The Gravity Recovery and Climate Experiment Follow-on (GRACE-FO) mission is a partnership between NASA and the German Research Centre for Geosciences (GFZ). GRACE-FO is a successor to the original GRACE mission, which began orbiting Earth on March 17, 2002.
Launched on May 22, 2018, GRACE-FO continues the work of tracking Earth's water movement to monitor changes in underground water storage, the amount of water in large lakes and rivers, soil moisture, ice sheets and glaciers, and sea level caused by the addition of water to the ocean. These discoveries provide a unique view of Earth's climate and have far-reaching benefits to society and the world's population.
All About the Gap
GRACE-FO raw data is a series of measurements showing how far apart two satellites are. The twin GRACE-FO satellites follow each other in orbit around the Earth, separated by about 137 miles (220 km). They constantly send microwave signals to each other to measure the distance between them.
From distance measurements between the two satellites, GRACE data can be used to estimate Earth's gravity field, the force that anchors us to the planet. Gravity is the attraction between two objects. Its strength varies depending on how much mass those objects have and how far apart they are.
Microwaves, Lasers and the Future
GRACE-FO is able to make accurate measurements thanks in part to two advanced technologies: a microwave ranging system based on Global Positioning System (GPS) technology, and a very sensitive accelerometer—an instrument that measures the forces on the satellites besides gravity (such as atmospheric drag).
Using the microwave ranging system, GRACE can measure the distance between satellites to within one micron -- about the diameter of a blood cell.
The two GRACE-FO satellites use the same kind of microwave ranging system as GRACE, and can achieve a similar level of precision. But they will also test and demonstrate an experimental instrument using lasers instead of microwaves, which promises to improve the precision of separation distance measurements on future generations of GRACE satellites by a factor of up to 20, thanks to the laser’s higher frequencies.
Once validated on GRACE-FO, the laser interferometer is envisioned to be used operationally for the next-generation version of GRACE. “It will be the first time we've ever done active laser ranging between two spacecraft,” said Deputy Project Manager Mike Gross, who is also Flight Systems Manager.
While the microwave instrument measures changes in distance between the spacecraft, the laser system is designed to also provide information in the angle between the two spacecraft. In combination with the increased precision of the separation measurement, and advances in the ground science data system, these improvements will enable future GRACE-like satellites to infer mass changes at the Earth's surface at significantly smaller scales.
The accelerometer measures the forces that move the satellite by pushing on its surface. “The measurement allows us to correct for anything that is related to drag or solar pressure, leaving just gravity,” JPL Director and former Project Scientist Mike Watkins said.
Together, these very precise measurements of location, force and orbital change translate into an observation of gravity with unprecedented accuracy. Flight engineers maneuver the satellites only if they separate by more than 155 miles (250 km), otherwise they are left alone and gravity ‘tugs and pulls’ on them. As the satellites circle the Earth, the ranging technology tells scientists exactly where each satellite is relative to the other. Supercomputers and scientists compare the positions of the satellites relative to each other and to previous orbits, noting every variation.
GRACE-FO was injected into a 304-mile (490-km) altitude, near circular polar orbit like many of the satellites in NASA's Earth Observing System. In this orbit, the satellite moves around the Earth from pole to pole, taking about 99 minutes to complete an orbit. During half of the orbit, the satellites view the daytime side of Earth. At the pole, the satellites cross to the nighttime side of Earth.
As the satellites orbit, the Earth turns underneath. By the time the satellite crosses back into daylight, it is over the region neighboring the area seen in its last orbit. In a 24-hour period, polar orbiting satellites will view most of the Earth twice: once in daylight and once in darkness.
Landerer, F. W., Flechtner, F. M., Save, H., Webb, F. H., Bandikova, T., Bertiger, W. I., et al. (2020). Extending the global mass change data record: GRACE Follow‐On instrument and science data performance. Geophysical Research Letters, 47, e2020GL088306. https://doi.org/10.1029/2020GL088306.