对于给定频率，在所示掠射角范围内的变化为20至25分贝。
For a givenfrequency, the variation with grazing angle over the range shown is 20 to 25dB.
对于给定的掠射角，本例中不同频率之间的反射率变化约为10分贝。
For a given grazingangle, the variation across frequency in this example is about 10 dB.
图2.21是固定频率下不同地形类型的σ0随掠射角变化的一个例子，在这种情况下是S波段。
Figure 2.21 is oneexample of the variation in σ0 versus grazing angle for differentterrain types at a fixed frequency, in this case S band.
Figure 2.21. S波段σ0随地形类型和掠射角的变化Behavior of σ0 versus terrain type and grazing angle at Sband. (Data from Currie, 2010.)
一般来说，反射率随着地形粗糙度的增加而增大，因此平坦的沙漠地形反射率相对较小，而复杂多变的城市地形反射率则较大。
Generally,reflectivity increase with terrain roughness, from the presumably smootherdesert terrain to the complex, rough urban terrain.
如这些图中所示，σ0受掠射角的影响很大。
As seen in thosefigures, σ0 varies significantly with grazing angle.
一般来说，反射率在非常低的掠射角下迅速减小，在非常高的掠射角（雷达视线垂直于杂波表面）下迅速增大，而在中间的“平坦区域”变化较小。
Generally, itdecreases rapidly at very low grazing angles, and increases rapidly at veryhigh grazing angles (radar look direction normal to the clutter surface), witha milder variation in a middle “plateau region.”
图2.22是这种行为特征的概念示意图。
Figure 2.22 is anotional diagram of this behavior.
Figure 2.22. 陆地杂波随掠射角变化的σ0曲线General behavior of σ0with grazing angle for land clutter. (After Long, 2001.)
在平坦区域描述σ0特性的一个通用模型是“常数Gamma”模型：
A common model forthe behavior of σ0 over the plateau region is the “constantgamma” model (Long, 2001):
其中，γ是特定杂波类型在感兴趣雷达频率和极化上的特征常数。
where γ is a characteristicof the particular clutter type at the radar frequency and polarization ofinterest.
该模型预测表明：σ0在垂直入射时达到最大，随着掠射角趋于零时，σ0逐渐变小。
This model predictsthat σ0 is maximum at normal incidence and becomes vanishingly smallas the grazing angle tends to zero.
然而，它不能充分反映在接近垂直或接近零入射角时观察到的σ0变化程度，因此在这两种极端情况下通常使用其它附加模型。
However, it does notadequately reflect the degree of change in σ0 often observed atnear-normal or near-zero incidence angles, and additional models are often usedat these two extremes.
文献中提出了各种关于σ0作为重要参数函数的预测模型。
Various predictivemodels for σ0 as a function of important parameters have beenpresented in the literature.
——本文译自Mark A. Richards所著的《Fundamentals of Radar Signal Processing（Second edition）》
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