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In January and February of 1981 and 1982, the Ballistic Research Laboratory (BRL) participated in SNOW-ONE at Camp Ethan Allen, VT, sponsored by the US Army Cold Regions Research and Engineering Laboratory (CRREL). The BRL conducted high-angle radar measurements there, measuring backscatter at 35 GHz. from various areas of undisturbed snow, acquiring time series data under varied weather conditions. A dual-polarized, 35 GHz radar was used to make the snow backscatter measurements. Polarity of the transmitted signal, vertical or horizontal, was controlled by means of an R.F. switch. The received signal was passed through an orthomode transducer to two receivers, allowing both parallel and cross polarized components to be recorded simultaneously. The sensor was supported 15 meters above the ground on an elevation-over-azimuth antenna mount. Elevation angle was adjusted so that radar beam angle was 30-degrees from vertical. Azimuth angle was varied so as to scan areas of undisturbed snow around the base of the support. The sensor was enclosed in an insulated box and heated to a fixed temperature. Signal returned to the sensor by the snow was, in large part, dependent upon the condition of the surface snow as it was affected by air temperature. When the temperature was well below freezing, sigma-zero was around -7dB with parallel polarization (-13dB, cross polarization). When the temperature increased to the freezing point, the snow became wet, packed easily, and had a lower sigma-zero (-17dB, parallel, and -22dB, cross polarization). The condition of the snow surface had a secondary effect upon the value of sigma-zero. A smoothed surface reflected less energy back to the sensor than a roughened surface. The combined effect of these two variables produced the changes that were measured in sigma-zero.
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