PROPAGATION CHARACTERISTICS UNDER VARIOUS METEOROLOGICAL CONDITIONS Physical Mechanisms The attenuation of laser radiation passing through the Earth's atmosphere is termed linear attenuation if the processes responsible are independent of the beam intensity. In general, molecular scattering, molecular absorption, aerosol scattering, and aerosol absorption contribute to linear attenuation. Beam attenuation and spreading due to turbulence, another linear mechanism, are negligible for space-to- Earth power transmission (1). If the attenuation depends on the beam intensity, however, the propagation is termed nonlinear. For example, estimates for the onset of thermal blooming are given in Ref. (1); if the molecular and aerosol absorption coefficients are sufficiently small, gross beam wander and self-induced spreading will not occur. Another nonlinear mechanism is aerosol droplet vaporization. With sufficiently large laser power densities, hole boring through various types of meteorological formations may be affected with a concomitant increase in transmission efficiency. Mitigation Techniques A number of high transparency spectral “windows” are present for which laser radiation will propagate from space to Earth with only minimal attenuation due to molecular absorption. During periods of heavy cloud cover or precipitation, however, a severe loss in transmission efficiency will occur because of aerosol absorption and scattering. The transmission efficiency may be improved during adverse meteorological conditions by [1] selection of a wavelength region which minimizes the effects of aerosol absorption and scattering, 12] increasing the elevation of receptor sites, [3] using a vertical propagation path (zenith angle 0 = 0°) rather than line- of-sight propagation from a satellite in geosynchronous equatorial orbit (0 — 50°), and [4] by hole boring, i.e., vaporization of the aerosol droplets within the beam path. Wavelength Selection. Preliminary information (2, 3, 4) indicates that operation in the spectral region around 11 /im may reduce the attenuation caused by light fog and light cloud cover. This phenomenon occurs for two reaaons: [1] the real part of the complex refractive index of water has a minimum at about 12 ^im and [2] the aerosol size distribution of certain fogs and clouds decreases more steeply than (particle radius)-2 above 7-10 p.m radius, thus reducing the scattering and absorption coefficients. Receptor Elevation. The selection of receptor sites at high elevation can reduce the deleterious effects of haze and can mitigate the problems caused at lower elevations (river valleys, coastal regions, etc.) by many types of advection and radiation fogs. In addition, if water vapor is an important molecular absorber for a specific laser line, high elevation receptor sites can “get above” a large fraction of the humid air in the lower troposphere and result in improved transmission efficiency. The clear-air transmission efficiency (considering molecular absorption only) is, unfortunately, undesirably low everywhere in the ll-/am window except for high- elevation receptor sites. Alternately, we have identified an extremely high- transmission region around 2 pm in which molecular absorption is negligible concurrent
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