High resolution Ka-band images of a small tree: measurements and models

Ka-band (27 GHz to 36 GHz) radar images of a 2.5-m red maple tree, measured in the Ohio State University ElectroScience Laboratory (ESL), Columbus, compact range, are presented. Radar images of the tree with and without leaves were obtained, as well as images of a 6 in diameter metal sphere obscured by the tree for investigation of attenuation effects. The large bandwidth (9 GHz) of these measurements provides a cm scale range resolution, so that backscatter from individual components of the tree canopy can be separated. In addition, measurements made as a function of azimuth angle in 0.1 degree steps allow inverse synthetic aperture radar (ISAR) images to be obtained with cm scale resolutions in down and cross range. A single scattering model, in which tree branches are represented as a deterministic collection of cylinders and leaves as a distributed collection of curved dielectric sheets, is combined with a detailed measurement of the tree geometry to provide theoretical ISAR images for comparison with the measured data. Although incomplete knowledge of tree geometry limits a direct quantitative comparison of the measured and modeled returns, a qualitative comparison of images shows that single scattering models may be sufficient for obtaining many features of foliage objects in Ka-band radar images