GRACE Satellite Program Observes Earth in Critical Ways

The GRACE satellite mission, directed by the university's Center for Space Research and Aerospace Engineering Professor Byron Tapley, measures changes in Earth's mean gravity field.

Byron Tapley stands in front of an image of the GRACE sattelite

Cockrell School Aerospace Professor Byron Tapley is the principal investigator and leader of the GRACE Satellite Program.

In 2009, NASA scientists examining data about water resources around the world noticed something unusual.

"One thing that jumped out at us was northwestern India," said Matthew Rodell, a hydrologist at the space agency.

The aquifer that supplies water for the area that's known as the Bread Basket of India was being emptied of water faster than it was being replenished.

Further investigation showed that the water table dropped about a foot a year from 2002 through 2008 because farmers are pumping more water out for irrigation than is going in.

"They've got a serious issue," Rodell said.

Following this observation, additional studies noted a similar loss of underground water in California's San Joaquin Valley, which supplies provide about 15 percent of the United States' agricultural needs.

The data used for these observations come from a pair of satellites orbiting 320 miles above the Earth.

The Gravity Recovery and Climate Experiment (GRACE) mission, which is directed by The University of Texas at Austin Center for Space Research, measures changes in Earth's mean gravity field, which most often is the result of the movement of water. The GRACE program's principal investigator is Cockrell School of Engineering Aerospace Engineering Professor Byron Tapley. More than 40 graduate students have worked on the GRACE project through the years.

"GRACE maps the Earth's gravity with extremely high precision," Rodell said. "Gravity is related to mass so when there's more mass (or water) somewhere, it affects the orbit of the satellites."

The satellites' instruments are sensitive to even small changes.

"A couple of centimeters of rainfall actually have an affect on the satellites," said Rodell, who uses GRACE to monitor the global water cycle. "It's really amazing."

Since its launch in 2002, GRACE has provided information about changes in the oceans, ice sheets at the poles and river deltas as well as regional aquifers like the ones in India and California.

Scientists working in hydrology, climate change, geology, geodesy and other disciplines from around the world are GRACE users. They've published scores of papers in a range of journals including Science and Nature.

The GRACE mission is a joint project between NASA and the German Aerospace Center (DLR).

The University of Texas at Austin's ties to GRACE go beyond Tapley and the Texas staff who monitor the daily activities of the satellites. Rodell and a number of other members of the mission's team received their Ph.D.s at the university.

One thing that makes GRACE so valuable to researchers is its capability to map the gravity field's fluctuations over time.

"Month-to-month you can see changes in the gravity field," Rodell said. By comparing month-to-month variations to long-term averages, one can see rainfall anomalies with GRACE.

That's how the situation in the India aquifer popped out.

Map of India's groundwater changes between 2002-08.

The map above shows groundwater changes
in India during 2002-08, with losses in red and
gains in blue, based on GRACE satellite
observations. The estimated rate of depletion of
groundwater in northwestern India is 4
centimeters of water per year, equivalent
to a water table decline of 33 centimeters per
year. Increases in groundwater in southern India
are due to recent above-average rainfall,
whereas rain in northwestern India was close
to normal during the study period. Click image to
enlarge. Credit: Isabel Velicogna/University of

GRACE maps the Earth's gravity fields by making accurate measurements of the distance between the two satellites, using geodetic quality Global Positioning System (GPS) receivers and an ultra-accurate intersatellite microwave ranging system.

The high accuracy ranging system measures variations in the distance between the two satellites, which are caused by small variations in the Earth's mass distribution. The accuracy is as fine as 1/10th the diameter of a human hair strand.

These extremely accurate measurements, collected over the entire Earth once every 30 days, have provided a new and extremely important capability for monitoring the Earth's changes.

The signals associated with gravity variations that GRACE measures are caused by: changes due to surface and ocean bottom currents in the ocean; water evaporated from the ocean and carried to land areas by the atmospheric circulation, flooding runoff and ground water storage over the land masses; exchanges between ice sheets or glaciers and the oceans; and variations of mass deep within the Earth.

The project provides data for 21st Century science, but its roots go back to the late 1960s.

Tapley said that the need for a gravity mission was defined at a NASA workshop in Williamstown, Mass., in 1967. He participated in several attempts of a GRACE-like mission during the following decades without success.

The delays were a blessing because the earlier missions would have used less sophisticated technology and probably would not have lasted nearly as long.

Development of technologies such as the global positioning system and space-qualified accelerometers came together in the 1990s to make GRACE a much more robust mission.

"That we proposed the mission at the right time was an essential part of the GRACE mission success," Tapley said. "The need for the mission was well understood. The opportunities for the new measurement concepts that we proposed in terms of time-variable gravity and the application of these measurements to observing climate effects made the mass flux measurements important on both the national and international level. The ability to use the measurements to look at the changes in the underground aquifers and to measure changes in the ocean bottom currents represent the first time that a satellite mission has been able to observe below the Earth's surface."

As the technologies came together, Tapley still had to arrange money and politics to get the mission going.

NASA put out the first call for principal investigators to lead Earth-observing missions with a $60 million to $90 million cost. The GRACE team submitted a proposal for the $90 million cost cap, but that still wouldn't cover building two satellites, equipping them with two sets of instruments and then launching them.

Colleagues in Germany were approached in an effort to establish a bi-lateral collaboration that would cover the launch cost. In the proposed approach, NASA would build the satellites and the German Space agency would launch them

In the initial discussions, Tapley said, their question was, "Why should we put money into launching what is essentially U.S. satellites?"

The answer was that the U.S. team would propose to buy the satellites from a German aerospace company. It was called Darnier Space Systems then, but was subsequently bought by Astrium.

"Darnier Space Systems, which was a component of Daimler-Benz, the Mercedes-Benz car manufacturer, was trying to establish themselves as a satellite builder," Tapley said. "They were very interested in getting their satellites selected for a U.S. mission to get them on the U.S.-approved satellite list. They joined our effort and were very active in encouraging the German government to go forward with the collaboration. With this collaboration our launch problem was solved. So this was a good arrangement for both parties."

The project took on a greater international flavor when Germany partnered with the Russians to launch from the Plesetsk Cosmodrome, about 125 miles north of Moscow.

The mission launched on March 17, 2002. Originally scheduled for five years, it has now exceeded nine years and has just been approved to fly through 2013, Tapley said. And though some battery problems have cropped up in one of the satellites, he's hopeful it can stay in operation until 2016. That's the earliest that GRACE Follow On, the recently approved successor to the GRACE mission, could be launched.

Connection to the following mission, he said, is critical to continue the important climate change measurements that GRACE started.

"Climate measurements need continuity over multi-decade time scales to be able to separate the yearly and decadal variability from the long-term trends," he said.

Support Dr. Tapley's work by making a gift to help establish the Byron and Sophie Tapley Graduate Fellowship. The endowment will ensure Dr. Tapley's legacy for generations to come as Tapley Fellowship recipients continue the teaching and research he began in 1958. Make a gift online, or contact Amanda Brown at 512-471-4046 or

Byron Tapley holds the Clare Cockrell Williams Centennial Chair in Engineering .

Data from GRACE has helped scientists:

  • Discover that ice sheets on Greenland and Antarctica are melting much faster than previously thought.
  • Find an ancient impact crater under the Antarctic ice sheets.
  • Determine the mass contribution to global sea level rise.
  • Identify depleting aquifers in the United States and Africa, as well as India.
  • Monitor the impact of major earthquakes.

GRACE Research Purposes

Members of the GRACE science team, and scientists from around the world who use GRACE data in their research, have identified several areas where the project could be put to immediate use:

  • Flood Potential Index — Grace can tell where areas are near maximum water storage. Places close to the maximum storage could expect a flood situation with a little more rain.
  • Streamflow/Snowmelt Forecast — GRACE is sensitive to accumulation and melting of the snowpack. This year there's more snowpack than normal and it can forecast that there's going to be more runoff than normal.
  • Drought Monitoring — GRACE can determine whether the land is dryer than normal, how it compares with previous dry episodes, and whether it is worsening or improving. The current drought maps shown on the evening news are an example of this application