## 5 Results and Analysis  
The selected region in Qinghai Province, China, was imaged using a four-satellite interferometric imaging mode, with PIESAT-01A as the master satellite and PIESAT-01B, PIESAT-01C, and PIESAT-01D as auxiliary satellites. Figure 4 is the optical imagery of the Qinghai research area on Google Earth. This location was chosen as the research area because it includes both mountainous and flat terrains, allowing for a clear evaluation of the adaptability and accuracy of the imaging and DSM algorithms under varying topographical conditions. The observation is conducted with a descending orbit left-looking configuration at an observation angle of 43.853. After decompressing the original echoes, the data were processed using the bistatic interferometric CS imaging algorithm. Figure 5 presents the imaging results of the selected region using the PIESAT-01 constellation. Interferometric processing is conducted on the imaged data. Initially, the complex SAR images are cropped and registered. The registration accuracies in the range direction after registration of A-B, A-C, and A-D pairs are 0.012, 0.045, and 0.031 pixels, respectively, while in the azimuth direction, the registration accuracies are 0.021, 0.056, and 0.014 pixels, respectively, meeting the requirements for interferometric processing. Coherence is a critical factor influencing terrain extraction. Figure 6 depicts the coherence map after regional registration. Statistical analysis reveals that the mean coherence values between each pair are 0.012 and 0.014 pixels, respectively, while the mean coherence values are 0.012, 0.056, and 0.014 pixels, respectively, while the mean coherence values are 0.012, 0.056, and 0.014 pixels, respectively, meeting the requirements for interferometric processing. Coherence is a critical factor influencing terrain extraction. Figure 6 depicts the coherence map after regional registration. Statistical analysis reveals that the mean coherence values between each pair are 0.012 and 0.014 pixels, respectively, meeting the requirements for interferometric processing. Figure 4: Qinghai research area optical image on Google Earth. The white box represents the scope of the research area. Figure 5: Amplitude figure of PIESAT-01 constellation imaging in four-satellite interferometric imaging mode. of images exceed 0.8, indicating excellent coherence of SAR images under the four-satellite interferometric imaging mode. Interferometric processing was conducted on the registered data to remove the flat phase, resulting in the phase changes induced by the topographic elevation variations, as depicted in Figure 7. The baselines corresponding to the A-B, A-C, A-D, B-C, B-D, and C-D paired data were measured as 38.90 m, 289.13 m, 342.31 m, 250.23 m, 381.21 m, and 631.43 m, respectively. Subsequently, the phase unwrapping of the filtered and flat-corrected phases was performed using the minimum cost flow (MCF) method to obtain the unwrapped phase  
Figure 6: Coherence illustration image. (A-B represents the coherence between PIESAT-01A and PIESAT-01B, A-C represents the coherence between PIESAT-01A and PIESAT-01C, A-D represents the coherence between PIESAT-01A and PIESAT-01D, B-C represents the coherence between PIESAT-01B and PIESAT-01C, B-D represents the coherence between PIESAT-01B and PIESAT-01D, C-D represents the coherence between PIESAT-01C and PIESAT-01D). Figure 7: Interference fringe after flat-earth phase removal. (A-B represents the interference phase between PIESAT-01A and PIESAT-01B, A-C represents the interference phase between PIESAT-01A and PIESAT-01C, A-D represents the interference phase between PIESAT-01A and PIESAT-01D, B-C represents the interference phase between PIESAT-01B and PIESAT-01C, B-D represents the interference phase between PIESAT-01B and PIESAT-01D, C-D represents the interference phase between PIESAT-01C and PIESAT-01D). results.