## 4 Simulation and Discussion
### Composite Scattering of Canyon/Valley Environment with Different Slope Angles  
In this numerical example, the bistatic RCS of the canyon and valley environment are investigated. The RMS height is \(h_{\rm rms}=0.2\lambda\), the wind speed is \(v_{19.5}=0.2\) m/s. The slope angles are denoted in the figures and table. The incident angle is \((\theta_{i},\varphi_{i})=(-30^{\circ},90^{\circ})\), the observation angles are \((\theta_{s},\varphi_{s})=(-90^{\circ}\sim 90^{\circ},90^{\circ})\). Figure 14 illustrates the main paths  
Figure 12: The geometry of the missile-like target above composite rough surface.  
Figure 13: Bistatic RCS for a missile-like target above the canyon and valley composite rough surface.  
of the EM wave propagation in canyon/valley composite rough surfaces with different slope-angle combinations. Numerical results are depicted in Figure 15, and the specific peak angles and peak values are listed in Table 2. The illustration of simulation geometry is the same as Section 4.3, shown in Figure 12.  
As shown in Figure 15, different slope-angle combinations have different peak angle positions because the EM waves have different propagation paths. First, these five combinations all have peaks at \(\theta_{s}=30^{\circ}\), which are caused by the reflection from the bottom. Secondly, when the incident waves strike the slopes with different slope angles, the main paths are different, which makes the RCS prediction difficult. Thirdly, the Brewster effect [16] will affect the peak values, sometimes even making the peak disappear. Comparing Figure 15a's and Figure 15b's \(45^{\circ}-45^{\circ}\) curves, both show three peaks at \(\theta_{s}=-60^{\circ},30^{\circ},60^{\circ}\), respectively. However, the peak value at \(\theta_{s}=60^{\circ}\) in VV polarization is much smaller than in HH polarization.  
Similarly, when the slope angles are \(30^{\circ}-45^{\circ}\), the peak at \(\theta_{s}=0^{\circ}\) disappears in VV polarization. The reason is that when the incident angle is equal to the Brewster angle of the environment, the reflection wave will be fragile, and even disappear. The Brewster angle of the terrestrial in this paper is \(60^{\circ}\)[42]. Note the path marked by yellow in Figure 14d. The incident wave hit the left slope and reflected to the bottom; as the incident angle is \(60^{\circ}\), because of the Brewster effect the reflection wave from the bottom is weak, as a result, the peak at \(\theta_{s}=60^{\circ}\) disappears. Then, as shown in Figure 14b, when the incident wave hit the left slope at the Brewster angle, the reflection wave decreased sharply, leading to the scattering wave at \(\theta_{s}=0^{\circ}\) disappearing. It is worth noting that the curves of the combinations of \(45^{\circ}-45^{\circ}\), in Figure 15b,d, have different performances at \(\theta_{s}=60^{\circ}\), even though they are both in VV polarization. The difference of the bottom can explain the phenomenon. The Brewster angle for the water bottom is about \(80^{\circ}\), while the incident angle is \(60^{\circ}\), so the reflection marked by the yellow of the valley bottom is still strong and makes the peak at \(\theta_{s}=60^{\circ}\) apparent. Hence, the structure of the canyon/valley environment decides the peak position; the surface type affects the peak values; Brewster effect produces special points.  
Figure 15: Bistatic RCS simulation results: (**a**) canyon, HH-polarization; (**b**) canyon, VV-polarization; (**c**) valley, HH-polarization; (**d**) valley, VV-polarization.