# ERS-1/2 and Sentinel-1 SAR Data Mining for Flood Hazard and Risk Assessment in Lima, Peru
## 2 Materials and Methods
### Synthetic Aperture Radar (SAR) Data Mining
#### 2.2.1 Selection of SAR Data  
As mentioned in Section 2.1, SAR data were chosen for their independence from weather conditions and owing to their availability with a certain continuity with respect to the dates of occurrence of the flood events under study (see Table 1). Figure 4 shows the timeline of El Ninfo events vs. the whole archive of SAR data from the satellite missions ERS and ENVIronmental SATellite (ENVISAT) of the ESA, and the Copernicus Sentinel-1 constellation. These data are free and their access is unrestricted for registered users, through the open-source Earth Observation Link-ESA (EOLi-SA) for ERS-1/2 and ENVISAT (now via the ESA Online Dissemination tool), and the Copernicus Open Access Hub for Sentinel-1. As can be seen in Figure 4, of the most intense El Ninfo-related events, those occurred in 1982-1983 and 1987 could not be investigated since there were no SAR data available to cover those years. Furthermore, given that no ENVISAT data covered any of the other two El Ninfo-related events, these data were not used in the end. Table 2 shows the main features of ERS-1/2 and Sentinel-1 data used for this research. The two ERS satellites, ERS-1 and ERS-2, were launched into the same orbit in 1991 and 1995 and ceased to collect data on 10/03/2000 and 05/09/2011, respectively [61]. Sentinel-1 is the SAR constellation made of the two twin satellites, Sentinel-1A (since 03/04/2014) and Sentinel-1B (since 25/04/2016), of the European Commission's Copernicus programme, aiming to provide C-Band SAR data continuity following the retirement of ERS-2 and the end of the ENVISAT mission. Of the four acquisition modes of the Sentinel-1 SAR sensor, in this study we used the Interferometric Wide swath (IW) providing 5 m \(\times\) 20 m (ground range by azimuth) resolution, over a 250 km swath [62]. Compared to the nominal satellite mission parameters reported in Table 2, it is worth noting that the number of ERS-1/2 images available over Lima and covering the entire Rimac River basin \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Satellite** & \begin{tabular}{c} **Launch and End** \\ **of Operations** \\ \end{tabular} & \begin{tabular}{c} **Product** \\ **Level** \\ \end{tabular} & \begin{tabular}{c} **Revisit Time** \\ **(Days)** \\ \end{tabular} & **Polarization** & **Mode** & **Band** & \
\begin{tabular}{c} **Swath** \\ **(km)** \\ \end{tabular} &
\begin{tabular}{c} **Ground Range** \\ **Resolution (m)** \\ \end{tabular} \\ \hline ERS-1/2 & 1991/1995 & Level 1 & 35 & VV & IM & C & 100 & 25 \\  & 2000/2011 & SLC & & & & & & \\ Sentinel-1B & 2016 & Level 1 & 12 & VV+VH & IW & C & 250 & 5 (single look) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Key technical features of the SAR data used in this study. Notation: IM—Image Mode; IW—Interferometric Wide; SLC—Single Look Complex; GRD—Ground Range Detected; V—Vertical; H—Horizontal. Figure 4: Flood events and El Niño phenomenon vs. availability of C-band synthetic aperture radar (SAR) satellite data from the European Remote-Sensing (ERS-1/2), ENVIronmental SATellite (ENVISAT) and Copernicus Sentinel-1 missions. The most intense events are indicated in blue. was less than expected, with revisit times sometimes higher than 35 days (i.e., 70 days at worst). On the other side, the ERS-1/2 mission was the only SAR data resource to investigate the 1997-1998 El Nino event. By contrast, the revisit time of Sentinel-1 data was almost regular, equal to 12 days as per the observation scenario over Peru in 2017-2018 [63]. Since the end of 2016, indeed, the programmed Sentinel-1 acquisitions over Peru account for a 12 days revisit frequency provided by the Sentinel-1B satellite only, with IW mode dual polarization data (Table 2). In total, 39 images (13 ERS-1/2 and 26 Sentinel-1) were selected to investigate the flood events listed in Table 1, and were categorized into working pairs according to the following three groups: \"Pre-event\", \"Cross-event\" and \"Post-event\" (Table 3).