Estimation of Canopy Attenuation at L-band by a Time Domain Analysis of Radar Backscatter Response

A new technique for determining the canopy attenuation that uses the measured stepped frequency radar backscatter response is proposed. It makes use of the details found in a transient solution where the canopy (volume scattering) and the tree-ground (double interaction) effects appear at different times. A coherent scattering model, based on a Monte Carlo simulation, is developed to calculate transient response from distributed scatterers over a flat surface such as a forested terrain. The influence that the different backscattering contributions have on the expected time-domain response of a forest stand is then described through numerical simulations. The proposed technique is based on separating the backscattering sources within a forest canopy in the time response. The frequency correlation functions (FCF) of the separated contributions are generated for difference frequencies in the bandpass of the radar system. The ratios of volume scattering to double interaction return for an array of difference frequencies over the system bandpass are computed to eliminate system characteristics such as antenna gain. It is shown that these ratios provide a system of equations depending only on the canopy thickness, the canopy attenuation and a combined parameter involving the forest scattering coefficients and the ground reflectance. A least squares method is used to solve for the attenuation and the combined parameter assuming the canopy thickness is known a priori. The technique has been used with L band data collected over deciduous trees to verify that the algorithm results match the simulated dat