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These are two false-color composites of Raco, Michigan, located at the eastern end of Michigan's upper peninsula, west of Sault Ste. Marie and south of Whitefish Bay on Lake Superior. The two images (centered at 46.39 degrees north latitude, 84.88 degrees west longitude) show significant seasonal changes in the mid-latitude region of mixed deciduous and coniferous forests.
The images were acquired by the Spaceborne Imaging Radar-C and X-band Synthetic Aperture Radar (SIR-C/X-SAR) aboard the shuttle Endeavour on the sixth orbit of each mission. In these images, red is L-band (23 cm) with horizontal/vertical polarization; green is C-band (6 cm) with horizontal/vertical polarization; blue is C-band with horizontal/horizontal polarization. The region shown is largely forested and includes a large portion of Hiawatha National Forest, as well as an agricultural region near the bottom of each image. In early April, the area was snow-covered with up to 50 centimeters (19.5 inches) of snow in forest clearings and agricultural fields. Buds had not yet broken on deciduous trees, but the trees were not frozen and sap was generally flowing. Lake Superior, in the upper right, and the small inland lakes were frozen and snow-covered on April 9, 1994. By the end of September, deciduous trees were just beginning to change color after a relatively wet period. Leaf loss was estimated at about 30 percent, depending on the species, and the soil was moist to wet after a heavy rainfall on September 28, 1994. Most agricultural fields were covered with grasses of up to 60 centimeters (23 inches) in height.
In the two images the colors are related to the types of land cover (i.e. vegetation type) and the brightness is related to the amount of plant material and its relative moisture content. Significant seasonal changes between early spring and early fall are illustrated by this pair of images. For the agricultural region near the bottom of the images, the change from snow-cover to moist soil with short vegetation cover is shown by the color change from blue to green and blue. The green color corresponds to significant increases in vegetation cover and field-to-field differences in blue are the result of differences in surface roughness and soil moisture. In the forested areas, many of the conifer forests appear similar in both images (red pine forests appear red in both images). However, there is more blue and green in the September 30, 1994 image as a consequence of greater foliage and more moisture in the forest crowns. Lowland conifer forests (spruce and northern white cedars) appear as bright green in both images. Deciduous forests produce very strong radar returns at these frequencies and polarization combinations, resulting in a nearly white appearance on the images (the specific color mix is related to the local species mix). In the September 30, 1994 image, the areas of deciduous forest appear darker than in the April image because of the weaker radar signal from the foliage in the crown layer. The clear-cut areas (shown in April by the irregularly shaped dark areas in the center) change dramatically in appearance due to loss of snow cover and increases in soil moisture and vegetation cover by the end of September.
SIR-C/X-SAR radars illuminate Earth with microwaves, allowing detailed observations at any time, regardless of weather or sunlight conditions. SIR-C/X-SAR uses three microwave wavelengths: L-band (24 cm), C-band (6 cm), and X-band (3 cm). The multi-frequency data will be used by the international scientific community to better understand the global environment and how it is changing. The SIR-C/X-SAR data, in conjunction with aircraft and ground studies, will give scientists clearer insights into those environmental changes which are caused by nature and those changes which are induced by human activity.
SIR-C was developed by NASA's Jet Propulsion Laboratory. X-SAR was developed by the Dornier and Alenia Spazio companies for the German space agency, Deutsche Agentur fr Raumfahrtangelegenheiten (DARA), and the Italian space agency, Agenzia Spaziale Italiana (ASI) with the Deutsche Forschungsanstalt fuer Luft und Raumfahrt e.v. (DLR) which is major partner in science, operations and data processing of X-SAR.
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