GRACE Follow-on satellites
The image above shows what the GRACE Follow-On satellites will look like.

More than 15 years ago, Byron Tapley, director of the Center for Space Research (CSR) at the Cockrell School of Engineering, set out to look at the earth in a different way — from a vantage point of 280 miles above the earth.

Tapley encouraged NASA’s Jet Propulsion Laboratory (JPL) to use twin satellites to measure small changes in the earth using its gravitational field. The joint U.S.-German project, called Gravity Recovery and Climate Experiment (GRACE), successfully collected detailed measurements that have resulted in a decade’s worth of valuable data. The GRACE satellites marked their 10th year in space in 2012.

CSR and Tapley’s way of peering at the earth has become critical to delivering accurate climate measurements and water resource data to the scientific community.

GRACE has enabled hydrologists, oceanographers, geographers and glaciologists to shed light on the causes of drought, melting polar ice caps and aquifer water depletion in California and India. Because of the GRACE mission, The University of Texas at Austin has had a leading role in advancing the field of space geodesy — the science of accurately measuring variations in the earth’s land, ice and mass.

Now, thanks to a new agreement with JPL for the new U.S.-German GRACE Follow-On mission, CSR at UT Austin will have a key role in shaping what space geodesy will look like over the next 10 to 15 years. Tapley said the mission will allow them to continue the measurements of water and mass movement that GRACE has been making during the last decade.

“It’s a log of our resources that will help us address practical concerns related to water usage in the upcoming decade,” he said. “Because water is, and will be, a global issue.”

NASA JPL intends to launch the GRACE Follow-On satellites, which will replace the existing twin GRACE satellites, in 2017. The mission is expected to last until 2022, but CSR expects the new satellites will likely have a lifespan of 10 years.

GRACE Follow-On has been “somewhat in its infancy over the last decade,” said Srinivas Bettadpur, research professor at CSR and in the Department of Aerospace Engineering and Engineering Mechanics. He is the principal investigator for UT Austin's efforts for GRACE Follow-On.

Bettadpur, who was part of the original GRACE mission, has been a key figure on the GRACE team for more than 17 years.

"I have a vision of what GRACE ought to look like in the next decade and two decades, with a constellation of superbly precise missions, where we have integrated space mission design and science, giving us ever new insights into the earth and other planets," he said.

CSR’s piece of GRACE Follow-On funding is expected to range from $1 million to $2 million annually for the next few years. CSR will support the mission design and development until the GRACE Follow-On is launched. After the satellites are in space, CSR will carry out the data analysis for science- and application-related studies.

GRACE Follow-On is an opportunity for CSR to collaborate with other schools on campus and graduate students who will bring fresh ideas to the mission, Bettadpur said.

CSR graduate researchers Chistopher McCullough and Carly Sakumura say GRACE has provided them with an invaluable learning experience.

“GRACE Follow-On presents an amazing opportunity for graduate student research,” Sakumura said, “not only because it will ensure the continuity of the GRACE data and effectiveness of the models and tools we work on now, but because it allows us to see the mission planning and design process first hand.”

McCullough agrees, adding that GRACE has given him the “chance to work with some of the world's experts in precise orbit determination, work with satellites that have some of the most advanced technology flown in space, and be at the forefront of earth science.”

At its core, GRACE Follow-On will act as a bridge linking the original GRACE mission to future missions, allowing each phase to build upon the knowledge of the last, Bettadpur said.

“Our engagement with the new project is long enough that we will be defining the next generation of these technologies and these techniques,” he added.

CSR has been working for three years to figure out how to seamlessly continue GRACE’s measurements with GRACE Follow-On.

“It’s important that we not lose the continuity of the GRACE data,” Tapley said. “If we have a decade and a half interval of continuous measurements, we will have enough data to separate the strong seasonal signals from the long-term trends.”

CSR’s experience with GRACE Follow-On will be different from its predecessor in two major ways: The implementation of new techniques of analysis and applications to take advantage of lessons learned from the first GRACE mission, and data from a new demonstration laser instrument.

“We are much more mature in the use of this data and its applications than we used to be,” Bettadpur said. “We can immediately start exploring new applications with the new data.”

The GRACE Follow-On measurements will continue to rely on radio waves exchanged between the two satellites. Similar to GRACE, the new satellites, which will each be about 10-feet long, will exchange signals as they fly in a fixed formation about 137 miles apart and some 280 miles above the earth.

When the satellites sense a subtle change in the earth’s landmass, it causes the satellites to rapidly close in or to drift apart. When they are closing in, the pitch of the radio waves increases, and when they drift apart, the pitch decreases. These fluctuations in landmass are primarily caused by the movement of water or the presence of ice.

“You have radio waves coming from one satellite and received by another satellite. Scientists watch the beat, like the Doppler, between the two satellites,” Bettadpur said. “The two satellites are talking to each other and what they are saying depends on what the earth is doing underneath.”

These measurements are then converted into usable data products using supercomputers at UT Austin’s Texas Advanced Computing Center that tell us at what rate ice is melting, ocean levels are rising or aquifers are being depleted.

Beyond continuing the collection of data, GRACE Follow-On will test a new instrument that will provide measurements based on lasers, or light, instead of radio waves. The laser instrument is expected to yield more precise measurements than existing instruments.

If new instruments prove successful, the GRACE Follow-On mission could one day provide more detailed geographic data, Bettadpur said.

“What you would like to do is peer closer and that requires a different class of instruments,” he said.

Tapley is also looking forward to the new measurement possibilities that the GRACE Follow-On mission will bring.

 “GRACE has been the capstone of my career, not only because of the challenge and the success we have had in fulfilling the original objectives of the mission, but because the community has viewed our work with an appreciative tone,” Tapley said. “They have embraced the measurements, and we are continuing to push it forward.”