Microwaves and Lasers
GRACE and GRACE-FO each map Earth's gravity field by measuring subtle changes in the distance separating a pair of satellites as they orbit the planet (see How It Works).
Using a ranging system based on microwaves, GRACE can measure that distance to within one micron -- about the diameter of a blood cell. GRACE-FO will employ a similar microwave system, but the two spacecraft will be able to point at each other with even more accuracy.
The enhanced pointing control is expected to improve the performance of the microwave instrument, but its real purpose is to support the testing and demonstration of an instrument which is planned for the next-generation version of GRACE: the laser interferometer. "It will be the first time we've ever done active laser ranging between two spacecraft," said Mike Gross. The laser instrument is expected to provide at least 20 times better precision than the microwave system. And while the microwave instrument measures only changes in distance between the spacecraft, points out Bill Klipstein, who manages the laser interferometer for JPL, the laser system will also measure changes in the angle between the two spacecraft. "So we will know where the other spacecraft is better than is currently known," he said. The improvements will enable the satellites to detect gravitational differences at significantly smaller scales that is currently possible.
Both the microwave and laser systems will be used simultaneously for as long as the laser system lasts. However, said Project Manager Phil Morton, the primary objective of this mission is to maintain continuity of measurements made with the microwave instruments. "At some point, when we have power issues or orbit issues, we'll have to turn the laser off," he said. "It will hopefully give you much better data for those periods of time (when it's on) and allow you to do the comparison that says this is a really great technology. Then let's really design a long-term laser-interferometer kind of mission" for the next generation of GRACE.
The laser instrument is an outgrowth of technology developed for the Laser Interferometer Space Antenna (LISA), a mission to detect the gravitational waves in spacetime that Einstein's General Theory of Relativity predicted. NASA has been working on LISA in partnership with the European Space Agency.