# Penetration Analysis of SAR Signals in the C and L Bands for Wheat, Maize, and Grasslands
## 3 Data Analysis
### Temporal Behavior of the L- and C-Bands According to the SSM and NDVI for Wheat and grasslands  
In this section, the temporal evolution of SAR backscattering in the L and C bands according to soil moisture and NDVI variations is analyzed first. Next, the behavior of the C-band backscattering during one complete growth cycle of wheat and grassland is discussed. Unfortunately, studying the behavior of the L-band backscattering during one complete growth cycle was not possible because the number of images was limited. Figure 4 shows the temporal variation of radar signals in the C and L bands according to the SSM and NDVI for one wheat reference plot (Figure 4a) and one grassland reference plot (Figure 4b). For the wheat plot, Figure 4a shows that between 19/02/2017 and 02/04/2017, the SSM increases by 6.0 vol.%, the NDVI increases from 0.34 to 0.93, the C-VV decreases by 3.2 dB, and the L-HH increases by 3.5 dB. Later, between 02/04/2017 and 09/05/2017 with an NDVI approximately 0.9, the SSM increases by 4 vol.%, the C-VV decreases by 2.1 dB, and the L-HH increases by 0.7 dB. For well-developed vegetation cover (NDVI > 0.7), the decrease in the C-VV as the SSM increases indicates that attenuation caused by the canopy dominates the soil contribution. In the other hand, the increase in the L-HH with the increase in the SSM was expected since the L-HH, as shown in Figure 2, is still sensitive to soil moisture even when the canopy is well-developed (NDVI > 0.7). Therefore, in contrast to the C-VV, the L-HH incident wave penetrates the canopy layer and the L-HH backscattered wave is sensitive to SSM under the well-developed vegetation layer (NDVI > 0.7). Similar results were observed for the grassland plot (Figure 4b). For instance, between 02/04/2017 and 09/05/2017, when the NDVI is higher than 0.7, the SSM increases by 6.5 vol.%, the C-VV decreases by 1.9 dB, and the L-HH increases by 1.1 dB (Figure 4b). The potential of the L-HH and the inability of the C-VV to penetrate the canopy for an NDVI > 0.7 were also demonstrated for all wheat and grassland reference plots. For each reference plot with an NDVI higher than 0.7, the difference in backscattering between two successive dates (\(\Delta\sigma^{0}\)) was computed (the backscattering coefficient of the previous date _t-1_ was subtracted from that of the later date _t_). Then, \(\Delta\sigma^{0}\) was plotted against of the difference of the SSM (\(\Delta\)SSM) (Figure 5). For the L-HH, Figure 5 shows that \(\Delta\sigma^{0}\) and \(\Delta\)SSM are well correlated (R\({}^{2}\) = 0.95). This high correlation indicates that for all reference wheat and grassland plots, the L-HH is sensitive to SSM for well-developed vegetation cover (NDVI > 0.7), and the sensitivity of L-HH to SSM is almost the same for all reference plots. This demonstrates the penetration ability of the L-band into vegetation cover for an NDVI > 0.7. For the C-band and an NDVI higher than 0.7, a \(\Delta\sigma^{0}\) close to 0 is obtained for all \(\Delta\)SSM between \(-\)15 vol.% and 15 vol.% (positive \(\Delta\)SSM-values correspond to an increase of SSM and negative \(\Delta\)SSM correspond to a decrease of SSM between two successive dates). Thus, for all reference plots, the incident C-band  
Figure 4: Temporal variation of the SAR signal in the L and C bands according to in situ SSM and NDVI. (**a**): wheat reference plot, (**b**): grassland reference plot. To display NDVI and SSM values on the same axis the NDVI-values were multiplied by 50 (for instance, an NDVI-value of 0.8 corresponds to a value of 40 in Figure 4a,b). The vertical lines in red represent the rainfall events. wave does not penetrate the vegetation layer for well-developed vegetation cover, or the C-band backscattered wave received by the SAR contains a negligible SSM contribution (impossible to detect an increase or decrease in soil moisture).