Remote Sensing - a tool for environmental observation

The dielectric constant or complex dielectric

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The dielectric constant or complex dielectric constant is a measure of the electric properties of the surface. It consists of two parts: permittivity and conductivity. In dry conditions most natural materials have a dielectric constant between 3 and 8. Water itself has a high dielectric constant around 80. Consequently, a change in moisture content generally results in a significant change in the dielectric properties of natural surfaces: increasing moisture content is associated with an increased radar reflectivity. The electromagnetic wave penetration in an object is an inverse function of water content. For vegetation, penetration depth depends on moisture, density and geometric structure of the plants: leaves and branches. Surface roughness is the terrain property that most strongly influences the strength of the radar returns. Surface roughness is not the same as topographic relief. Surface roughness is measured in centimetres and is determined by the textural features of the surface comparable in size to the radar wavelength such as leaves and branches of vegetation and sand, gravel and cobble particles. For most surfaces the vertical relief or average height of surface irregularities is an adequate approximation of surface roughness. Surfaces are mostly grouped in smooth and rough surfaces (figure 3.9) according to the Rayleigh criterion. The Rayleigh criterion considers a surface to be smooth if: h < λ / (8 sin γ ) where h is the vertical relief (cm), λ is the radar wavelength (cm) and γ is the depression angle of the radar system. For a radar system with a wavelength of 23.5 cm (e.g. Seasat) and a depression angle of 70 ° the surface relief below which the surface will appear smooth on the radar image is 3.1 cm. An example of the effect of surface roughness can be observed in the zones of contact between land and water. Inland water bodies tend to be relatively smooth (little or no waves), most radar radiation is reflected away from the radar system and the water body appears as a dark spot on the image. The sea with many waves and hence a rough surface has many exposed and shadowed surfaces resulting in a pattern of dark and bright spots or lines. Look direction and feature orientation are closely related and their effect on the radar image depends on the chosen aircraft track and the look direction of the radar system and the topographical properties of the terrain. Linear geologic features such as faults, outcrops and lineaments that are oriented at a normal or oblique angle to the radar look direction are enhanced by highlights and shadows in the image. Features oriented parallel with look direction are suppressed and are difficult to recognize in the image.
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49 The wavelength of the incident radar radiation determines: - the penetration depth of the waves into the objects at the surface - the relative roughness of the surface considered (see section on surface roughness).
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