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The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is one of several instruments on the EOS AM1 platform (which has since been renamed ("Terra" on the grounds that it is a more public-friendly name). Originally scheduled for launch in June of 1998, a long string of delays and problems were finally overcome, and Terra launched from Vandenberg AFB, a few seconds before 10:57 AM PST on Saturday, December 18, 1999.
The ASTER instrument has 3 downlooking radiometers, one for the visual and near infrared range (VNIR), which covers the wavelength range 0.52 - 0.86 microns; the short wave infrared (SWIR) telescope, 1.6 - 2.4 microns, and the thermal infrared radiometer (TIR), 8.1 - 11.3 microns. I am working with the TIR instrument team. The goal of the TIR is to retreive the land surface temperature and emissivity. Water is relatively easy to do because of its homogenous nature; it's pretty much the same everywhere. Not so for land, which can be bare rock, sandy desert, forest, prarie, and so on. Temperature measurements can tell us about energy flow from the ground to the air, or vice-versa. Emissivity measurements can tell us about mineralogical composition. The latter is especially poorly known on a global scale, and ASTER will be the first attempt to create reliable satellite based mineral maps on a global scale. The small pixel size (90 meters) allows for detailed examination of relatively small areas, and coupled with the temperature data, makes ASTER TIR an excellent instrument for locating and tracking thermal pollution.
In order for all this to work right, the effect of the atmosphere on the thermal infrared observations must be removed. Although we call this atmospheric correction, a better phrase would be atmospheric compensation. We will use the MODTRAN atmospheric transmission code package, currently managed by the Battlespace Environment Division of the Air Force Research Laboratory, but originally developed at the USAF Phillips Laboratory, as the kernel for our own atmospheric correction package. Using data from other instruments on the same platform (notably MODIS and MISR), or other available data, we will construct a chemical-physical model for the atmosphere along the instrument line of sight to the ground, compute the absorption and emission along that path, and use the results to remove the effect of the atmosphere from our data. If it works as intended, this method will allow us to retreive the ground-leaving infrared radiance directly, and from that another team will derive temperature and emissivity.
A complete, technical description of this task can be found in the relevant Algorithm Theoretical Basis Document (ATBD), which you can download in PDF format. You may have to download ADOBE Acrobat Reader (it's free) to read the PDF files.
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