Millimeter-wave measurements of foliage attenuation and ground reflectivity of tree stands at nadir incidence

In this paper, the phenomenology of wave propagation through foliage and forest ground reflectivity is investigated for assessing the feasibility of foliage-covered target detection at millimeter-wave frequencies. An experimental procedure for simultaneous measurements of foliage attenuation and ground reflectivity is outlined. This measurement procedure is implemented for two different tree stands, one mostly coniferous and the other deciduous, using a nadir-looking, high-resolution, 35-GHz radar positioned above the tree canopy. Statistics of the two-way attenuation and ground reflectivity for these two well-characterized stands are derived. Strong spatial and angular fluctuations in the two-way foliage attenuation coefficient are observed. The mean, standard deviation, and median of the measured two-way attenuation factor at Stand 1 (mostly coniferous trees with 0.140 trees/m² stocking density and 45.45 Kg/m² green biomass) are -25.4, -18.3, and -48.2 dB, respectively, while the mean, standard deviation, and median of the measured two-way attenuation factor at Stand 2 (deciduous trees with 0.1055 trees/m² stocking density and 30.90 Kg/m² green biomass) are -15.4, -12.7, and -33.6 dB, respectively. The mean attenuation rates of Stand 1 and Stand 2, derived from the measured two-way attenuation factor, are 0.40 and 0.24 Np/m, respectively. Only a small percentage of the data had two-way foliage attenuation values exceeding 70 dB. The mean, standard deviation, and median of the power reflectivity of the forest floor at Stand 1 are -14.2, -11.0, and -21.1 dB, respectively, while for Stand 2, the same statistical measures are -16.0, -14.3, and -22.2 dB, respectively. The results demonstrate the potential for using MMW nadir-looking radars for the detection of targets underneath foliage-cover.