A tremendous amount of data is collected on each flight, and must be processed by investigators prior to conducting research on the imagery. The data must be corrected for geometric abnormalities due to flight path variations, and must be radiometrically calibrated. These raw sensor scan lines are then reconstructed into a two dimensional image data set.
Flight path and data
(Click to view higher resolution data)
Now that the data is an image, the scientist is able to begin data inquiries. This is typically done by creating false color images based on 3 spectral channels from the sensor. Dependent on whether vegetation health,
|The ATLAS scanner|
Our remote sensing is driven by the biology and physics of the crop and the soil. We need visible and near IR bands for several reasons. Vegetation has a very strong reflectance feature at 0.7 microns (see the plot of typical reflectance curves). A pair of bands which bracket this can be used to determine the amount of green vegetation. Clay minerals have a strong absorption feature in the 2.2 micron region. Comparison of a 2.2 micron band with a 1.6 micron band is therefore sensitive to clay. Iron, as hydrated iron oxides, has a high reflectivity in the red portion (near 0.7 microns) of the spectrum and a reflectance depression near 0.8 microns. Bands which cover these features can measure iron content.
Thermal bands are used for completely different reasons. As anyone who has stood in the full sun light of a summer day knows, the sun dumps a lot of energy on every exposed surface. To stay alive, plants must cool themselves or their basic chemical processes would fail. They cool themselves by evaporating water. Plants which are warm and plants which are cool show up differently in thermal bands.
|Typical reflectance curves (blank areas are where the atmosphere is not transparent to that wavelength)|
|Figure 2.2 from Settle, Mark, et al.; 1985; Sensor Assessment Report, in The Joint NASA/Geosat Test Case Project Final Report, ed. Abrams, M., Conel, J., Lang, H., Paley, H.; American Association of Petroleum Geologists, Tulsa, OK.; p 2-5.|
Other parameters address a host of practical aspects of the problem. Crop stage, spatial resolution, seasonal and daily weather conditions must be controlled. For example spatial resolution in the 2-5 meter range was chosen, avoiding many complexities caused by aliasing crop row spacing at higher resolutions yet finer than the harvester's tightest recording rate. This dictates use of an airborne system. Use of an airborne system also makes scheduling around weather much simpler than use of satellite data. Active video calibration was recognized as essential if quantitative measures were ever to be obtained or reproduced. The system would also have to have onboard geometry recorded during data acquisition. There are a limited number of sensor/aircraft combinations that provide the needed features. The only system which is currently available that meets all of these criteria is the Atlas scanner, flown out of NASA's Stennis Space Center.
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