Review of synthetic aperture radar frequency, polarization, and incidence angle data for mapping the inundated regions

Abstract

The use and importance of synthetic aperture radar (SAR) data for flood area mapping studies have been proved beyond the doubts as SAR signals are able to penetrate the thick formation of clouds and are able to receive the reflected signals of surface objects even during extreme weather conditions. At the same time, the accuracies of an SAR image-based flood area mapping model has a direct relationship with the frequency of the source SAR signal, the polarization mode that has been used, and the incidence angle by which the disaster region has been sensed. In addition to this, while evolving SAR image-based flood area mapping models, it is a must to understand the response of the inundated regions of the different geographical regions, as well as the response of the same geographical region during different climatic and seasonal periods as the same object, produces different signatures in varying situations. As of this date, there is no single article that can synchronize such information, which is widely disseminated across various research publications. This article mainly focuses on gathering and reviewing such vital information as well as bringing out the details about the physical basis of the interaction of the radar signal with various water surfaces of different geographical conditions, the response of inundated regions under different frequency, polarization, and incidence angle. Such information is mainly used to understand the difficulties that arise when mapping the inundated regions using the SAR image. In the end, the significant observations of the literature reviews are highlighted, which is very useful for young researchers who are interested in building flood area application models using different sets of SAR data.

Mapping Floods in Lowland Forest Using Sentinel-1 and Sentinel-2 Data and an Object-Based Approach

“…Namely, the same observation object in different geographical locations, different periods, or different climate and seasonal variations does not have the same image. Moreover, the signal's backscattering and the measuring quality of the imaged object also change with the change in the SAR sensor parameters: polarization, wavelength, and local angle of incidence [26,38]. Furthermore, assuming the constancy of SAR sensor parameters, which is true for SAR systems with a repeated trajectory (orbit) [19], the intensity of reflected SAR signals is a function of heterogeneity (roughness) of the measured surface and conductivity and dielectricity of the Earth's surface [32].…”

“…The characteristics of the observed object and sensor parameters, namely, the wavelengths, frequency, polarization of the transmitted and received signal, incidence angle, and direction of observation, determine the strength of the feedback signal (scattering) [38]; therefore, they must be taken into account when interpreting and analyzing the SAR data [7,19]. Researchers [7,30,[40][41][42][43] have described the interaction of SAR impulses with forest and forestland.…”

“…In this regard, the authors of [41] describe the interaction of the forest and the L band, which consists of the following: (i) diffuse scattering from the forest floor, (ii) volume scattering from the forest canopy, (iii) scattering from the canopy signal to the soil and trunks, and (iv) specular reflection from the forest soil and trunks (Figure 1). The characteristics of the observed object and sensor parameters, namely, the wavelengths, frequency, polarization of the transmitted and received signal, incidence angle, and direction of observation, determine the strength of the feedback signal (scattering) [38]; therefore, they must be taken into account when interpreting and analyzing the SAR data [7,19]. Researchers [7,30,[40][41][42][43] have described the interaction of SAR impulses with forest and forestland.…”

“…In general, longer wavelengths have a greater capacity for the SAR signal to penetrate vegetation (forest) canopy. The authors of [4,7,13,32,37,38,44] indicate that composite feedback SAR signal (Figure 1) increases in the flooded forest compared to non-flood conditions. The basic explanation of the increased (amplified) feedback signal of flooded forests is the double-bounce reflection (rectangular or full).…”

“…By contrast, a shallow incidence angle increases interaction with the forest canopy and consequently increases volume reflection [48,50]. Thus, the impact of the incidence angle on SAR data quality is significant [38].…”