Polarimetric Ka-band backscattering measurements at grazing-angle extremes

Polarimetric millimeter-wave backscattering data has been measured in true operational environments for perpendicular illumination and for grazing angles below one degree. Manmade objects, such as vehicles and light poles, were found to give a 5-10 dB lower response for vertical polarization, and the cross-polarized returns were about 10-20 dB below the reference level obtained with horizontal polarization. Forest and soil backscattering showed similar characteristics of -7 to -9 dB (vertical) and -14 to -17 dB (cross-polar). An anomaly was observed for horizontally transmitted/vertically received data, where gravel soil showed only about 2 dB attenuation compared to the co-polar reference level. The dynamic variations of mobile target signals at different polarizations indicated that millimeter-wave cross-polarized returns call for high transmitter power if true polarimetric target classification is attempted. Close-to-ground radars seem to benefit from multipath and thus from using horizontal polarization if only maximum range is defined. Variations in the cross-polarized echo amplitudes of moving manmade objects were found to be within 3-5 dB of their typical mean values