A Single-ended Atmospheric Transmissometer

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Abstract
It has been shown experimentally that a measurement of the Raman shifted component of the laser backscatter from atmospheric nitrogen will give a direct determination of transmission as a function of range. This type of single-ended device, when operating in a radar-like mode, can satisfy the need to accurately measure atmospheric transmission. A major difficulty in the interpretation of pulsed lidar backscatter data is that unless a priori information is available concerning the relationship between the volume backscattering coefficient and the attenuation coefficient, the received intensity cannot be easily evaluated as transmittance. The backscatter coefficient for Raman scattering, however, depends only on the Raman cross-section of the specific molecule used and the number density of that molecule. In the lower atmosphere the density of atmospheric nitrogen is constant. A measurement of Raman scattering from nitrogen will therefore give a direct determination of transmission as a function of range. Experiments were conducted over a 1/4-mile range and produced consistent results for transmissions down to as low as 2 percent when compared with simultaneous double-ended reference transmissometer data. The laser Raman transmissometer system is now computer controlled and produces real time data displays.

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