# Bounding Volume Hierarchy-Assisted Fast SAR Image Simulation Based on Spatial Segmentation
## 4 Discussion of Simulation Results
### Analysis of Simulation Results  
The test first takes the Japanese helicopter aircraft carrier model as an example and refers to the material parameter list in Table 3 to assign values to the surface material parameters of the target's main functional parts in turn. This test is mainly based on the actual geometric structure of the target and the amount of information related to the material. It then disassembles, assigns parameters, and splices the target in a refined way. As shown in Figure 8a, the test disassembles the Japanese helicopter carrier into four main functional parts. The target model is in OBJ file format, and its surface cell size is dual scale. Large surface facets are used for planar structure components and small surface facets are used for curved structure components, which not only ensures that the target has a detailed geometric texture but also effectively prevents the surface facets from being overdivided, which reduces the complexity of the calculation of the target multi-backscatter field. The total number of target model facets (length 166 m \(\times\) width 22 m \(\times\) height 57 m) is 204,362, according to the sampling rate in Table 1 (No. I). The interval of lattices is set to half the system sampling space, in compliance with the Naquist sampling law. The number of target lattices is 125,972. As shown in Figure 9, in order to facilitate the demonstration of the process of fast SAR image simulation based on spatial segmentation proposed in this paper, the target latticed radiation area is initially segmented into six latticed blocks, and the located subspaces are labeled with different colors. The CUDA kernel function designed is executed in parallel on the CUDA platform to quickly obtain the amplitude and phase of the echo signal corresponding to each latticed block. The simulation time consumption of the test is detailed in Table 4, and compared with the CPU simulation time, the acceleration ratio can reach 210 times. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Material** & **Relative** & **Diffuse** & **Specular** & **Specular** & **Energy Decay** \\ **(Main Component)** & **Permittivity** & **Coefficient** & **Coefficient** & **Index** & **Coefficient** \\ \hline Aluminum & 8.00 & 0.75 & 0.80 & 50.00 & 0.20 \\ Fiber-reinforced plastics & 8.50 & 0.80 & 0.60 & 50.00 & 0.10 \\ Special steel & 9.50 & 0.65 & 0.80 & 30.00 & 0.25 \\ Copper–nickel & 12.00 & 0.70 & 0.50 & 50.00 & 0.15 \\ Inconel & 10.50 & 0.75 & 0.40 & 30.00 & 0.10 \\ Nickel–titanium & 15.00 & 0.65 & 0.70 & 40.00 & 0.20 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Material property parameters. Figure 8: Japanese helicopter carrier model and latticed radiation area. (**a**) Assignment of the material property parameter. (**b**) Latticed radiation area of the Japanese helicopter carrier. Figure 9: Splicing of the latticed block echo signal. In order to better demonstrate the amplitude difference between single and multiple scattering, the amplitude difference of both is taken. In Figure 10, it can be visualized that multiple scattering is mainly concentrated in the command tower, secondary scattering mainly occurs at the edge of the hull enclosure, and single scattering mainly occurs on the flight deck, and the flight deck is also arranged with a number of dihedral angular of the catapult and the flame plate, which can be a secondary scattering. After the above comparative analysis, the reliability of the calculation method of multiple backscatter fields proposed in Section 2.2 of this paper can be effectively verified. The real SAR image of the Japanese helicopter carrier is shown in Figure 11 (left), with the theoretical resolution of 1 m, and other system parameters as shown in Table 1 (No. I). The simulation results obtained by the method proposed in this paper are linearly stretched in amplitude to highlight the target silhouette; see Figure 11 (middle).