## 4 Advancements in Remote Sensing Technologies for Coastal Monitoring
### UAV Technology for Precise Coastal Erosion Monitoring  
Modern developments in remote sensing technologies have made shorelines observable by satellite-based observations and Unmanned Aerial Vehicles (UAVs). UAVs provide an ultra-high spatial resolution at a centimeter scale, which is well-suited for collecting small-scale features such as beach cusps, berms, and micro-topography [23]. UAVs equipped with LiDAR or RTK-GPS photogrammetry have demonstrated shoreline mapping precision within 0.1 m [88]. In contrast, satellite imagery, such as Sentinel-2 (10 m), Landsat (30 m), or even PlanetScope (3 m), covers a wider area at a lower resolution, with positional errors generally ranging from 5 to 10 m in favorable situations according to their resolution [54; 89]. Satellites generalize the shoreline edge through pixel averaging, whereas UAVs are able to capture small changes, which are useful for local investigations like post-storm beach erosion or dune recession. The temporal consistency of monitoring is also what separates the two methods. Satellites are on repeatable revisit intervals--5 days for Sentinel-2 and 16 days for Landsat--due to which they are particularly useful in building long-term shoreline change records over a span of decades [90]. In comparison to satellite images, UAVs provide centimeter-level spatial resolution, with much finer orders of magnitude compared to Sentinel-2 (10 m) or Landsat (30 m). UAVs are well-suited for identifying fine-scale changes in shorelines like dune crest movement, micro-erosion, or loss of plant cover. Nevertheless, in comparison to the larger spatial extent and less weather dependency, their restricted spatial scope makes them better for local-scale assessments. When used together, satellite data provide an extensive temporal and spatial scope, whilst UAVs provide detailed snapshots for in situ validation and calibration. Previous studies have illustrated this multi-resolution complementarity well, like that of [PERSON] et al. [44] in Ghana where, in addition to satellite observation of a rapidly eroding beach, UAV photogrammetry was used [23]. The work of [PERSON] et al. [44] focused on the application of UAVs in coastal erosion monitoring in connection with other techniques, such as satellite imagery and VCS (video camera systems), using the case of Elmina Bay in Ghana. For this purpose, UAVs were deployed to ensure high-resolution month-to-month beach monitoring, offering detailed information about erosion and deposition along the shoreline. The results showed that UAVs could observe localized event-driven shoreline changes more accurately and frequently than satellite imagery, which does not have the necessary temporal resolution to capture such changes effectively. Complemented by a VCS, UAV data correspond reliably [91] as a value for near real-time monitoring, indicating serious erosion was happening in extensive parts of the beach not covered in the front by jetties. UAV imagery helped emphasize how complicated the engineering solutions of sea defenses were in affecting sediment distribution and, therefore, contributing to downdrift erosion [44]. The conclusion of this study underlined the effectiveness of drones in operation with other remote-sensing platforms for coastal management; thus, UAVs have become very critical for high-resolution monitoring, particularly for event-based changes that satellite data cannot capture. While fixed satellite scheduling and cloud cover restrict satellite use during peak storm or tidal windows or generally rapidly occurring events, UAVs are able to overcome these by enabling on-demand deployment and exact timing control (i.e., flying at low tide or shortly after the event occurred). Although UAV campaigns are logistically complex and spatially limited--numerically only a few kilometers per flight--truly timely, focused information is provided. An example is the research by [PERSON] et al.