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and Infrared Imaging Spectrometer (AVIRIS)aboard the N A S A U-2at an elevation of 20 km. The image is a composite of three IO-nm wide bands centered at 544,633, and 803 nm. The faces of the parallelpiped show the color-coded, relative reflectance spectra for each of the 358 pixels along the top and 614 pixels along the right-hand edge of the image. Each spectrum covers the 0.4 to 2.5 p m region from front to back. Black and blue represent low values; red and white represent high values of reflectance. Atmospheric water absorption bands at 1.4 and 1.9 rm are represented by two black stripes. North is to the right. (Processed by the Center for the Study of Earth from Space, University of Colorado,Boulder.)
Imagine an animal that sew IIUIIUTBUS of colors well beyond the reddest red. It views not only the visible universe but also the invisibly colorful displays from molecular motions-an infrared choreography of stretching, bending, and vibrating atoms. Imagine further that the beast flies high above the Earth and sees a t once large areas of the planet’s surface. I t then would remotely view the chemical signals coming from a forest canopy or mineral outcrop. Individual chemicals, with their own troupe of molecular dancers, would have unique displays of humanly unimaginable colors that might advertise their presence to this beast. You are imagining a remote-sensing beast that already is beyond scientific imagination; it has taken on electronic, glass, and metallic flesh. In its first form, the instrument-the Airborne Imaging Spectrometer (AIS)-flew aboard the National Aeronautics and Space Administration’s (NASA) huge C-130 plane between 1983 and 1987. Although not yet ready for routine operation, high-flying spectrometers with spectral and spatial resolution so fine that they can function as geochemical imagers are becoming a novel category of earth-watching tools. The scope of their vision is just what the research community needs for learning how chemical cycles work on a global scale, says Alexander F. H. Gaetz, who, as NASA’s former imaging spectrometer program chairman, brought AIS and its instrumental progeny into this world. Now mothballed, the AIS served mostly as a testbed for new detector technologies, says Goetz, who now heads the Center for the Study of Earth from Space, a part of the Cooperative Institute for Research in Environmental Studies (CIRES). CIRES is run jointly by the National Oceanic and Atmospheric Administration and the University of Colorado a t Boulder, where Goetz also is professor of geolog-
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ical sciences. “ w i m nd,we were testing the concept of doing reflectance spectroscopy like you do in a laboratory, but from an airborne platform instead,” Goetz told ANALYTICAL CHEMISTRY. AIS begat the Airborne Visible and Infrared Imaging Spectrometer (AVIRIS),which is beginning to gather spectral data from its perch on NASA’s super-high-flying ER-2, a modified U-2. The third-generation imagersHigh-Resolution Imaging Spectrometer (HIRIS) and Moderate-Resolution Imaging Spectrometer (MODIS)-will embody lessons learned from their instrumental precursors. They are slated to scan the planet in the mid-1990s from a space platform as part of the Earth Observing System (Eos). “Ideally, these instruments will ‘see’ chemistry of large areas,” Goetz said. With more and more urgency, scientists are striving to develop a global perspective on the distribution of materials in the oceans, lands, animals, plants, and skies and determine how materials move within and between these vast reservoirs. Detailed field studies of, say, a small forest here and a desert area there, provide a crucial, though patchwork, picture wrought with vast data holes that are filled by model-guided extrapolation. Reliable remote sensing using imaging spectrometers could help scientists to fill in the gaps with hard numbers. Such spectrometers would enable researchers to sample huge contiguous areas of the surface, says Walter Westman, an environmental scientist a t the Environmental Policy Analysis Unit of Lawrence Berkeley Lab in Berkeley, CA. Westman, who has worked with the AIS instruments, stresses that the ideal remote-sensing imager is years away. “Research to tie remote-sensing variables to global ecological models is now in its early stages,” he wrote in a recent article that appeared in Trends in Ecology & Evolution ( I ) . It’s one thing to interpret the unperturbed transmission spectrum of a liquid in a cuvette, Westman says, but quite another to understand the meaning of a reflectance spectrum that has traveled through trees, leaves, soil, and air. The new breed of imaging spectrometers AIS. When NASA began funding the imaging spectrometer program in 1981, engineers at the Jet Propulsion Laboratory (JPL) in Pasadena, CA, built the firsttwo of a series of high-spectralresolution imaging spectrometers: AIS I and AIS 11. These flew between 1983 1340A
and 1987 before giving way to the newer AVIRIS that now flies aboard the ER-2. “AIS was the first imaging spectrometer to look at the Earth,” said Goetz. Although the orbiting Landsat scanning systems can view the Earth in seven spectral hands, the resolution within the bands is insufficient for distinguishing particular materials in a mixture. Not so for the AIS instruments. They viewed the spectral ranges from 1.2 to 2.4 pm in 128 contiguous bands at about a 10-nm resolution-good enough for distinguishing hundreds of geochemically,ecologically, and biologically interesting compounds. AIS-I used a 32 X 32 array detector, AIS-I1 a 64 X 64 array. Each element was made out of infrared-sensitive HgCdTe sandwiched with a silicon cbarged-coupled device multiplexer. The toasteroven-sized instruments flew in the belly of a C-130. A stepping motor nudged the spectrometer gratings through the 128 bands during the time it takes for the plane to fly 1 pixel width on the ground. This translates into a minimum ground resolution of 12 X 12 m when the plane is flying at 8 km. This resolution is crisp enough for many geological and geobotanical applications, researchers say. AVIRIS. The Airborne Infrared Imaging Spectrometer is the next instrnment in the series. It was completed last year and a t press time was starting t o gather remotely sensed spectral data. Unlike AIS, AVIRIS covers both the infrared and visible regions. Instead of area array detectors, which re-
quire vast computer support, AVIRIS is built with line arrays of silicon and indium antimonide elements. They image 550 pixels across in 224 contiguous spectral bands ranging from 0.41 to 2.45 pm, again at 10-nm resolution. Any diagnostic benefit gained by a finer spectral resolution would be more than offset by a decreased signal-tonoise ratio and an unmanageable deluge of data, Goetz says. The spectral range is analyzed with a set of four spectrometers connected to the scanning head by fluoride polymer optical fibers (see Figure 1). Because AVIRIS flies in the ER-2. which cruises a t 65,000 f t or more, the instrument can see larger swaths than its lower flying predecessors. The AVIRIS “sees” roughly 11 km2 of ground with a pixel resolution of about 20 m2,fine enough to detect segregated regions of tree death in a forest or mineralogical variations in a region such as Nevada’s Cuprite mining district. HIRIS. The High-Resolution Imaging Spectrometer will be part of the Eos, a multisensor platform designed to obtain Earth sciences data on the global scale that is planned for the middle of the next decade. It will have a spatial resolution of 30 m, a purview of 30 km, and a spectral range of 0.4-2.5 pm at a 10-nm resolution, allowing for roughly 200 discrete bands. Experiments with the AVIRIS will contribute to the final design of the HIRIS.
Geltlng the hang of remote sensing In a paper published this year in Nature, Carol Wessman and colleagues (2) reported using spectral data ac-
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Diagram of the AVlRlS sensor showing four spectrometers connected to the scanning head with optical fibers. Flgure 1.
ANALYTICAL CHEMISTRY, VOL. 60, NO. 23, DECEMBER 1, 1988
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quirea with the A b 1 1 to make “the first accurate estimation of lignin content of whole forest canopies by remote sensing.” Lignin, a stable carbohydrate polymer, is a major constituent of plant cell walls. It makes the plants rigid and unpalatable to insects that might otherwise eat them. Lignin also correlates well with a region’s fertility, presumably because its decomposition makes the cell wall leaky and allows nitrogencontaining nutrients inside the cells to re-enter the ecosystem. The amount of bioavailable, or mineralized, nitrogen in an area, in turn, relates to the area’s potential productivity. By remotely sensing signals linked to the lignin content of a forest canopy, and then hooking up these signals to a model describing the relationship between lignin and nitrogen, the researchers hope to be able to remotely sense nitrogen availability and forest productivity. The results of the study demonstrate the promise of remote sensing of key biogeochemical indicators, according to and Other researchers. But uncertainties about how the detected raw signals relate to canopy knin demand interpretational caution.. Wessman notes that the “lignin spectrum has not been characterized adequately because of difficulties in extracting lignin (an amorphous polymer) from foliar tissue in a pure, isolated form.” In addition, the two spectral lines and one broader spectral region that the researchers used as a lignin signature were empirically selected. “We aren’t certain that we are actually seeing lignin features in the spectrum,” Wessman said. Walter Westman adds that the “vibrational overtones of the C-OH bond will occur as a result of the abundance of such bonds in cellulose, starch, sugar, lignin, and other carbohydrates in the leaf.” The provisional lignin signature that Wessman used in her study also is highly vulnerable to spectral interference by water. consequently, Westman says he doubts that the remotely sensed signals contain enough information to distinguish lignin from other materials that have similar reflectance spectra. One way to mitigate the danger of such spectral monkey wrenches is to learn how in situ interferences actually affect the reflectance spectrum and then work these findings into the computer program that filters, corrects, and manages data coming from the imaging spectrometer. For example, Westman has stacked dried leaves from different tree species in a cuvette to see how leaf, anatomy, and thickness 1342A
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L‘anopy lignin concentration tor fllackhawk Island, W1, as predicted by a linear combination of three transformed Airborne Imaging Spectrometer bands. Changes in concentrationsfrom left to right (west to east) across the island reflect a soil texture gradient resultingfrom sediment sorting when the island area was an Dostglacialflood Dlain, .
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