## 6 Discussion  
The concept of formation flying satellites has been introduced for many years [33; 34; 35], and the utilization of multi-baseline methods to enhance the accuracy of DSM is regarded as one of the most promising approaches. Bistatic SAR imaging methods and multi-baseline DSM processing techniques have also been the focus of research in recent years. However, studies typically utilize simulated data, repeat-pass data, or data from two formation-flying satellites for investigation. The PIESAT-01 satellite, as the world's first four-satellite formation flying mission, exhibits superior imaging quality, and its interferometric results  
Figure 19: PIESAT-01 DSM of Heilongjiang Region.  
Figure 20: PIESAT-01 DSM and TanDEM DSM difference histogram of Heilongjiang Region.  
meet the accuracy requirements of China's 1:50,000 scale requirement. The data from the PIESAT-01 satellite can provide high-quality data for multi-baseline SAR imaging and application research.  
This paper employs the bistatic imaging CS algorithm, which also finds widespread application in the 3D algorithm of multi-baseline SAR imaging and holds considerable potential for further development [36; 37]. However, with the advancement of multi-baseline interferometric SAR technology, the demand for scene 3D imaging extends beyond merely extracting surface elevation information from DSM. Instead, there is a shift towards achieving true 3D SAR imaging of scenes from the perspective of resolution. In this context, multi-baseline interferometric SAR imaging systems typically project the scattering characteristics of 3D target scenes onto the 2D range-azimuth plane, treating the resulting sequence of 2D images as echo data in the height direction, followed by height direction focusing, which has been a recent research focus. PIESAT-01, as a dataset with simultaneous multi-baseline observations, can provide high-precision and effective data for such studies.  
Moreover, the DSM processing method proposed in this paper involves multi-baseline DSM fusion. For complex terrains containing steep slopes or discontinuous surfaces (e.g., numerous buildings, canyons, and steep mountains), severe undersampling phase phenomena may occur, where adjacent phase differences in interferograms can exceed \(\pi\). In such cases, multi-baseline phase unwrapping techniques are considered effective means to overcome the limitations of the phase continuity assumption and generate absolute phases for complex terrains [38; 39]. Their potential advantage lies in leveraging multiple interferograms corresponding to different baseline lengths, thereby circumventing the strict constraint of adjacent phase differences not exceeding \(\pi\). Commonly used multi-baseline unwrapping algorithms include coarse-to-fine phase unwrapping and maximum likelihood phase unwrapping.  
Lastly, it is meaningful to analyze error interference terms in the processing of multi-baseline InSAR data, as this can be an effective method to further enhance the computational accuracy of DSM. For instance, studying the impact of the atmospheric phase on interferometric phase and elevation accuracy is one such avenue. By utilizing real-time DSM and DEM data, it becomes possible to more effectively eliminate terrain phases and obtain more accurate deformation results.