Simulation of Radar Backscattering Coefficients Using IEM - A Tool for Surface Soil Moisture Retrieval

The present paper attempts to simulate the radar backscattering coefficients of bare surface at steep and shallow incidence angle applying the integral equation model (IEM). Using the simplified form of IEM with the known input parameters of incidence angle, dielectric constant of the surface soil and corresponding surface roughness parameters; the backscattering coefficients have been simulated. This model has been implemented on the bare and sparsely vegetated fields of alluvial plains of India. The two selected test sites are located at Batala (Punjab Plain) and Diamond Harbor (Kolkata, West Bengal). Synchronized with the RADARSAT 1 satellite overpass, the field campaigns have been carried out during September 2003 for Batala test site and two campaigns for Diamond Harbor test site in February and December 2005. Here, the terrain surface is assumed to be exponential in nature and the associated autocorrelation function of the surface has been computed. As both the study sites are characterized by gently sloping terrain, only the single scattering term of the model has been considered for the estimation of the backscattering coefficients. The modelled backscatter values for HH polarization is validated using RADARSAT 1 SAR (Synthetic Aperture Radar) data in order to evaluate the performance of the simulation. The sensitivity experiments are performed considering the near, mid and far range of the incidence angle. Initially, the experiments are carried out for Batala test site using both steep (Extended Low) and shallow (Standard 5) incidence angle beam mode data. An rms error of 1.79 dB and 2.96 dB is achieved while comparing the simulated and observed backscattering coefficients for the extended low and standard beam modes respectively. The computation of the index of agreement is also revealed that the indices are 0.80 and 0.66 respectively for steep and shallow incidence angle. Hence, only the steep incidence angle beam mode data has been used subsequently for - the model evaluation of Diamond Harbor coastal alluvial test site where the rms error is observed to be 1.98 dB with 0.79 index of agreement for the entire sampling dataset. The present study has utilized 102 point observation samples in the entire analysis. The overall results have demonstrated that about 76% of the simulated backscatter data are within the radiometric resolution of RADARSAT 1. It is observed that the model result provides consistently good agreement with the measured backscattering data which indicates a scope to retrieve the surface soil moisture using SAR observations.