plotted in Figure 3-3, and combined with estimates of clear air gaseous absorption and plotted in Figure 3-4 for two elevation angles. Also shown in Figure 3-4 are transmission efficiency estimates for propagation through an extremely heavy rainfall (50 mm/hr) in a 4 km high cloud and a severe thunderstorm.(8) The latter results are based on the measured rainfall rates (9) shown in Figures 3-5 and 3-6. A Laws and Parsons distribution and Mie theory were used to compute the attenuation coefficient. The variation in efficiency with frequency for four different propagation paths through the thunderstorm are depicted in Figure 3-7. The curve in Figure 3-4 corresponds to path through the storm center. Note the height and lateral extent of the storm and the variation in efficiency with distance from the region of peak rainfall. A more accurate estimate would take into account the variation in loss across the beam cross-section. The results presented herein provide a lower bound on the transmission efficiency. Based on the results in Figure 3-4, it is concluded that atmospheric attenuation due to molecular absorption and heavy rain is tolerable at frequencies below 3 GHz. Under the most severe conditions it is found that at 3 GHz. In the 10 to 30 cm band, attenuation due to clouds is less than one-half percent. Attenuation due to dry snow is also negligible in this band as can be seen by evaluating Equation (3-5), using the Rayleigh approximation to the cross-section and an empirical particle size distribution (see Table 6.4, pg. 74, in Battan). Figure 3-4. Transmission Efficiency - Molecular Absorption and Rain
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