A coherent electromagnetic model developed for estimating the radar backscatter from rice crops is presented. This model is based on the first order solution of the polarimetric backscatter response as a function of the sensor parameters and the physical description of the rice plants. This paper presents a comparison
between simulations obtained by the model and experimental data collected at the European Microwave Signature Laboratory (EMSL). This laboratory is currently carrying out several campaigns of measurements over different growing stages of a rice crop sample. For the first stage of growing, simulated and measured backscattering coefficients, at HH, HV and VV channels, show a reasonable agreement. The sensitivity and dependence of the radar signal with respect to the input parameters (incidence angle, frequency and morphological characteristics of the rice plants) has been studied in order to achieve a future parameter inversion. The final aim of this work is to establish a reliable inverse algorithm to retrieve biophysical parameters of rice crops from radar measurements.
Some recently developed algorithms known as Non-Uniform FFT's (NUFFT), which enable the computation of efficient FFT's with unequally spaced data in the time or frequency domain, have been applied to SAR imaging in this study. The main objective has been to analyze the potential improvement of the computational efficiency and/or image accuracy of seismic migration SAR processing techniques, like the ω-k algorithm. Our approach consists in substituting both the Stolt interpolation and the final range inverse FFT by a single NUFFT. Numerical simulations illustrate the performance of the new method and the influence of the selection of NUFFT parameters in the precision and computation time of the SAR imaging algorithm. The new method is especially suited for near-field wide-band configurations, such as inverse SAR (ISAR) and ground-based systems, where a very precise imaging algorithm is required.
This paper presents a new near-field 3-D synthetic aperture radar (SAR) imaging algorithm. This algorithm is an extension of the 2-D chirp scaling algorithm (CSA). First, the original formulation of the CSA has been extended to the 3-D case. Then, some processing steps have been reformulated in order to incorporate additional terms (up to n-th order) in the approximations assumed by the algorithm. These extra terms are required to maintain the accuracy of the method in the so-called near-field conditions (large coherent integration angle and/or high bandwidth-to-center-frequency ratio), but do not entail a significant increase of the computation time. The algorithm has been also optimized for stepped-frequency radars. The performance of the method has been illustrated with numerical simulations and experiments.