Radar, specifically imaging radar, represents a powerful tool for remote sensing that has the advantages of around-the-clock capability (due to independence from solar illumination) and all-weather performance (due to cloud penetration). The concept of imaging radar dates to 1951, as defined by Carl Wiley, and practical systems followed fairly shortly thereafter. The first satellite test was the NRO experimental Quill satellite, launched in 1964, using the Corona satellite systems. Radar can penetrate modest depths into the earth and allow glimpses below the surface—useful when detecting buried objects such as pipelines or mines. Imaging radar is an essential tool for maritime concerns, from tracking ships to tracking sea ice. This chapter develops the physics and nature of imaging radar.
Figure 9.1 illustrates some of the radar data characteristics of interest. This image from an airborne sensor is created by a radar that collects data at two wavelengths, 6 and 24 cm. Shades of yellow and blue differentiate regions of different surface roughness, and vegetated areas (golf courses and city parks) show up particularly well.
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