Nonstationary natural media analysis from polarimetric SAR data using a two-dimensional time-frequency decomposition approach

In synthetic aperture radar (SAR) polarimetry, it is generally assumed that the sensor has a fixed orientation with respect to objects and illuminates a scene with monochromatic radiations. Modern high-resolution SAR sensors have a wide azimuth beam width, however, and a large bandwidth in range. During SAR image formation, multiple squint angles and radar wavelengths are integrated to synthesize the full-resolution SAR image, and variations in the polarimetric signatures due to changes in the azimuthal look angle and in the wavelength commonly remain unconsidered. In this paper, a fully polarimetric two-dimensional (2D) time–frequency analysis method is introduced to decompose processed polarimetric SAR images into range–frequency and azimuth–frequency domains. This 2D representation permits characterization of the frequency response of the scene reflectivity, observed under different azimuth look angles and wavelengths. For the case of Bragg resonance in agricultural areas, the influence of anisotropic scattering and frequency selectivity on the polarimetric descriptors is pointed out in detail and compared with theoretical predictions from a quasi-periodic surface model. Lastly, a statistical analysis of polarimetric parameters is presented, which permits clear discrimination of media showing nonstationary behavior during the SAR integration.