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. GRACE-FO will carry on the extremely successful work of its predecessor while testing a new technology designed to dramatically improve the already remarkable precision of its measurement system.
GRACE-FO, scheduled for launch in 2017, will continue 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 will be a series of measurements showing how far apart two satellites are. The twin GRACE 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.
On a more sophisticated level, GRACE measures gravity, 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 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 will use the same kind of microwave ranging system, and can expect to achieve a similar level of precision. But they will also test an experimental instrument using lasers instead of microwaves, which promises to make measurements of their separation distance at least 20 times more precise.
The testing and demonstration of the laser interferometer is envisioned 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 also Flight Systems Manager.
The laser instrument is expected to provide at least 20 times better precision than the microwave system. And while the microwave instrument measures changes in distance between the spacecraft, the laser system will also measure changes in the angle between the two spacecraft. The improvements will enable the satellites to detect gravitational differences at significantly smaller scales than currently possible.
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. Unlike most satellites, which are put in a carefully controlled orbit, the GRACE satellites were launched into a high orbit and then largely left alone. Flight engineers maneuver the satellites only if they separate by more than 155 miles (250 km). 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 will be start in the same orbit as its predecessor, at an altitude of more than 300 miles (500 km), building on the datasets gleaned from GRACE.
To relate changes in satellite motion to changes in gravity, scientists start with what the satellites’ paths ought to look like. They know where the mountains are, and where the oceans grow deep. They know the path of the Sun and the Moon, and the related fluctuation of ocean tides. They even account for large weather systems moving through the atmosphere. They determine how much all of these things should pull on the satellites if all our models were perfect. The next step is to compute the distance (or relative speed) between the two satellites based on these models. We then use the differences between the distance (or speed) actually measured and the modelled one to improve the models, one of which is the gravity field change for that month.
Researchers use mathematics and fundamental laws of physics to translate measurements of the satellite paths into measurements of gravity. Located at the Texas Advanced Computation Center, a supercomputer crunches billions of arithmetic operations on half a million new observations collected every month. The result is a single set of numbers that represent how much gravity has shifted compared to previous months.
GRACE-FO will be 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.