# Penetration Analysis of SAR Signals in the C and L Bands for Wheat, Maize, and Grasslands
## 2 Study Site and Dataset Description
### Dataset Description  
#### 2.2.1 SAR Data  
A total of 49 SAR images obtained from the Sentinel-1 (S1) constellation operating in the C-band (wavelength \(\sim\)6 cm) were used. S1 images were downloaded from the Copernicus web site ([[https://scihib.copernicus.eu/dhus/#/home](https://scihib.copernicus.eu/dhus/#/home)]([https://scihib.copernicus.eu/dhus/#/home](https://scihib.copernicus.eu/dhus/#/home))) for the period between January 2017 and May 2017. The S1 images were in the Interferometric Wide (IW) swath mode (primary conflict-free mode for land) with VV and VH polarizations, and they were generated from the high-resolution Level-1 Ground Range Detected product with a spatial resolution of 10 m \(\times\) 10 m. For all S1 images, the study site was imaged with an incidence angle (\(\theta\)) of approximately 39\({}^{\circ}\).  
Moreover, 12 SAR images in the L-band (wavelength \(\sim\)24 cm) acquired by the Phased Array Synthetic Aperture Radar (PALSAR-2) onboard the Advanced Land Observing Satellite (ALOS-2) were used. The ALOS-2 SAR images are in mono-, dual-, and quad-polarization (Table 1). ALOS-2 data in mono- and quad-polarization have pixel sizes of 3 m \(\times\) 3 m, whereas ALOS-2 data in dual-polarization  
Figure 1: Location of the study site.  
have a spatial resolution of 6 m \(\times\) 6 m. In this study, the L-band in VV polarization was not used because only two images contained the VV polarization.  
S1 and ALOS-2 images were calibrated using the Sentinel-1 Toolbox (S1 TBX) developed by the European Spatial Agency. The calibration aims to convert digital number values of ALOS-2 and S1 images into backscattering coefficients in linear units. For each reference plot, the mean backscattering coefficient in linear scale was calculated from each calibrated SAR image by averaging the \(\sigma^{\circ}\) values of all pixels within that reference plot.  
The mean of L-band backscattering coefficients acquired at various incidence angles (\(\theta\) between 27.4\({}^{\circ}\) and 40.5\({}^{\circ}\)) were normalized at a given reference incidence angle (\(\theta_{ref}\)). The normalization was done using the following equation [34; 35]:  
\[\sigma^{0}_{ref}=\frac{\sigma^{0}_{\theta}\ cos^{n}\Big{(}\theta_{ref}\Big{)} }{cos^{n}(\theta)} \tag{1}\]  
where \(\sigma^{0}_{\theta}\) is the backscattering coefficient at the \(\theta\) incidence angle, and \(\sigma^{0}_{ref}\) is the backscattering coefficient at the \(\theta_{ref}\) incidence angle. The power index \(n\), which depends on the nature of the target imaged and polarization (for a given radar wavelength), is defined as the slope of a linear fit between \(\sigma^{0}_{\theta}\) in dB and the \(10\log(\cos\theta)\)[34; 36; 37; 38]. In this study, \(\theta_{ref}\) was considered equal to 37\({}^{\circ}\). The normalization of the backscattering coefficient was performed using Equation (1) with \(n\)-values of 3.1 for HH and 2.0 for HV. To determine these n-values, first the slope of the linear fit between \(\sigma^{0}_{\theta}\) in dB and \(10\log(\cos\theta)\) was computed for each target condition (elements of our dataset with close values of SSM, root mean square surface height (\(Hrms\)), and NDVI) and each polarization. Then, for each polarization, all slope values calculated for different target conditions were averaged to determine the \(n\)-value for that polarization [34; 36; 37; 38]. Using SAR data in the C-band, [PERSON] et al. [34] showed from a study in a wetland that the power \(n\) has higher values for HH-polarization (\(n\sim\) 2.5) compared to HV-polarization (\(n\sim\) 1.7). For the L-band in HH-polarization, [PERSON] et al. [38] found that the \(n\)-value varied between 0.2 and 3.4 depending on vegetation type and season with the highest occurring in savanna areas.