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Remote Sensing - a tool for environmental observation

3.5 sar synthetic aperture radar in the previous

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Unformatted text preview: 3.5 SAR: Synthetic Aperture Radar In the previous section it was discussed that apart from wavelength, the antenna length was an important factor to determine the spatial resolution of a radar system: spatial resolution improves with longer antennae. Traditional radar systems used an antenna of the maximum practical length or real aperture. The Synthetic Aperture Radar or SAR employs a relatively small antenna that transmits a broad beam. The Doppler principle (and special software) are employed to synthesize the azimuth resolution of a very narrow beam. Hence, a technical trick is used to increase the antenna length. Figure 3.5 shows the SAR principle. 43 Figure 3.5 Synthetic Aperture Radar (SAR) system (Sabins, 1987). 44 Figure 3.5 Concept of Synthetic Aperture Radar (SAR) system cont’d. 3.6 Geometry of Radar Images The nature of the radar remote sensing systems causes specific geometric characteristics in the images such as shadows, and bright spots, slant-range distortion and image displacement. Generally, topography has four effects on radar images: corner reflection, long shadows, terrain overlay and terrain foreshortening: Corner reflection occurs when a microwave pulse is reflected straight back to the antenna as a result of double reflection from a horizontal and then a vertical surface as shown in figure 3.6. The returning pulse has so much energy that it saturates the system and produces a white spot on the radar image. Long shadows are the results of an object shielding an area from the oblique microwave pulse (figure 3.7). Long shadows are an advantage for the geomorphologists as they give an impression of the relief of the terrain. On the other hand it is a loss of data, no observations can 45 be performed in the shadow portions of the terrain. Terrain layover is the result of a microwave pulse hitting a high point like the top of a steep mountain before it hits a low point like a river valley, which is further from the aircraft ground track but nearer to the antenna (figure 3.8). As a result all tall objects seem to lean towards the aircraft’s ground track. Terrain foreshortening is a result of a microwave pulse hitting a point like a valley before or at the same time, that the pulse hits a higher point such as a mountain (figure 3.8). As a result a slope facing the antenna will be compressed on a radar image. Figure 3.6 Corner reflection: the microwave radiation is reflected straight back to the antenna (Curran, 1985). 3.7 Radar Return and Image Signatures Radar images are always characterized by speckle. Speckle refers to the grainy or salt and pepper pattern of dark and bright spots in radar images. Speckle is a result of the antenna transmitting many minor pulses along with each major pulse. These pulses often reinforce or suppress the backscatter of the major pulse and speckle in the image is the result. Speckle is tedious for visual interpretation of radar images and it hampers seriously automatic analysis and classification. classification....
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3.5 SAR Synthetic Aperture Radar In the previous section it...

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