Retrieval of wheat biophysical parameters using hybrid polarimetric SAR decompositions from RISAT-1 data

Accurate estimation of crop biophysical parameters such as biomass and phenological stage is critical for large-scale agricultural monitoring. Synthetic Aperture Radar (SAR), particularly in hybrid polarimetric mode, offers enhanced capability for vegetation characterization under all-weather conditions. This study investigates the retrieval of wheat biophysical parameters using hybrid polarimetric decomposition techniques applied to RISAT-1 C-band SAR data acquired during the 2015–2016 rabi season over Junagadh district, Gujarat, India. Ground-based measurements, including crop biomass and age, were collected in synchronization with satellite overpasses. Stokes parameter-based decompositions, m-δ and m-χ, were implemented alongside Raney decomposition to examine their sensitivity to biomass, crop growth progression, and maturity. Additionally, the Radar Vegetation Index and Alpha Angle were derived from C2 and pseudo-C3 matrices to assess their correlation with ground-truth parameters. Results indicate that the m-δ decomposition provides the most consistent response, with odd-bounce components decreasing and diffuse scattering increasing as biomass and crop age progress. Radar Vegetation Index values ranged from 0.27 to 0.52, while Alpha Angle values indicated a transition from surface to volume scattering with crop development. The diffuse component of the m-δ decomposition showed the strongest correlation with field-measured biomass, outperforming both m-χ and Raney methods. The study confirms the effectiveness of hybrid polarimetric SAR for estimating crop biophysical parameters and presents a transferable decomposition framework for operational crop monitoring. The findings are directly applicable to national and global agricultural monitoring programmes and provide methodological benchmarks for hybrid SAR missions, including RISAT-1A/1B and NISAR.