Mission Overview
The Gravity Recovery and Climate Experiment (GRACE) was a pioneering twin-satellite mission that measured changes in Earth’s gravity field with unprecedented accuracy. These observations revealed how mass is distributed and transported through the Earth system, including changes in surface water, groundwater, ice sheets and glaciers, the oceans, and other hydrological and terrestrial features.
Mission Objectives
The primary objective of GRACE was to produce accurate global models of both the static and time-varying components of Earth’s gravity field. By observing how mass moved across and within the planet, GRACE provided a new way to study seasonal water storage, long-term climate processes, ocean circulation, ice loss, and changes in the solid Earth.
How GRACE Worked
GRACE inferred changes in the gravity field by measuring extremely small variations in the separation between two satellites flying in the same low-Earth, near-polar orbit. Local variations in Earth’s gravity affected the trajectories of the two spacecraft differently, producing measurable changes in the distance between them.
The mission continuously tracked those changes using a precision microwave ranging system. Combined with precise orbit information and measurements of non-gravitational forces acting on each spacecraft, the inter-satellite ranging observations were used to estimate monthly gravity field solutions and track the movement of mass through the Earth system.
Instruments
Each GRACE satellite carried a set of precision geodetic instruments that worked together to measure inter-satellite distance changes, determine the spacecraft orbits, and account for forces unrelated to gravity.
- K/Ka-band Microwave Ranging System (KBR): Continuously measured changes in the distance between the two satellites, providing the primary observation used to recover variations in Earth’s gravity field.
- GPS Receiver: Supplied precise spacecraft positioning and timing information for orbit determination and gravity field estimation.
- Accelerometer: Measured non-gravitational forces acting on each satellite, including atmospheric drag and solar radiation pressure, so those effects could be separated from the gravity signal.
- Star Cameras: Determined the orientation of each spacecraft, allowing the ranging, accelerometer, and orbit measurements to be interpreted in a consistent reference frame.
- Laser Retroreflector (LRR): Supported independent satellite laser ranging from ground stations for orbit validation and calibration.
Mission History and Partnership
GRACE launched on March 17, 2002, as a joint mission of NASA and the German Aerospace Center (DLR). It was selected in 1997 as the second mission in NASA’s Earth System Science Pathfinder program, with Dr. Byron Tapley serving as Principal Investigator.
Although originally designed for a five-year mission, GRACE returned valuable measurements for more than 15 years. The mission was decommissioned in 2017 after the loss of battery capacity on one of the spacecraft.
“Using cutting-edge technology to make exquisitely precise distance measurements, GRACE improved our scientific understanding of our complex home planet, while at the same time providing information — such as measurements related to ground water, drought and aquifer water storage changes worldwide — that was used in the U.S. and internationally to improve the accuracy of environmental monitoring and forecasts,”