*2.2. Data*

In this study, Sentinel-1 level-1 images provided in the Single Look Complex (SLC) format [63] by the Alaska Satellite Facility (https://asf.alaska.edu, accessed on 2 December 2019) were used in DInSAR processing. The Copernicus Sentinel-1 mission, developed by the European Space Agency (ESA), is based on a constellation of two satellites: Sentinel-1A, launched in April 2014, and Sentinel-1B, launched in April 2016. The Sentinel-1 constellation operates at the C-Band frequency (5.405 GHz, 5.5 cm wavelength), with a 12-day repeated acquisition for a single mission, and 6-days in the case of a two-satellite combination. Sentinel-1 imagery was selected in this study, thanks to its free accessibility with a regularly repeated acquisition at a 6-day interval.

A total of 115 (Sentinel-1A/1B) images were acquired on ascending Path 34, Frame 54/55, in Interferometric Wide (IW) swath mode, with dual (VV + VH) polarization between 2 January 2018 and 29 December 2019. Moreover, 108 images were acquired on descending Path 99, Frame 530/532, in IW swath mode [64–66], with dual polarization between 6 January 2018 and 27 December 2019. Only Sentinel-1 VV polarization bands were used, since co-polarized bands provide higher coherence than VH polarization. The main characteristics of the Sentinel-1 SLC data used in this study for DInSAR processing are listed in Table 1.

**Table 1.** Characteristics of the Sentinel-1 SLC data used in this study in DInSAR processing. The nominal spatial resolution is specified for single-look data. N is the number of used SAR images, and I is the number of interferograms for each dataset calculated and analyzed in this study.


The external datasets used for the SAR SLC data interferometric processing include the Shuttle Radar Topographic Mission (SRTM) 1-arc-second (30 m) DEM [67] and Copernicus Sentinel-1A and Sentinel-1B precise orbital files (AUX\_POEORB products), obtained from the Copernicus Open Access Hub.

Satellite data available for mapping and monitoring flood coverage were obtained by passive (e.g., onboard Landsat, Aqua, Sentinel-2, Resourcesat-2) and active (e.g., Sentinel-1, RADARSAT, ENVISAT) sensors [68,69]. Sentinel-1 level-1 ground range detected (GRD) images were obtained from the Alaska Satellite Facility and used to identify flooded areas. Sentinel-1 level-1 (GRD) products consist of focused SAR data that has been detected, analyzed, and projected to the ground range using a ground ellipsoid model. The pixel values of the Sentinel-1 level-1 (GRD) image represent the detected amplitude of the backscattered signal, without phase information. Two pairs of GRD images with dualpolarization from descending orbital pass were acquired, covering the flood events in 2018 and 2019. Only VV polarization bands were used because, for flooded area detection, the co-polarization comparison gives better results than the cross-polarization one, while the

use of VV polarization is recommended over the use of VH data [70]. The characteristics of the Sentinel-1 GRD products used in this study are presented in Table 2.


**Table 2.** Characteristics of the Sentinel-1 GRD data used to identify flooded areas.
