Wavelength-Resolution SAR Speckle Model

This study investigates both fully and nonfully developed speckle models for wavelength-resolution synthetic aperture radar (SAR). The resolution of the wavelength-resolution SAR systems is on the order of wavelengths of the radar signal. Therefore, the equivalent number of independent scatterers per resolution cell is so small that the speckle is not fully developed. Based on the central limit theorem, the relationship between the number of scatterers and the nonfully developed speckle is first clarified in detail by using a characteristic function and moment generating function. Considering the surface scattering and volume scattering for the radar observations, both fully and nonfully developed speckles are demonstrated in wavelength-resolution SAR images by testing three probability density functions (pdfs), namely, K-distribution, Rayleigh, log-normal, and Weibull distributions. The use of airborne coherent all radio band sensing II (CARABAS-II) very high frequency (VHF) (20–90 MHz) SAR data for the nonfully developed speckle has been investigated. This study discovers that the speckle is not fully developed when the equivalent number of independent scatterers is less than eight. Wavelength-resolution SAR data exhibit nonfully developed speckles on regions of interest in heterogeneous surface scattering and volume scattering. In contrast, for homogeneous surfaces with specular scattering on lakes and some roads, the data follow a Rayleigh distribution due to systematic background thermal noise and very little reflected received signal. In this case, it corresponds to the absence of effective independent scatterers within the resolution cell.