## 1 Introduction  
Interferometric synthetic aperture radar (InSAR) measurement technology, an active quantitative microwave remote sensing technique developed over the past half-century, has been verified as an important technical means for Earth observation [1, 2, 3, 4, 5, 6, 7, 8, 9]. It originated from \"[PERSON]'s double-slit interference experiment\", proposed by [PERSON] in 1801. By leveraging both amplitude and phase information from SAR imagery for interferometric processing, InSAR plays a decisive role in global topographic mapping, marine hydrological observation, disaster prevention and mitigation, etc., featuring high precision and wide coverage. [PERSON] and [PERSON] [10] were the first to apply interferometry techniques to terrain observation of Venus and the Moon. In 1974, [PERSON] [11] proposed the technical principle of using InSAR technology for topographic surveys, demonstrating the feasibility of InSAR for topographic surveying for the first time through X-band airborne radar implementations. Subsequently, the National Aeronautics and Space Administration (NASA)'s Jet Propulsion Laboratory (JPL) in the United States conducted the first experimental research on the airborne InSAR three-dimensional reconstruction of terrain using airborne side-looking radar, obtaining a digital elevation model (DEM) of San Francisco [12]. In 1988, [PERSON] et al. [12] processed SEASAT satellite observation data using InSAR technology, and the topographic maps of Death Valley obtained by them were consistent with the results published by the United States Geological Survey. With the launch and operation of numerous of SAR satellites, scholars conducted a large number of studies on repeated-pass InSAR technology, effectively promoting the rapid development of related theoretical methods and application technologies. Due to the serious atmospheric delay differences, temporal decorrelation, and low baseline measurement accuracy between multiple passes, repeated-pass InSAR is primarily limited to surface deformation monitoring and cannot satisfy digital elevation accuracy requirements [13]. To address these constraints, single-pass interferometric SAR can simultaneously obtain multiple coherent SAR images. For instance, the pioneering Shuttle Radar Topography Mission (SRTM) generated DEM within the range from 56\({}^{\circ}\) south latitude to 60\({}^{\circ}\) north latitude, which conformed to the Digital Terrain Elevation Data level 2 (DTED-2) standard [14]. The development of InSAR has gone through the process of \"ground detection radar-imaging synthetic aperture radar-synthetic aperture radar interferometry\", which also indicates that InSAR is the integration of synthetic aperture radar remote sensing imaging and electromagnetic wave interference technologies. Key milestones in onboard InSAR systems and technical development history are shown in Figure 1.  
China's spaceborne SAR missions have continuously advanced and accelerated, gradually forming a relatively complete technical system and application framework. Since the launch of HuanJing-1C (HJ-1C), the HJ-1C satellite equipped with an S-band SAR system in 2012, China has officially entered the era of spaceborne SAR. The GaoFen-3 (GF-3) satellite, launched in 2016, is China's first C-band fully polarimetric SAR imaging satellite, with a resolution reaching 1 m [15; 16; 17], providing crucial support for high-resolution imaging and surface deformation monitoring of the land and ocean [18; 19]. Subsequently, the LuTan-1 (LT-1), the world's first L-band distributed-formation multi-polarization InSAR altimetry satellite system, achieved meter-level ground observation resolution and successfully carried out China's first on-orbit bistatic interferometric observation, marking a key breakthrough in high-precision interferometric measurement technology.