February 2000

Grounded in fact

ESD’s Distributed Active Archive Center puts Terra satellite’s data on firm footing

Just before Christmas, NASA launched Terra, the flagship satellite for its Earth Observation System, to monitor environmental and climate change on Earth. Researchers in the Environmental Sciences Division, working closely with the space agency and global change researchers, are helping to verify the data that come from the satellite’s instruments.

ORNL’s role is to assist in the validation of data coming from Terra’s MODIS (or Moderate Resolution Imaging Spectroradiometer) by comparing these data with actual observations from the ground. In other words, the researchers in the field are going to help confirm what the spacecraft is seeing and how it should be interpreted.

“Sensing climatic and environmental conditions on the ground from a satellite requires a lot of interpretation,” says Dick Olson, with ESD’s Distributed Active Archive Center, or DAAC. “We’re helping NASA determine how well satellite-derived images relate to the information on the ground.”

The DAAC is funded by NASA to assemble, archive, and distribute information on terrestrial ecosystems useful to global change researchers. The center will compile statistics from 24 sites around the globe, including ORNL’s Walker Branch Watershed, and use that knowledge to help Terra researchers accurately interpret what the MODIS instrument is measuring.

“Another satellite, the Hubble space telescope, gave us tremendous ability to look out in space. Terra gives us an ability to look back at the Earth,” says Olson. “The science community sees Terra as a big jump in its ability to monitor, understand, and predict global environmental conditions and the impact they may have on the environment.”

The bus-sized satellite’s instruments will produce copious streams of data, which will be managed by a system of centers around the country. ORNL’s capabilities with data management, specifically the DAAC’s ability to handle ground-based information, will be used to help NASA validate its space-based observations with real conditions on the ground.

“It’s an incredible investment by NASA,” says ESD’s Bob Cook. “The streams of data from the five instruments on Terra are unprecedented. The NASA researchers had to develop a receiving and management system. We handle the field data, which NASA will use to support the satellite data.”

The 24 sites are located around the world—many in the United States and Canada—in a variety of environmental settings and ecosystems. What the DAAC provides to NASA will help them extrapolate conditions on the ground from the space-based views.

“For instance,” says Cook, “the Walker Branch area is one of the sites being looked at from space. Data are collected from small plots, say a few square meters. What you see from space is an image one kilometer on each side. Data on the amounts and types of vegetation and other ecosystem characteristics enable the scientists to scale up from meters to kilometers, so that researchers can interpret the satellite data.”

“We’re more than an archive,” notes Cook. “We’re working with the scientists so that the data required to perform the validation—data on climate, plant species and soils—are available to compare with the satellite measurements.”

Olson further explains that the ORNL-NASA collaboration could eventually add up to an eye-in-the-sky model for environmental and climate effects.

“What’s new is predicting, from these chunks of landscape, how much carbon dioxide is removed from the atmosphere and stored as plant biomass,” Olson says. “The more leaf area, for instance, the more carbon is stored. The less leaf area, the less is stored.

“Varying amounts of leaf area give different reflective patterns to Terra’s MODIS instrument. An algorithm takes into account leaf area, climate and other variables to estimate the net amount of carbon stored.”

Terra will spend roughly the first three months being calibrated, with DAAC’s help, to the realities on the ground. One way they’ll do it is by pointing MODIS at the moon, at salt flats located around the world, and at Lake Tahoe, areas where reflectance characteristics are uniform and well documented.

“The complexity of this project is beyond most I’ve been associated with,” says Olson. “It involves a very large community and large amounts of information. On the other hand, it makes it fun, because it involves an international group of researchers in different fields—geographers and computer modelers, for instance—and we know they will use our data.”—B.C.

Learn more about the Terra project at terra.nasa.gov and ORNL’s role at www-eosdis.ornl.gov/eos_land_val/valid.html.