## 2 Remote Sensing Technologies for the Monitoring of Coastal and River Delta Regions
### Sea Surface Monitoring through SAR-Based Approaches
Surface current velocities can be estimated using two different SAR data processing techniques: the Along-Track Interferometry (ATI) [271] and Doppler Centroid Anomaly (DCA) method [120]. Both methods only measure the radial component of the sea surface velocity, i.e., the radar-to-target line-of-sight (LOS) component. However, it is worth noting that the radar Doppler frequency results from several factors, e.g., ocean currents, windfields, ocean waves, and wave-current and wave-wave interactions [120; 269; 270], and extra work is required to separate the various mechanisms. For ocean current estimation, it is necessary to integrate data acquired from SAR with wind models, ocean models, and real data obtained from in situ instruments (when available), such as buoys, to get a quantitative and precise description of the current sea state [276; 277]. Currently, only a limited number of operational services exist; thus, research in this field is substantial. Consequently, many efforts are still required to foster SAR data for coastal applications, considering the potential social and economic returns: regional coastal protection, ship traffic monitoring, electrical energy generation from the sea, etc. Additionally, the results obtained in this framework could also be helpful for the analysis and characterization of meteorological phenomena. ATI requires that the SAR images be achieved from along-track displaced antenna phase centers [278]. The ATI technique typically operates on a SAR data pair, which allows for the combination of two (or more) SAR data points collected over an identical area; data are collected from different orbital positions but separated by a physical baseline along the azimuth direction. The estimated phase differences are proportional to the Doppler shifts of the backscattered radar echoes [279]. More specifically, when two SAR antennas are dislocated along the radar flight track (azimuth direction), the radar signal related to the identical target is collected by the different antennas within a short time lag. Accordingly, the targets' movements translate to phase shifts on the interferometric phase difference [268; 280]. In the framework of marine observation, examples of fast-varying phenomena are sea current surface velocity and vehicle fluxes in marine environments; hence, their monitoring necessitates a technique, such as ATI, able to measure changes in the observed scene over a short time scale [280; 281]. This technique has undergone significant development in recent years. Still, it requires unique sensor characteristics: namely, the possibility of having two antennas spaced along the azimuth direction, which can achieve images with a short time lag. For this reason, this technique is often successful in using SAR sensors mounted on airborne platforms. The DCA technique, unlike ATI, exploits a single SAR image and relies on measuring the Doppler shifts induced on the SAR images by the LOS component due to the sea surface velocity [120]. The DCA is obtained by subtracting from the Doppler Centroid the term corresponding to a \"stationary\" scene, e.g., the scene where all scatterers move at the same velocity associated with the Earth's rotation [275]. Depending on the radar wavelength, the sea surface velocity can be obtained from DCA using a given factor. Furthermore, in the literature, some studies have used DCA to analyze the sea state by considering surface currents and wind influence [273; 274; 275; 276]. The DCA has the potential to obtain information on the surface current with a single SAR image, acquired with any sensor and platform. Therefore, the use of this technique is very flexible. Sea state is generally defined for the ocean where statistical characteristics of the wave field do not vary appreciably [262]; therefore, ocean areas typically represent the starting point. Hence, several SAR technology applications consider case-study areas in oceans or coastal regions [120; 263; 270].