ASTER gives geophysicists new eyes

New satellite data in an unprecedented mix of wavebands and high resolution are beginning to yield results for scientists studying such varied areas as climatology, urban development, and the health of coral reefs.

The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), one of five instrument packages on NASA's Terra satellite, produces the data. Terra was launched in December 1999, and the first scientific results were released at the American Geophysical Union's 2001 spring meeting (29 May to 2 June; Boston, MA). The researchers praised ASTER's high-resolution and multiple spectral bands, which provide information they've never had before.

Andrew French of the U.S. Department of Agriculture's Agricultural Research Service (Beltsville, MD) is using ASTER to measure the spatial distribution of water vapor as it moves among soil, plants, and the atmosphere. He does that by measuring radiometric surface temperature with the satellite and combining those results with common meteorological data such as air temperature and wind speed.

French makes his measurements with the five bands of ASTER's thermal infrared subsystem—8.125 to 8.475 µm, 8.475 to 8.825 µm, 8.925 to 9.275 µm, 10.25 to 10.95 µm, and 10.95 to 11.65 µm. The system uses a telescope and scanning mirror to take images at 90 m resolution, and the data are collected by mercury-cadmium-telluride detectors—10 per band, each cooled to 80 K.

Measuring the temperature requires some computational sleight of hand to subtract other components the instrument picks up. The atmosphere between the ground and the satellite has its own temperature, for instance. Plus, it attenuates the radiation from the surface. It also propagates some radiation down to the earth's surface, which is then reflected back toward the satellite. Even after correcting for the atmosphere, the different IR bands show different temperatures. "Land surface is not close to a black body at all, so you have to know something about the emissivity of the surface to get a good temperature, and you have to know something about the atmosphere," French says.

The team uses an algorithm called Temperature Emissivity Separation, which makes assumptions about what the actual temperature is and uses that to derive the emissivity over and over until a single temperature emerges from all bands. French says it takes at least four bands for this to work, but he eventually gets a temperature accurate to about 1 °C. ASTER, which has more bands than some higher-resolution instruments yet has reasonable resolution, provides just what he needs.

Using ASTER, William Stefanov of Arizona State University (Tempe, AZ) measures visible light reflectance from 100 cities around the globe to determine if there is any pattern to the way urban areas develop. He performs variance texture analysis to highlight changes in the edge density of recorded pixels, measuring the transition from bright to dark pixels. The variance in brightness, he says, correlates with variations in the density of buildings and vegetation. "It gives us some sense of how humans like to engineer their environment," he says. So far, he has tentatively classified cities into three types—centralized cities with a Gaussian distribution pattern, decentralized areas, and intermediate areas. Stefanov plans to perform a frequency analysis of his data to make sure that the variation his eyes see is supported statistically.

Stefanov is using data collected by the visible and near-infrared subsystem, which covers 520 to 600 nm, 630 to 690 nm, and 760 to 860 nm, and has a spatial resolution of 15 m. The subsystem contains two telescopes, one looking backward along the satellite's track and one looking straight down. The telescopes contain arrays of 5000 silicon charge-coupled device detectors, one array for the backward-looking telescope, and three for the other.

A shortwave infrared system covers six more wavebands—1.6 to 1.7 µm, 2.145 to 2.185 µm, 2.185 to 2.225 µm, 2.235 to 2.285 µm, 2.295 to 2.365 µm, and 2.36 to 2.43 µm. These have a resolution of 30 m, and data are detected by a platinum-silicide-silicon Schottky barrier linear detector array in each channel, all cooled to 80 K.

ASTER was built by NASA and by Japan's Ministry of Economic Trade and Industry, which are providing much of the data to researchers for free. French hopes scientists will take advantage of the satellite's existence. "It's something that people haven't seen before," he says. "There's just nothing else like it."