itself, including the response of Earth’s crust to the retreat of glaciers since the last Ice Age, and the impact of large earthquakes.

According to Frank Webb, GRACE-FO project scientist at JPL, the new mission will provide invaluable observations of long-term climate-related mass changes.

“The only way to know for sure whether observed multi-year trends represent long-term changes in mass balance is to extend the length of the observations,” Webb said.

An Orbiting Cat and Mouse

Like its predecessors, the two identical GRACE-FO satellites will function as a single instrument. The satellites orbit Earth about 137 miles (220 kilometers) apart, at an initial altitude of about 305 miles (490 kilometers). Each satellite continually sends microwave signals to the other to accurately measure changes in the distance between them. As they fly over a massive Earth feature, such as a mountain range or underground aquifer, the gravitational pull of that feature tugs on the satellites, changing the distance separating them. By tracking changes in their separation distance with incredible accuracy – to less than the thickness of a human hair – the satellites are able to map these regional gravity changes.

A global positioning system receiver is used to track each spacecraft’s position relative to Earth’s surface, and onboard accelerometers record non-gravitational forces on the spacecraft, such as atmospheric drag and solar radiation. These data are combined to produce monthly maps of the regional changes in global gravity and corresponding near-surface mass variations, which primarily reflect changes in the distribution of water mass in Earth’s atmosphere, oceans, land and ice sheets.

In addition, GRACE-FO will test an experimental Laser Ranging Interferometer, an instrument that could increase the precision of measurements between the two spacecraft, by a factor of 10 or more, for future missions similar to GRACE. The interferometer, developed by a German/American instrument team, will be the first in-space demonstration of laser interferometry between satellites.

“The Laser Ranging Interferometer is an excellent example of a great partnership,” said Frank Flechtner, GFZ’s GRACE-FO project manager. “I’m looking forward to analyzing these innovative inter-satellite ranging data and their impact on gravity field modeling.”

GRACE-FO will be launched into orbit with five Iridium NEXT communications satellites on a commercially procured SpaceX Falcon 9 rocket from Vandenberg Air Force Base in California. This unique “rideshare” launch will first deploy GRACE-FO, then the Falcon 9 second stage will continue to a higher orbit to deploy the Iridium satellites.

GRACE-FO continues a successful partnership between NASA and Germany’s GFZ, with participation by the German Aerospace Center (DLR). JPL manages the mission for NASA’s Science Mission Directorate in Washington.

For more information on GRACE-FO, visit:

https://www.nasa.gov/gracefo