GRACE (2002-2017) was 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 measured 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. 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.
It will be the first time we've ever done active laser ranging between two spacecraft.
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 the 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 detect gravitational differences 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 GRACE-FO Science Lead 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 will 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.