# Performance Analysis of Ground Target Detection Utilizing Beidou Satellite Reflected Signals
## 4 Experimental Results
### Results of the First Experiment
As can be seen from Figure 11, there were two distinct changes in the phase difference information in the PLL loop when tracking the PRN9 satellite reflected signal. When the target appeared in the coverage of the antenna, the phase difference information of the reflected signal was kept near 90\({}^{\circ}\), and the duration of the phenomenon depended on the existence time of the target. If the target was removed, the phase difference value would return back to the normal level--near 20\({}^{\circ}\). After selecting the maximum value, the two periods of significant changes were obtained--about 12-22 s and 41-56 s. In the experiment, the target placement times were 10-20 s and 40-55 s.  
Compared with the processing results, there was a deviation of 1-2 s. There are two main reasons for this phenomenon. Firstly, there were some errors in the time of target placement and removal. Figure 11: Results of the BeiDou satellite (PRN9) reflected signal: (**a**) phase difference information (in ms); (**b**) phase difference information (in s). Figure 12: Results of the BeiDou IGSO satellite (PRN16) reflected signal: (**a**) Phase difference information (in ms); (**b**) Phase difference information (in s). Secondly, there were buffer data in the dual channel collector when receiving BeiDou signals. In order to avoid its impact, it is necessary to skip this data segment in the process of calculating the phase difference information of the BeiDou reflected signal. The number of data bytes skipped in the software receiver was \(4\times 10^{7}\). However, the results are very consistent with the actual situation when the duration time of phase difference information change is observed. The phase difference information of PRN16 showed no obvious change, which can be seen in Figure 12. Except for the abnormal protrusions near 11 s, most of the phase difference values were in a stable range. The analysis results of PRN24 from Figure 13 show that the phase difference information increased abruptly to \(90^{\circ}\) four times, near 10, 22, 41, and 58 s, respectively, and then it returned to its normal value; this process lasted for about 1 to 2 s, while the phase difference of other time periods remained in a small range. These moments were close to the target's placement time, but there was no continuous time period. This phenomenon was caused by the influence of the signal from the placement of the metal boxes. By comparing and analyzing the results of the phase differences of the three BeiDou satellites, it was concluded that the position relationship between the target and the specular points determines whether the ground target can be detected by the reflected signal or not. It is well known that the specular point determines the location of the first Fenier reflection region [35; 36]. This phenomenon shows that when the location of the target is not only in the coverage area of the LHCP antenna but also in the first Fresnel area of BeiDou satellites, it means that the LHCP antenna can receive the reflected signal from the target surface. In this case, the phase difference information of the reflected signal will be greatly affected, and the time of target placement can be calculated by its change in duration, as demonstrated by the results obtained from PRN9 in the experiment. On the contrary, if the location of the target does not satisfy this relationship, as occurred for BeiDou PRN16 and PRN24, it will not be detected by the method mentioned. By calculating the carrier-to-noise ratio of the three satellites [37; 38], the variation trend of the PRN9 satellite was found to be more obvious than that of the other two satellites, which is consistent with the time when the target appeared (see in Figure 14). Compared with the situation when the target did not appear, the CNR of the BeiDou reflected signal decreased by about 7 dB.