High resolution ka-band backscattering measurement of deciduous and coniferous tree

In this paper, we present Ka-band backscattering measurement of deciduous and coniferous pine trees on National Central University (NCU) campus. The coherent radar system, designed and configured based on a vector network analyzer N5245A PNA-X (10 MHz-50 GHz) is capable of measuring the return power from 26.5 GHz to 50GHz for VV, HH, HV polarizations at incidence angles between 30 and 80 degrees. In measuring process, the radar system has typical trace of the frequency ranges from 28 to 38 GHz of the received power for a given footprint of tree canopy. For this high resolution, the wind-induced Doppler effects can be ignored. Undesired returns from outside the antenna beam volume are conveniently rejected by gating in time domain. By this feature, the propagating depth through the tree can also be precisely determined so that the effective propagation constant and attenuation factor can be estimated. A reference measurement was taken at normal incidence using a conducting sphere as a calibration target. The calibration was conducted both in anechoic chamber and in-situ measurements. Ensemble average of 2000 samples was taken to estimate the mean return power and later by inverting the radar equation to obtain the backscattering coefficient as functions of frequencies and incident angles are then estimated. To simplify the matrix inversion, a narrow beam is realized with dish antenna. Based on the measured results, backscattering characteristics and statistical properties were analyzed for the cases of deciduous (banyan) and coniferous (pine) trees.