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By Lee J. Nelson
The International Space Station affords astronaut photographers a unique perspective of ongoing events. Their visible imagery is oblique—taken from a skewed angle as opposed to the “straight down” (nadir) line-of-sight which is typical of most automated satellite sensors. Sometimes, oil is difficult to see in high seas and among floating debris where it blends in with dark backgrounds. Municipal assets report surface color, reflectance, temperature, and roughness. When oil’s presence modifies one or more of those parameters, it more readily can be verified.
Sun glint customarily obscures targeted objects; but, in this case it actually abets identification of the oil spill. The slick smoothes surface texture and the mirror-like reflection of the sun accentuates differences between the glossy oil-covering and rougher waters. Still, while satellites document the overall extent of the disaster, they are unable to separate surface sheen from denser oil patches.
NASA (the National Aeronautics and Space Administration; Washington D.C.) dispatched its Earth Resources-2 research aircraft (ER-2) to map the oil slick and adjacent coastline. The agency is providing images acquired by MODIS, the Moderate Resolution Imaging Spectroradiometer (405nm ≤ λ ≤ 2155nm) carried on the Aqua and the Terra satellites; by ASTER, the Japanese Advanced Spaceborne Thermal Emission and Reflection Radiometer (520nm ≤ λ ≤ 11.65μm) also aboard Terra; and by the Advanced Land Imager (433nm ≤ λ ≤ 2350nm) and the Hyperion Hyperspectral Imager (375nm ≤ λ ≤ 2576nm), both on EO-1, the Earth Observing-1 satellite. All observations are funneled to the U.S. Geological Survey/Eros Data Center’s Hazards Data Distribution System (Sioux Falls S.D.), a digital repository which users can access via a browser-based interface to view and download full-resolution, disaster-response imagery and data.
As we go to press, NASA continues watching the Gulf and threatened wetlands from ER-2, outfitted with AVIRIS, the Airborne Visible/Infrared Imaging Spectrometer (400nm ≤ λ ≤ 2500nm) and with the Cirrus Digital Camera System (510nm ≤ λ ≤ 990nm). AVIRIS measures how water absorbs and reflects light to help determine oil concentration. Researchers also plan to evaluate changes in vegetation along coastal areas and assess where and how oil is affecting bayous, beaches, marshes, and swamps. The Cirrus Digital Camera System’s pseudocolor infrared imager supports habitat mapping and ecosystem monitoring.
The directionality, intensity, and wavelength of ambient illumination—whether natural (and transcribed by the solar elevation and azimuth angles) or artificial—remain primary variables that impact the prospective value of any image...regardless of its intended purpose.