Propagation modeling for VHF/UHF-radar detection of targets behind a forest

The detection of targets below line-of-sight has been one of the driving subjects in the research, related to the VHF-experimental radar LARISSA (Large wavelength Radar with Interference dependent Spectral Signal Adaptation), developed and operated at FGAN-FHP. A realistic prediction of the attenuation of electromagnetic waves caused by propagation through and above forest areas plays an important role for the configuration and performance evaluation of a VHF radar dedicated to detect hovering helicopters beyond terrain obstacles. In order to evaluate the transmission attenuation above a forest, measurements have been performed on a selected forest area near FGAN. The measurement results have been compared to a propagation model, which regards the forest as a sequence of ideally conducting diffraction screens. This study was conducted to develop, a generalized model for radio wave propagation above forests under the constraint of easy applicability to a coverage prediction model based on a digital terrain database. The measurements and calculations were restricted to horizontal polarization, as being of major importance for the detection of helicopter rotor echoes.