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Characterization of wave propagation through rain at sub-terahertz frequencies

The fact that millimeter-wave radars can be designed to be compact in size and light in weight, while at the same time be broadband and narrow beam systems, have led to a proliferation of these radars over a wide range of applications. These applications include high resolution synthetic aperture radar imaging, target surveillance and tracking, vehicle collision avoidance, autonomous navigation of robotic ground and air vehicles, and assistive landing of airborne vehicles. Recently, there has been an increased interest in use of radar systems operating at frequencies in the upper millimeter-wave and sub-terahertz portion of the spectrum (90 GHz <; f <; 300 GHz). At these frequencies, wave propagation through the atmosphere is of concern, especially during inclement weather, such as brownouts due to sandstorms, whiteouts due to snow storms, and reduced visibility due to heavy rain. Distortions introduced by rain, in the form of extinction, depolarization, and increased backscatter, are of particular concern due to the fact that water droplets have a higher permittivity and larger size than other airborne particles. The size of a water droplet may extend anywhere between few microns and 8 mm in diameter. For example, at a moderate rain fall rate of 4 mm/hr, the median diameter of water droplets is 1.6 mm, which is larger than the wavelength of a radar operating at 200 GHz (wavelength = 1.5 mm). These relatively large particle sizes, coupled with high permittivity of liquid water, are expected to result in strong scattering at these frequencies. Furthermore, the shape of a falling water droplet depends on its size. When the droplet size is small (i.e. diameter <; 1 mm) then its shape is essentially spherical. However, as the droplet size increases beyond 1 mm, its shape becomes increasingly distorted, resembling a deformed oblate spheroid. The radar cross section of large droplets can be quite different compared to spherical particles of the same volume. In addition, the distorted water droplets will result in cross-polarized return of the reflected signal and depolarization of the forwardly-scattered signal as well. Hence, a polarized radar signal propagating through rain may suffer from significant attenuation as well as from polarimetric distortions in the form of depolarization and dispersion. As a result, radar signatures of targets may appear significantly different from reality, and detection features may not work efficiently. It should be noted that the degree of severity of such effects increases with the increase in the rain fall rate. No thorough study of wave propagation through rain has been reported to date at sub-terahertz frequencies.