random number RN between 0 and 1 is assigned to this probability, and the angle a is then determined from the solution of Water vapor and molecular oxygen absorption coefficients are taken from Ref. 15. Oxygen absorption is based upon Van Vleck's theory (16) with line parameters determined by Carteret al. (17). Calculations of water vapor absorption are based on parameters derived from the data of Becker and Autler (18). Absorption from other trace constituents such as NH;i and O3 is relatively small (19). The mid-latitude summer and winter atmospheres given by McClatchey et al. (20) are used for the vertical water vapor distribution. Oxygen and water vapor extinction coefficients (km-1) are given as a function of height and frequency in Fig. 1 for a clear atmosphere. Absorption by both oxygen and water vapor increases strongly with decreasing altitude. At a height of 10 km, absorption by oxygen is significantly larger than absorption by water vapor, except in the 20-25 GHz region. However, absorption by water vapor increases rapidly with decreasing altitude. At a height of 1 km, absorption by water vapor is less than that of oxygen only for frequencies less than 10 GHz. The indices of refraction for ice and water are strongly dependent upon temperature as well as wavelength within the microwave region. The complex permittivity e = (e' - ie") is determined as a function of temperature, from which the complex index of refraction n = (nr - in^ is then calculated (21). The indices of refraction of
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