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.

It will be the first time we've ever done active laser ranging between two spacecraft.
- Deputy Project Manager Mike Gross

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 is 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.

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