## 3 Status and Application
### Repeat-Pass Interference
#### 3.1.1 D-InSAR
However, the excessive density of fringes in the epicentral area challenges the accurate estimation of the ground surface deformation amount in areas using visual interpretation methods. In D-InSAR data processing, phase-unwrapping technology is adopted to restore the absolute phase, thereby accurately measuring the ground surface deformation in the epicentral area. Figure 3: Flow of the measurement of surface elevation change by D-InSAR [4]. The most common application of D-InSAR deformation monitoring is in the measurement of co-seismic deformation fields of earthquakes [29]. Since the 1990s, many research institutions have begun to use InSAR to measure the co-seismic deformation fields of earthquakes that have occurred worldwide and have carried out inversion work on sliding fault models [30; 31; 32]. [PERSON] and [PERSON] [33] showed the mosaic map of the co-seismic deformation fields of the four 7.7-8.4 magnitude earthquakes that occurred in Chile from 1992 to 2000 calculated from the ERS-1/2 SAR images, and Figure 5 shows the co-seismic deformation fields and the inversion results of the sliding fault of the 6.8 magnitude earthquake in Bam, Iran, in 2003, calculated from the ENVISAT ASAR images [34]. A large number of studies have shown that the application of InSAR in the monitoring and analysis of co-seismic deformation fields of earthquakes has obvious technical advantages, such as high deformation measurement accuracy and wide coverage. This high-density surface deformation data provides unique key basic data for earthquake mechanism research and model inversion. Figure 4: The 1999 Turkkiye Mw7.1 Izmit earthquake by D-InSAR [28]. (**a**) Interferogram for the Izmit earthquake in the 35-day period between the two image acquisitions; (**b**) Interferogram calculated using the elastic dislocation model; (**c**) Residual interferogram, obtained after subtracting (**b**) from (**a**). Figure 5: The 2003 Bam Mw6.8 earthquake and deformation by D-InSAR [34]. (**a**) Detail of Envisat ASAR interferogram; (**b**) Interferogram of the same area based upon best-fitting two-fault distributed-slip model; (**c**) Interferometric correlation; (**d**) The distribution of slip on the main right-lateral strike-slip fault. It is worth noting that some scholars are exploring the joint monitoring of pre- and post-seismic deformation through combining geodetic techniques such as global navigation satellite system (GNSS) with InSAR technology, attempting to understand the earthquake mechanism more deeply and making new attempts for crustal deformation monitoring and even earthquake prediction [32; 35; 36]. Compared with conventional geodetic deformation monitoring techniques such as precise leveling and GNSS, InSAR offers characteristics of high accuracy, high resolution, wide coverage, low cost, safety, and continuous observation. It has incomparable superiority to conventional deformation monitoring methods [23; 37]. Table 2 lists the comparison between InSAR and precision level measurement and GNSS technologies. Compared with geodetic technologies based on point observations, InSAR provides unique space geodetic measurement technology based on surface observations, which can supplement the existing low-spatial-resolution geodetic measurement technology. Therefore, InSAR provides an economic and effective space Earth observation approach for geophysical research and deformation disaster monitoring. China is also providing outstanding results in the field of D-InSAR applications, with a number of research teams producing a series of representative research outcomes. Figure 6 shows in May 2022, Wuhan University released \"a map of ground surface deformation across the country with a resolution of 40 m and an accuracy of 5 mm/year\" [38], which was produced using Sentinel-1A/B time-series InSAR data covering China in 2021. This achievement was verified by Qianxun Position Network Co., Ltd. through the distribution of more than 2000 base stations of the Beidou ground-based augmentation system in the terrestrial area.