Abstract-The results of measurements of backscatter cross section per unit volume and attenuation for falling snow and rain at 96, 140, and 225 GHz are presented. The attenuation due to rain is almost independent of the measurement frequency, but for snow the attenuation is considerably greater at 225 GHz than at 96 GHz. The rain attenuation generally varies with the rain accumulation rate in accordance with an aRh relationship for a Laws and Parsons drop-size distribution. The attenuation at all three frequencies is about 3 dB/km for a rain rate of 4 mm/h. The attenuation due to snow varies linearly with airborne snow-mass concentration, with the average rates of increase being 0.9, 2.5, and 8.7 ( d B / k m ) / ( g / m 3 ) at 96, 140, and 225 GHz, respectively. Generally the attenuation for snow is lower than that for rain. The backscatter cross section per unit volume for rain at 96 GHz is about -35 dB m'/m3 for a rain rate of 4 mm/h. The backscatter from snow at 96 GHz is much lower than that for rain under equivalent accumulation rates or airborne mass concentrations. Snow backscatter at 140 and 225 GHz is comparable but higher than that at 96 GHz.

Abstract-This study, consisting of three complimentary topics, examines of the millimeter-wave backscattering behavior of terrain at incidence angles extending between 70 and 90 , corresponding to grazing angles of 20 to 0 . The first topic addresses the character of the statistical variability of the radar backscattering cross section per unit area A. Based on an evaluation of an extensive data set acquired at 95 GHz, it was determined that the Rayleigh fading model (which predicts that A is exponentially distributed) provides an excellent fit to the measured data for various types of terrain covers, including bare surfaces, grasses, trees, dry snow, and wet snow. The second topic relates to the angular variability and dynamic range of the backscattering coefficient 0 , particularly near grazing incidence. In this paper, we provide a summary of data reported to date for each of several types of terrain covers. The last topic focuses on bare surfaces. A semi-empirical model for 0 is presented for vertical (VV), horizontal (HH), and cross (HV) polarizations. The model parameters include the incidence angle , the surface relative dielectric constant , and the surface roughness ks, where k = 2= and s is the surface root mean square (rms) height.

This paper presents two designs for phased arrays for collision avoidance radar operating at the 76 GHz band. The two designs for the antenna use a linear waveguide horn array that is fed with a waveguide powerdividing network and is tapered to produce low side-lobe levels for low-error detection. The taper is based on a 40-dB Taylor distribution. The first design uses passive phased arrays that are stacked and bore-sighted in different directions to produce scanned beams through switching. The second design uses a single phased array with MEMS phase shifters embedded in the last stage of the waveguide power divider. The phase shifters are integrated with the help of MIC/MMIC-to-waveguide or waveguide-tocoplanar-waveguide (CPW) transitions that are housed in the waveguide sections that feed the array elements. The antenna and detection system measurement results of the passive array design are presented, along with the simulation results of the active array design.

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