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Measuring the earth from space

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  • GRACE satellite mission has collected 15+ years of detailed measurements of the Earth
  • Resulting data analysis has unlocked mysteries of how water moves around our planet
  • HPC provides the ability to pose questions whose solutions would not otherwise be feasible

A twin satellite system called the Gravity Recovery and Climate Experiment (GRACE) was launched in 2002 to take detailed measurements of the Earth. The nearly two decades of data it collected have significantly advanced understanding of global climate patterns.

For 15 years, the GRACE mission has unlocked mysteries of how water moves around our planet. It gave us the first view of underground aquifers from space, and shows how fast polar ice sheets and mountain glaciers are melting. Courtesy NASA JPL.

Among the many contributions that GRACE has made:

  • GRACE determined that the annual ice loss of Greenland is 258 gigatons. The Antarctic is losing an additional 137 gigatons. This is three times the loss in the pre-GRACE mission period.

  • The resulting mass added to the oceans contributes 2.5 mm/year of the 3.7 mm/year increase in global sea level. GRACE observations also confirm that the majority of the warming occurs in the upper 2,000 meters of the oceans.

  • Of the 37 largest land-based aquifers, 13 have undergone critical mass loss. This loss, due to both climate-related and human-induced effects, documents the reduced availability of clean, fresh water supplies for human consumption.

  • The information gathered from GRACE provides vital data for the United States Drought Monitor and has shed light on the causes of drought and aquifer depletion worldwide.

Intended to last just five years in orbit to measure small changes in the Earth's gravitational fields, GRACE ultimately operated for more than 15 years. The mission provided unprecedented insight into our global water resources, from more accurate measurements of polar ice loss, to a better view of ocean currents, and the rise in global sea levels.

<strong>Trend signals</strong> in GRACE data from April 2002-January 2017. Courtesy Byron Tapley.

This collaboration between NASA and the German Aerospace Centre was led by researchers in the Center for Space Research (CSR) in the University of Texas's (UT) Cockrell School of Engineering.

UT's Texas Advanced Computing Center (TACC) also played a critical role, according to Byron Tapley, principal investigator of the GRACE mission.

"As the demand for the GRACE science deliverables have grown, TACC's ability to support these demands has grown. It’s been a seamless transition from the initial requirements supporting a demonstration of the concept to a much more demanding computational regime, supporting a much richer suite of scientific results," Tapley said.

By measuring the changes in gravity associated with changes in the Earth's mass distribution—water systems, ice sheets, atmosphere, land movements, and more—the satellites can measure small changes in the Earth system interactions.

<strong>Ice loss.</strong> The Greenland and Antarctic ice sheets are losing mass at an accelerating pace, three times faster than that of mountain glaciers and ice caps. Courtesy Eric Rignot, NASA JPL."By monitoring the physical components of the Earth's dynamical system as a whole, GRACE provides a time variable and holistic overview of how our oceans, atmosphere and land surface topography interact," Tapley said.

The data system for the mission is highly distributed and requires significant data storage and computation through an internationally distributed network.

Although the final data products are generated at TACC, the Geophysics Center in Potsdam and the NASA Jet Propulsion Laboratory (JPL) in California also contribute.

The final analysis at TACC starts with a data downlink from the satellites to a raw data collection center in Germany. The data is then transmitted to JPL where the primary measurements are converted into geophysical measurements consisting of GPS, accelerometer, attitude quaternions, and the high accuracy intersatellite ranging measurements collected by each satellite during a month-long observation span.

"The challenging analysis of the collection of information data from this international community is enabled by the outstanding computing capability and operational philosophy at TACC. The consequence of this data collection and analysis effort has led to a paradigm-shifting view of the Earth's interactions," Tapley said.

Despite being a risky venture operating on minimal funding, the GRACE mission surpassed all expectations and continues to provide a critical set of measurements.

<strong>Global representation</strong> of trends and variability in ice and water mass recovered by GRACE over 15 years. Courtesy Cockrell School of Engineering, The University of Texas at Austin."The concept of using the changing gravimetric patterns on Earth as a means to understanding major changes in the Earth system interactions had been proposed before," Tapley said. "But we were the first to make it happen at a measurement level that supported the needs of the diverse Earth-science community."

One of the remarkable benefits of working with TACC, according to Tapley, is the ability to pose questions whose solutions would have not otherwise been feasible:

"When we began the GRACE mission, we found it difficult to determine gravity models that were characterized by approximately 5,000 model parameters, whose solution was obtained at approximately yearly analysis intervals,” says Tapley. “The satellite-only GRACE models today are based on approximately 33,000 parameters that we now have the ability to determine these solutions at a daily interval.”

Tapley continues, “Future concerns involve assimilating the global GRACE mass changes into coupled atmospheric, ocean and land data models. As we consider these questions, our interaction with TACC has always been in the context of: ‘If the answer to a meaningful question requires extensive computations, let's find a way to satisfy that requirement.'"

Now that the GRACE Follow-On mission has launched successfully, the chance to continue the GRACE record for a second multi-decadal measurement of changes in mass across the Earth system is possible.

Engineers and scientists anticipate that the longer data interval will allow them to see an even clearer picture of how the planet's climate patterns behave over time.

Read the original article on TACC's website.

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