# Unmanned Aerial Vehicle (UAV) Robot Microwave Imaging Based on Multi-Path Scattering Model
## 5 Discussion  
From above simulation results, based on the imaging method proposed in this study, it can be seen that the larger the receiver's zenith angle is, the closer the distance between two parts of energy becomes. This is because the distributions of scattering energy in these images are related to the travel path and the imaging algorithm. When the receiver's zenith angle is larger, the lengths of propagation paths from direct scattering and single bounce become closer. For this imaging configuration, it can be inferred that when the receiver's zenith angle becomes large enough, the two parts will overlap. In addition, it can be observed that the range resolution of Figure 6c-f is relatively clear among the six images. The resolution of the image shown in Figure 6b is somewhat worse than that in Figure 6c, but better than that in Figure 6a. These results might be due to the variation in the receiver's zenith angle. Equation (14) presents the range resolution formula [21], where \(c\) is the velocity of an electromagnetic wave propagating in free space, and \(B\) is bandwidth. \(\theta_{t}\) and \(\theta_{r}\) are the zenith angles of transmitter and receiver, respectively. The range resolutions as a function of zenith angle of receiver are calculated based on Equation (14), as shown in Figure 7. The range resolutions of six zenith angles of receivers shown in Figure 6 are listed in Table 4. The calculation results contribute to the understanding of the aforementioned imaging results. \[\Delta L_{range}=\frac{c}{B(\sin(\theta_{t})+\sin(\theta_{r}))} \tag{14}\] \begin{table}
\begin{tabular}{c c} \hline \hline
**Zenith Angle of Receiver (\({}^{\circ}\))** & **Value (m)** \\ \hline
26 & 0.87 \\
36 & 0.77 \\
46 & 0.70 \\
56 & 0.65 \\
66 & 0.62 \\
76 & 0.60 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Range resolution of different receiving angles. Figure 6: UAV imaging results of trees at different receiver zenith angles: (**a**) receiving zenith angle = 26\({}^{\circ}\); (**b**) receiving zenith angle = 36\({}^{\circ}\); (**c**) receiving zenith angle = 46\({}^{\circ}\); (**d**) receiving zenith angle = 56\({}^{\circ}\); (**e**) receiving zenith angle = 66\({}^{\circ}\); (**f**) receiving zenith angle = 76\({}^{\circ}\). Furthermore, from Figure 6, it is noticed that the zenith angle of the receiver also affects the signal strength received from the ground surface. Direct scattering from the ground surface is clearly shown in Figure 6a. However, as the zenith angle of the receiver increases, the signal strength from the ground surface becomes weaker and weaker. For Figure 6c, the signal strength received from the ground surface is too weak to be distinguished. In addition, it can be seen from the figures that the scattering strength of a single tree from the left part is gradually stronger than that on the right part. We have suggested in Section 4.2 that the image of a single tree includes two main parts: the left part is mainly from direct scattering of the scatterer in the tree, and the right part is the summation of a single bounce between the scatterer and ground. For a large receiving angle, the signal strength received from the ground surface is relatively weak. Furthermore, the strength from right part in the figure will also weaken because the right part depends on a single bounce between the scatterer and the ground. Therefore, it can be seen that the signal strength from the left part becomes gradually stronger, as shown in Figure 6a-f.  
For imaging results of multiple trees in a stand as shown in Figure 6, it can be found that when the receiver zenith angle is large, the distribution of trees in this area is relatively clear, and different scattering components of a single tree are relatively close. When the receiver zenith angle is small, the scattering component received from the ground surface is strong. To a certain extent, some scattering components of the tree are suppressed by direct scattering components of the ground surface, as shown in Figure 6a.