# Space-Based Displacement Monitoring of Coastal Urban Areas: The Case of Limassol's Coastal Front
## 2 Materials and Methods
### Data Processing
#### 2.2.2 SNAP-STAMPS PSI Processing  
The PSI processing was divided into two independent workflows. The first workflow concerned the preprocessing of the images and the single-master DInSAR processing using ESA SNAP and'snap2 stamps', and the second workflow comprised PSI processing using StaMPS/MTI. Initially, the image acquired on 17 July 2019 was selected and used as the'master' image in the preprocessing procedure. The master image minimizes the sum decorrelation of all interferograms by eliminating the perpendicular baseline values and the Doppler phenomenon among all images and concordantly maximizing the expected coherence in the interferometric stack [49]. This results in the geometric and interferometric coregistration of the dataset with respect to a common frame (master image). In each of the subswatths in the preprocessing workflow, three bursts were separated in the azimuth direction. The orbital information demonstrated accurate satellite position and velocity information following the SNAP image processing. Apart from that, the geocoding of all images was applied, using the SRTM-1m [50]. Equally important was the enhancement of the spectral diversity, as well as the extraction of the topo-phase and the generation of the interferograms for each date. Subsequently, after the phase noise estimation (SNR) and the phase unwrapping, a mean velocity map of all images was computed, as shown in Figure 3.  
An important omission was the conversation of phase displacements of LoS to vertical displacements. Specifically, according to [PERSON] et al. [51], the conversion from the unwrapped phase to displacements are in LoS and are relative between the images. In order to calculate the vertical displacement, a formula was applied to SNAP using the band maths module including the incidence angle as presented in equation 1. Overall, as was expected, lower estimated mean velocity values were found all across the coastal line, along which the buildings are located. \[Vertical\ Displacement=\frac{(unwrapped\ phase\text{-}wavelength)}{[-4\pi\cdot \cos(incidence\ angle)]} \tag{1}\] \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Date** & **2017** & **2018** & **2019** & **2020** & **2021** \\ \hline January & - & 17 & 18 & 13 & 13 \\ \hline February & - & 16 & 17 & 18 & - \\ \hline March & - & 18 & 19 & 19 & 14 \\ \hline April & 16 & 17 & 12 & 18 & 13 \\ \hline May & 16 & 17 & 18 & 18 & 13 \\ \hline June & 15 & 16 & 17 & 17 & - \\ \hline July & - & 16 & **17**1  & 17 & - \\ \hline August & 20 & 15 & 16 & 16 & - \\ \hline September & 19 & 14 & 15 & 15 & - \\ \hline October & 19 & 20 & 15 & 15 & - \\ \hline November & 18 & 19 & 14 & 14 & - \\ \hline December & 18 & 19 & 14 & 14 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Calendar of Sentinel-1 images that were used for PSI. After completing the procedure, the Persistent Scatterers (PS) were created. Specifically, the number of the final persistent scatterers was computed in three main stages. Firstly, all the candidate scatterers were identified in the study area. Persistent scatterers are pixels or points characterized by stability over time of the backscatter electromagnetic signal. Following this step, extracting the possible scatterers with a minimum value of coherence threshold equal to 0.80 was performed. Due to the density of the buildings and the small area, the coherence value should at the maximum extent possible to avoid the matching of the same PSs to more than one buildings [50]. Last but not least, the final scatterers that corresponded to tall buildings or skyscrapers in every pixel were isolated. The number of persistent scatterers is affected by various factors, including the size of the building, the material, and structures of the roof, the orientation of the building relative to radar look direction, and nearby vegetation [52]. Figure 4a represents an example of a distribution and the number of PSs within a buffer zone over a specific chosen point.