## 4 Trends and Challenges
### Multidimensional
#### 4.1.4 Multipolarity  
In 1997, [PERSON] and [PERSON] [103; 104] first studied the impact of frequency and polarization on coherence using SIR-C/X-SAR data, and found that coherence strongly depends on polarization, which was pioneering research in the field of multipolarization SAR interferometry. Polarimetric SAR interferometry (PolInSAR) offers unique advantages that conventional SAR cannot match, and has gradually become a new research hotspot in the field of SAR data processing [80; 105; 106; 107]. [PERSON] et al. [106] applied this technology for forest height inversion and validated its effectiveness under different forest structures and seasonal conditions. In recent years, significant progress has been made in the application of PolInSAR technology for surface vegetation height inversion [108]. With the continued use of both full-polarization and dual-polarization synthetic aperture radar data, PolInSAR has expanded its applications in forest biomass estimation, carbon stock assessment, and tropical forest monitoring. In 2015, [PERSON] et al. [109] proposed a PolInSAR technique based on dual-polarization SAR data and successfully applied it to forest height estimation by extending the three-stage inversion process and coherence optimization algorithm. As shown in Figure 24, experimental results show that dual-polarization PolInSAR performs comparably to the full-polarization mode, and the proposed volume-only coherence search method significantly enhances inversion accuracy. Figure 25 shows the grayscale of Sentinel-1A under different polarization modes. Figure 23: Deformation monitoring of the Muruntau Gold Mine in Uzbekistan. Figure 24: The inversion results of dual polarization SAR data are processed by the interference method [109] (black area is water bodies and yellow area is roads). In recent years, China has made significant progress in the development and application of spaceborne PolInSAR systems, gradually establishing a domestic SAR satellite fleet with multi-polarization observation and interferometric measurement capabilities. Studies have shown that the GF-3 satellite's DInSAR technique can achieve sub-centimeter-level ground deformation monitoring. The integration of polarimetric information further improves the accuracy of surface change detection, showing high consistency with optical monitoring results [110]. Moreover, change detection combining polarimetric data accurately delineates collapse areas in mining regions, with mapped areas closely matching public monitoring data, demonstrating the potential of GF-3's multi-polarization data in dynamic surface monitoring. Although PolInSAR applications for forestry or agriculture based on GF-3 remain limited, its high-resolution full-polarimetric capability provides a valuable data foundation for future biomass estimation studies. The LT-1 constellation, as China's first L-band fully polarimetric PolInSAR system, has achieved remarkable results in polarimetric calibration and interferometric measurement performance. Relevant studies conducted field validation using four Polarimetric Active Radar Calibrators (PARCs) deployed in the Inner Mongolia test site. The results showed that the amplitude error was less than 0.6 dB, phase mismatch was controlled within 5\({}^{\circ}\), and cross-talk in the sidelobes was lower than \(-33\) dB, demonstrating the system's precise calibration capability and high signal-to-noise imaging performance [111]. Additionally, the LT-1 satellites provide stable multibaseline interferometric observations. Under a multibaseline configuration, the acquired data effectively capture forest canopy scattering characteristics, making LT-1 a critical data source for spaceborne forest height retrieval studies [112]. These advancements indicate that China's PolInSAR technology and applications are rapidly evolving. Satellite platforms with multi-baseline and multi-polarization observation capabilities are providing novel remote sensing tools for forestry monitoring, agriculture, wetland protection, and related fields.