Experimental Characterization of Polarimetric Radar Backscatter Response of Distributed Targets at High Millimeter-Wave Frequencies

Subterahertz frequencies between 100 and 300 GHz remain an untapped portion of the frequency spectrum for many radar- and radiometer-based remote sensing applications. This can be attributed in part to the lack of knowledge of the phenomenology of signal interaction with terrain at these frequencies. This paper examines recently acquired polarimetric radar backscatter data of different types of surfaces using a newly constructed polarimetric instrumentation radar operating at 222 GHz. At this frequency bare surfaces such as asphalt and dirt are electrically rough with subsurface aggregate sizes comparable with the wavelength resulting in substantial volume and surface scattering. The data show strong backscatter responses from bare surfaces with angular dependence proportional to the cosine square of the incidence angle along with significant depolarization (between -8 and -4 dB). The data for vegetation-covered surfaces show weak dependence on incidence angle and appreciable depolarization (between -12 and -6 dB). Empirical models for bare and vegetation-covered surfaces are proposed.