tailieunhanh - Radio Propagation and Remote Sensing of the Environment - Chapter 6

Nhiều bề mặt tự nhiên, chẳng hạn như bề mặt đất hoặc mặt nước của đại dương, có thể được coi là mịn chỉ trong trường hợp nhất định. Nói chung, các bề mặt này nên được xem xét để được thô, và tương tác của họ với radiowaves nên được xem như là một quá trình tán xạ. Cho dù chúng ta giả định rằng bề mặt thô nói chung hay không phụ thuộc vào vấn đề xây dựng, đặc biệt, về tỷ lệ quy mô gồ ghề và bước sóng. Bản chất của độ nhám khác nhau tùy. | 6 Wave Scattering by Rough Surfaces STATISTICAL CHARACTERISTICS OF A SURFACE Many natural surfaces such as the soil surface or water surface of the ocean can be regarded as smooth only in certain circumstances. In general these surfaces should be considered to be rough and their interaction with radiowaves should be seen as a scattering process. Whether or not we assume that the surface is rough generally depends on the problem formulation and particularly on the ratio of the roughness scales and the wavelength. The nature of the roughness varies depending on the type of surface. Sea surface roughness is a result of the interaction of the wind with the water surface. This interaction has a nonlinear character. A great number of waves with different frequencies and wave numbers are generated as a result and their mixture leads to oscillations of the sea surface height according to the stochastic function of coordinate and time. However the velocity of the sea surface movement is small compared to the velocity of light so time dependence cannot be taken into account in the first approximation. Soil roughness can form as a result of wind erosion urban activity and other causes. The soil roughness is also a random function of coordinates. Again a dependence on time is not considered in the beginning and we are dealing generally with wave scattering by random surfaces. The specific surface will be described by a random function of the elevation c s where s x y . The average value c s 0 so it is assumed that the average surface is given by the plane z 0. The function Z s is supposed to be statistically homogeneous. It simplifies the problem substantially as the statistical homogeneity of the real natural rough surfaces take place in the restricted cases. The correlation function K z s -s c s c 4 depends in this case on the coordinate difference of the points involved. In many cases the surface may be assumed to be statistically isotropic. Then the correlation .