# Space-Based Displacement Monitoring of Coastal Urban Areas: The Case of Limassol's Coastal Front
## 1 Introduction  
The notable urban and infrastructure growth of the last five years in Limassol, Cyprus, highlighted the necessity to carry out a detailed investigation on potential land subsidence in the coastal zone. Following the crisis events of 2013 [1], the Cyprus Government promoted incentives for land development to revive the Cyprus economy. Consequently, Limassol became the fastest-growing city in Cyprus in construction, with skyscrapers and tall buildings, built one after the other along the coastal front, on a stretch of almost 20 km. This massive development attracted foreign investments, generating increased concern that a combination of factors, such as overexploitation of groundwater, the structures' load, the sea level rise driven by global climate change, and the intense earthquake activity, may holistically trigger land subsidence phenomena with severe impacts [2]. Land subsidence can be defined as the differential sinking of the ground surface with respect to the surrounding terrain or sea level. The later poses an imminent threat for the socioeconomic equilibrium of the country, as well as an incremental factor for the risk of possible floods in the specific area [3]. The integrated use of multiple space-borne InSAR (Interferometric Synthetic Aperture Radar) techniques is, undoubtedly, among the most effective and accurate methods to monitor land subsidence [4] and, therefore, assess the impact of urban infrastructures on the coastal zones [5]. Techniques such as Persistent Scatterer Interferometry (PSI) presentedby [PERSON] et.al [6] and Differential Interferometry (D-InSAR) and Small Baseline Subset Interferometry SBAS [7] became indispensable parts in ground deformation monitoring analysis of urban areas as they may provide cm- to mm-level-accuracy products [8]. Investigating the land subsidence in an urban environment using D-InSAR may be inadequate due to the lack of coherence in multitemporal images in cities, caused by temporal-geometrical decorrelations [9; 10] and atmospheric inhomogeneities [11]. Thus, the use of PSI could be characterized as inevitable. The PSI technique detects and measures specific points (buildings, stable rocks, roads, etc.) on the surface of the Earth that are phase-coherent and stable over a period of time [12; 13]. The displacements are only measured in the direction of the axis connecting the target and sensor (LoS); this axis has a different orientation in space for ascending and descending orbits [14]. Only descending orbits are considered in the current research, since the ascending ones were affected by various illumination effects arising from SAR images' side-looking scene illumination. Specifically, in densely built-up areas with tall buildings, large portions of the data can be interfered with by the shadowing, layover, and foreshortening effects due to the slope orientation, geometry, and height of the buildings in an urban environment, as well as the double-bouncing phenomenon of the radar signal [15]. A plethora of studies has proved the ability of land subsidence monitoring in coastal urban areas in many nations worldwide, combining different space-based and ground-based techniques, such as PSI and SBAS and GNSS, using various satellite constellations and leveling techniques. As shown in Table 1, Sentinel-1, provided by the ESA satellite mission, has been most usable technology in the past few years, as long as the PSI technique is the most applicable for quantifying land subsidence in urban cities. Specifically, according to the geology of cities, almost all locations are faced with land subsidence worldwide, consisting of alluvial soil, and the subsidence rates are ranged between mm and cm/year, depending on their development status and the years of the study. Utilizing technology and analyzing the findings so far on these landslides, it can be relatively concluded that many coastal cities are highly affected and may even be at high risk in the future.