# ISAR Imaging Analysis of Complex Aerial Targets Based on Deep Learning
## 4 Analysis of Simulation Results and Experimental Results
### Measurement Results  
In order to verify this design, this paper made a microwave anechoic chamber measurement experiment model. A microwave anechoic chamber refers to a space where the majority of incident electromagnetic waves are absorbed when they reach the walls, ceiling, and floor, with very little transmission or reflection. Microwaves share certain characteristics with light. Borrowing this concept from optical darkrooms, a microwave anechoic chamber can be created. A microwave anechoic chamber uses absorbing materials to create an enclosed space, thereby creating a pure electromagnetic environment within the chamber. The main working principle of a microwave anechoic chamber is based on the law that electromagnetic waves propagate from a low magnetic permeability to a high magnetic permeability in a medium. High-permeability absorbing materials are utilized to guide the electromagnetic waves. Through resonance, a large amount of the radiated energy of the electromagnetic waves is absorbed, and then the energy of the electromagnetic waves is transformed into heat energy through coupling. The angular reflection material in the model was aluminum with a thickness of 3 mm, which was assembled by welding process. The lower support base was made of acrylic wave-transmitting material, and the back was pasted with wave-absorbing material. The photo of the object is shown in Figure 9.  
Figure 8: Recognition results of the aircraft simulation ISAR imaging. The test system, illustrated in Figure 10, primarily comprises a computer (which includes test control and data processing software), a vector network analyzer, a power amplifier, a transceiver antenna, a turntable controller, a turntable, and a support structure. To enhance measurement accuracy, time gating technology is employed. The two antennas are positioned side by side and perpendicular to the target, facilitating the measurement of the RCS distribution characteristics under far-field conditions, as well as the data required for ISAR imaging. The specific test parameters are detailed in Table 4. \begin{table}
\begin{tabular}{c c} \hline \hline
**Item** & **Parameter** \\ \hline Test frequency & 8–12 GHz \\ Step frequency length & 0.01 GHz \\ angular range & \(-180\)–\(180^{\circ}\) \\ Sweep interval & 0.5\({}^{\circ}\) \\ Polarization mode & HH \\ Transmit–receive antenna spacing & 20 mm \\ Target distance & 9 m \\ Maximum power & 20 dBm \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Test parameter. Figure 10: Test system. Figure 9: Physical model of aircraft simulation assembly. As shown in the ISAR imaging model in Figure 1 that after translational motion compensation, no matter what kind of non-stationary and complex motion the ISAR conducts, the maneuvering target can still be equivalent to a turntable imaging model that rotates around the reference center O. If it is assumed that the reference origin O is consistent with the self-focusing center of the ISAR target, and the two-dimensional imaging projection plane is perpendicular to the rotation axis, when the maneuvering target conducts non-stationary motion, the angle rotated by the ISAR target within the same time interval is a non-constant variable parameter, which leads to different Doppler frequency values generated by each scattering point of the target during the rotation process. When the target has a stable motion, since the turntable works in a uniform rotation mode, the Doppler frequency of each scattering point is proportional to the azimuth coordinate x value. When sampling at equal intervals, the angle rotated by each scattering point remains constant. Therefore, the azimuth imaging processing process can be directly achieved by separating these Doppler frequencies in the frequency domain. Then, performing DFT transformation on the echo signal can obtain the two-dimensional imaging of the target, which is the ISAR imaging result.