ESTO explores cutting-edge technique for measuring Earth’s mass

For more than 20 years, scientists have maintained an unbroken record of geodetic measurements describing Earth’s mass. That record is critical for studying everything from melting glaciers to groundwater availability.

ESTO’s “Simplified Gravitational Reference Sensor for Future Earth Constellations” (S-GRS) accelerometer may allow researchers to measure Earth’s mass with unprecedented accuracy. S-GRS features a novel charge management system, as well as a larger test mass and a wider gap between the test mass and its electrode housing when compared to traditional accelerometers. Together, those elements make S-GRS more precise than current accelerometers by a factor of 50.

A video depicting ice mass changes in Greenland, made using geodetic data from GRACE and GRACE-FO. S-GRS will help researchers measure Earth’s gravity field and mass-distribution more accurately. (Image Credit: NASA and JPL/Caltech)

Accelerometers use a test mass as a reference to allow these gravity-measuring instruments to record any non-gravitational forces acting on their host spacecraft. Researchers remove those forces from their data analysis to create superior gravity measurements.

John Conklin, an Associate Professor of Mechanical and Aerospace Engineering at the University of Florida, Gainesville, and Principal Investigator for S-GRS, explained that while laser interferometers have come a long way over the past few years, accelerometers haven’t advanced at the same rate.

“Up until now, they’re still flying the same accelerometers they’ve flown in the past,” said Conklin. “We don’t really get the full advantage we would out of that improved performance with the laser interferometer unless we also improve the accelerometer.”

S-GRS overcomes a number of technical limitations holding back traditional accelerometers. One key innovation is the creation of an ultra-violet grounding system, which keeps S-GRS’s test mass from acquiring an electrical charge.

“If the spacecraft is jittering or moving around then that motion will impart a force on the test mass through the wire itself, and just some of the dynamics of that little wire create forces on the test which is another source of noise,” explained Conklin. “When we shine small amounts of UV light on the sensor, we can keep the test mass grounded, essentially without using a grounding wire.”

Removing the grounding wire from accelerometers has other benefits, too. There’s more space, which means Conklin and his team can include a larger test mass in their design that makes it easier for S-GRS to provide improved measurements of non-gravitational acceleration.

“The mass of the test mass, and how much the volume it has to move around inside the sensor, are both major drivers of the performance of the instrument. And so it’s a combination of those things that that provide us, let’s say, improvement by a factor of 50 over previous accelerometers,” said Conklin.

Adapting the gravitational reference sensor he originally developed for LISA to suit Earth observation missions was no easy task, and Conklin is grateful to his team for taking S-GRS to the next level.

“We have a number of graduate students, postdocs, and scientists in our lab that work on this project and also work on LISA so they understand this type of inertial sensor technology and applying that knowledge from LISA into this project,” said Conklin.

Conklin is also grateful to his teammates at Ball Aerospace, Inc., which is developing the mechanical sensor head and flight hardware for S-GRS.

S-GRS is still in development, but Conklin and his team are working towards a complete, space-ready prototype capable of supporting future NASA geodetic science missions.

“I think there is a great opportunity to use the SRS in future GRACE-like missions, and based on National Academies recommendations and the importance of GRACE science, I foresee these missions flying for decades into the future, so that’s the end goal,” said Conklin.

Gage TaylorNASA Earth Science Technology Office