## 2 Synthetic Aperture Radar  
A satellite radar instrument produces and transmits its own energy using a known microwave signal pattern, then records the reception of that signal reflected back after interacting with the earth's surface. When this instrument moves with a known velocity relative to the earth's surface, the reflection also adds azimuthal information due to the Doppler frequency deviation and is referred to as SAR data collection. SAR data must be interpreted differently from optical images because the signal responds to surface characteristics such as structure and wetness rather than being a static image. Compared to optical technology, SAR technologies can \"see\" through the darkness and can operate at any time of the day. Moreover, for longer microwave bands such as the L-band, the SAR instrument can also see through clouds, fog, and rain. This robust operation allows for tracking the trends in habitat, water, and moisture levels, the consequences of natural or human disturbance and variations in the earth's surface as a result of quakes or sinkhole openings. These products are created by analyzing the reflections of signals off a target location and measuring the two-way transit time back to the satellite, its frequency deviation, and the polarization changes. The SAR interferometry technique \"interferes\" (differences) with two SAR images of the same area, producing maps called interferograms that reveal ground-surface displacement (range change) here between the two time periods. The phase differences are used to extract information about the captured objects (in comparison to a single image). As a result, at least one aspect (\"Baseline\") must differ between the images. Future SAR missions will benefit from increased capability, reliability, and flexibility as a result of this spatial separation [28]. Applications for multistatic SAR systems include single-pass cross-track and along-track interferometry, spaceborne tomography, wide-swath imaging, resolution augmentation, ground-moving target acquisition, interference suppression, and multistatic SAR imaging. Simultaneous data collection from numerous satellites reduces temporal and atmospheric disruptions, enhances performance, and allows the identification of rapid changes. Figure 3: Chief-deputy classification (**a**) leader-follower, (**b**) pendulum, (**c**) cartwheel, (**d**) helix configuration, (**e**) same-ground track. AT-InSAR systems are employed to estimate the radial velocity of targets moving on the ground by combining the interferometric phases, which are acquired by combining the two intricate SAR images obtained by two antennas spatially separated along the platform moving direction [29]. The AT-InSAR can be used in various applications such as monitoring real-time traffic management, ocean currents, coastal surveillance, ice drift, etc. In AT-InSAR, the baseline difference is an along-track distance, with a magnitude depending on the mission type, and determines the time difference associated with the pass of the satellite over the particular target, hence measured in seconds for single pass interferometry (two consecutive satellites looking at the same target) or days/years for multiple-pass interferometry (i.e., to process a stack of images taken on different passes over the same scene by the same satellite, other different satellites in a SAR constellation). Figure 4 shows different types of SAR in a simplified classification. The two main branches, interferometric and polarimetric, can also be combined as in the polarimetric SAR interferometry (POLInSAR) techniques [30]. The interferometric SAR missions can be implemented by multi-static SAR, which is characterized by their relative position or equivalent time, known as the baseline. This subfield of SAR categorization will serve as the primary focus of the analysis. Some baseline types usually implemented on SAR interferometric missions are shown in Table 1, and the baseline position difference is shown in Figure 5 for Along-Track interferometry.