## 3 Status and Application
### Single-Pass Interference
#### 3.2.2 AT-InSAR  
The Along-Track Interferometry method involves placing two antenna pairs along the flight path to image the same scene. Each antenna processes its own image. Due to the ocean surface movement, there will be a phase difference between the two images. This phase difference can be used to measure the line-of-sight velocity of ocean surface currents, as shown in Figure 9b. In 2000, [PERSON] discussed the feasibility and limitations of ATI flow measurement through model simulation [69], and in 2010, he applied AT-InSAR to TerraSAR-X. The data processing results verified the effectiveness of the along-track interferometry method in current field measurement and inversion [70]. In February 2000, the SRTM, which was mainly used for the precise mapping of global terrain, began its mission. The aircraft placed the main antenna inside the cabin and the secondary antenna outside, forming a 60 m long cross-track interferometry baseline and a 7 m long in-track interferometry baseline [61]. [PERSON] and [PERSON] conducted numerical simulation studies on the imaging mechanism of AT-InSAR for ocean surface currents, proving the feasibility and limitations of AT-InSAR in measuring the radial velocity of ocean surface currents; that is, the along-track interferometry phase is not simply proportional to the radial velocity of ocean surface currents in the line-of-sight direction. It also includes phase deviations caused by Bragg waves, ocean waves, and sea surface wind fields [69]. [PERSON] also proposed a composite ocean surface model for calculating the normalized radar backscattering cross-section (NRCS) of the ocean surface under medium incidence angles. This model incorporates the analysis of sea surface wind fields and Bragg waves [71; 72]. Based on this, [PERSON] et al. demonstrated the feasibility of airborne AT-InSAR for ocean surface current measurement using SRTM interferometric data from the Waddenzee region in the Netherlands and KUSTWAD current field data [73]. They further demonstrated its application potential with SRTM interferometric data from the Orkney Islands in northern Scotland, launching the DLR's AT-InSAR mission (TerraSAR-X) [74]. They comprehensively discussed the theoretical ocean surface current measurement capabilities of TerraSAR-X AT-InSAR mode, and the results proved that the ocean surface current measurement capability of TerraSAR-X AT-InSAR mode was at least comparable to that of SRTM [75]. At the European Conference on Synthetic Aperture Radar (EUSAR) in 2022, [PERSON] et al. presented an analysis report on the Radarsat-2's MODEX-1 ATI mode and Doppler centroid anomaly (DCA) for estimating the radial velocity of ocean surface currents. The paper processed the Agulhas current data obtained by Radarsat-2 MODEX-1 in 2021, and the processing results are shown in the following figure. Under the condition of approximately equal accuracy, the spatial resolution of ATI is ten times higher than that of DCA [76]. To obtain the two-dimensional ocean surface current, two sets of mutually perpendicular radial velocity components of the ocean surface current need to be simultaneously acquired. In 1994, [PERSON] et al. [77] first mentioned the extended method of ATI-SAR for estimating ocean surface current, and in 2001, [PERSON] and [PERSON] named it dual-beam interferometer (DBI). It can achieve the acquisition of two-dimensional ocean surface current with a single pass [78]. The observation geometry of the DBI method is shown in Figure 12. In the coherent radar, a pair of dual-beam antennas are adopted, and each antenna generates a forward and a backward beam. Due to the forward motion of the flight platform, the echo frequency of the forward beam increases, and the echo frequency of the backward beam decreases. The time required for the spatially registered backward antenna echo and forward antenna echo to move between each other is determined according to the traverse antenna baselinedistance B.