The effects of tropospheric refraction, turbulence and index of refraction gradients across clouds or weather fronts are to produce negligible displacement or dispersion of the high power beam for frequencies of 3 GHz and below. Their effects on the pilot signal are such as to not degrade the signal or the shape of the associated reference phase front significantly. The effects of the high power beam at power densities between 10 and 2 100 mW/cm on portions of certain layers of the ionosphere (D, E and F layers) may be significant in terms of increasing electron temperature and modifying (both increasing and decreasing) the plasma density. This in turn may be significant in modifying the performance of systems involving the F layer such as Over the Horizon Radars, HF communications, UHF radar (backscatter clutter and measurement accuracy). Also systems involving the D layer such as VLF (Omega and LORAN) navigation systems may be effected. 3. 2 ATMOSPHERIC ATTENUATION AND SCATTERING Estimates of atmospheric transmission efficiency are developed in this section as a function of frequency for various meteorological conditions. Gaseous absorption and attenuation by hydrometeors, such as rain, cloud droplets, snow, and hail, are the principal sources of propagation loss. 3. 2. 1 MOLECULAR ABSORPTION Attenuation of decimeter and centimeter radiation in clear air is due to the excitation of collision broadened rotational lines in water vapor and oxygen. (1,2,3,4) Absorption occurs because these molecules have permanent dipole moments that couple the electric or magnetic components of the microwave field to rotational energy levels. Most of the absorption is due to the 1. 35 cm (22 GHz) line of water vapor and the 0. 5 cm (60 GHz) line of oxygen. The strength of the interaction, and hence the frequency dependence is a function of the inherent line shape and the line width of the induced transition, which are in turn functions of the atmospheric model. Battan contains a figure that illustrates the variation of the attenuation coefficient with frequency (and wavelength) for O9 and uncondensed H2O. This figure is duplicated in Figure 3-1 to point out the rapid rise in absorption above 10 GHz and the weak dependence on water vapor content in the decimeter range of the spectrum. The same variation with frequency is seen, of course, in the
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