## 4 Experimental Results
### Experimental Setup  
The constructed experimental platform is shown in Figure 10. It included a GNSS signal high-speed acquisition receiver, which adopted a superheterodyne structure design and directly stores the original intermediate frequency signal to facilitate subsequent flexible data analysis and processing. It also included L5 high-gain reflected signal receiving antenna, full-frequency GNSS direct signal receiving antenna, and supporting power supplies, cables, mobile workstations, etc. Considering that GNSS signals are right-hand circularly polarized electromagnetic waves, the platform used the left-hand circularly polarized antenna to receive echo to maximize the reflected energy and suppresses the direct signal energy.  
The GNSS-BSAR imaging experiment was carried out at the Xueyuan Road Campus of Beihang University in Beijing, China. The experiment collected data at three different times in the morning, middle and evening of a certain day, namely 7:59 on 17 April 2023,  
Figure 10: The constructed experimental platform.  
Figure 9: The diagram of the proposed two-step azimuth phase compensation processing.  
11:55 on 17 April 2023, and 23:04 on 17 April 2023 (UTC + 8). It is worth mentioning that the weather on the experiment day was clear. However, the intensity of ionospheric scintillation is influenced by many complex factors, including solar activity, geomagnetic activity, geographic location, and seasonal variations. The recorded data and the proposed processing method in this paper only provide a preliminary validation of the diurnal variation of ionospheric scintillation, conducting a qualitative study. Therefore, the atmospheric and weather conditions at the moment of obtaining the three sets of experimental data are not described in detail here. The on-site photos of the experiment are shown in Figure 11. Taking the first set of experiment as an example, the illustration of the experimental scene is shown in Figure 12. For ease of system setup, the left-hand circularly polarized receiving antenna was placed at a height within 1 m above the ground. Additionally, considering the obstructions in the surrounding environment and the polarization characteristics of the reflected navigation signals, we set the cutoff elevation angle for GPS satellites at 30 degrees. The receiver was placed on the playground stand, and the reflect antenna pointed east to observe nearby buildings within about 1.5 km. The GPS satellite PRN4 was selected as the illuminator, which formed a typical backscattering configuration with the receiver and imaging area. Each set of experiments carried out 5 min of continuous observation. Table 1 lists more experimental parameters. When using GNSS signals for positioning, multipath interference elimination is often considered. In this paper, we used the GNSS signals for bistatic SAR imaging, and the influence of multipath effects is not considered in this application. After long-time coherent processing, multipath effects may be reflected in the final SAR imaging product. This requires subsequent potential image interpretation and analysis, which is beyond the scope of this paper.  
Figure 11: On-site photographs of the experiment taken at different times of the day: (**a**) in the morning, (**b**) at midday, (**c**) in the evening.  
Figure 12: The illustration of the experimental scene.