# Performance Analysis of Ground Target Detection Utilizing Beidou Satellite Reflected Signals
## 4 Experimental Results  
### Results of the First Experiment  
The BeiDou software receiver was used to process the collected BeiDou reflected signals. Three satellites were captured and tracked, which were PRN9, PRN16, and PRN24 respectively. Among them, PRN9 is an IGSO satellite, while PRN16 and PRN24 are MEO satellites. BeiDou IGSO satellites have the same orbital altitude as the GEO satellite, so they have the same orbital period as the Earth's rotation period. Because the orbital inclination of the BeiDou PRN9 satellite is \(54.583^{\circ}\), its sub-satellite point trajectory has an \"8\" shape distribution with the equator (about 120\({}^{\circ}\) east longitude) as the symmetrical axis on the ground, which can cover most of the area of China [32]. In particular, due to the constellation characteristics of the BeiDou IGSO satellite, its visible time in China is much longer than the other MEO satellites. In this paper, the antenna was facing west to verify the reliability of the BeiDou IGSO satellite in the field of target detection. It should be noted that if we want to monitor whether there are targets in a fixed area for a long time, we need to adjust the direction of the antenna and choose BeiDou GEO satellite as the signal source. This is because, in the application of reflected signals, the GEO satellite can maintain a stable geometric relationship. According to the longitude and latitude of the experimental site, the BeiDou satellite skylplot at the time of data acquisition, \(2018\_11\_15\_15\_35\_32\), can be obtained as shown in Figure 9. Based on this, the azimuth and altitude information of the three BeiDou satellites can be extracted, as shown in Table 3.  
Figure 9: BeiDou satellite skylplot at the data collection time in the first experiment. The latitude and longitude are \(39.69^{\circ}\) and \(116.69^{\circ}\). In the remote sensing field, when using GNSS reflected signals, the peak power of the received signal is typically reflected from the surface surrounding the specular point, where the incident and reflected angles are equal [33]. In addition, the first glistening zone surrounding the specular point contributes most of the power of the reflected signal. According to the method presented in [34], the specular points on the ground surface between the three BeiDou satellites and the LHCP antenna can be calculated, the relationship of which is shown in Figure 10. Blue represents PRN16, red represents PRN9, and green represents PRN 24. The ellipses depict the first Fresnel zone, and the pots, numbered 2, 3, and 4, depict the specular reflection point. The bold dark ellipse is the LHCP antenna's footprint. The bold dark pot stands for receiver number 1, considering the origin of the reference system. The blue rectangle represents the mental box. As can be seen from Figure 10, the target was located nearest to the specular point of the PRN9 satellite and farther away from the specular points of PRN24 and PRN16 satellites. By tracking the reflected signals of the three satellites, the phase difference information in the tracking loop was obtained, as shown in Figures 11a-13a. In the tracking loop, the output interval of the phase difference information was 1 ms. In the experiment, the time units of the box placement were seconds. In order to achieve the same time units, the maximum value of each 1000 phase difference information values was selected, as shown in Figures 11b-13b. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Satellite Orbit** & **PRN** & **Elevation Angle** & **Azimuth** \\ \hline IGSO & 9 & 61\({}^{\circ}\) & 304\({}^{\circ}\) \\ MEO & 16 & 70\({}^{\circ}\) & 348\({}^{\circ}\) \\ MEO & 24 & 45\({}^{\circ}\) & 189\({}^{\circ}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Status information of the three acquired BeiDou satellites in the first experiment. Figure 10: Reflection points of each BeiDou satellite on \((x,y)\) planes in the first experiment.