various considerations involved in the SPS concept reaching maturity has been presented by Glaser (10). The American Institute of Aeronautics and Astronautics (AIAA) has recently completed a detailed position paper on space power system options (11). They report that “The major questions regarding these transmission schemes center on their environmental impact. Much more needs to be known about the interaction of such beams with the ionosphere, and there is considerable divergence of opinion about safe intensity levels for birds, other wildlife, vegetation, or humans living in the area surrounding a receiving antenna.” One of the prime reasons for the selection of a microwave transmission frequency is the fact that microwaves interact only weakly with cloud drops and aerosol particles. However, scattering and absorption of microwaves by rain drops increases rapidly with increasing size of the drops and with increasing beam frequency. It is the interaction of the microwave beam with rain drops which constitutes the main topic of this paper. Surface microwave power densities are calculated for various rain drop size distributions and at frequencies ranging from 2.45 to 10 GHz. II. COMPUTATIONAL PROCEDURE Even though absorption dominates scattering in the microwave region, it is important to determine the spatial distribution of power scattered out of the beam in order to determine probable system efficiencies and design criteria, as well as to evaluate health and ecosystem impacts. The Monte Carlo radiative transfer technique is ideally suited for the study of both radiance and irradiance patterns in finite clouds, or for beamed energy propagating through clouds (12,13). The present model consists of gaseous and droplet absorption superimposed on the basic Monte Carlo scattering procedure (14). Absorption by water vapor and oxygen is calculated from the top of the atmosphere to the point of entry into the scattering zone. The scattering routine considers the path of photons undergoing pure scattering by rain drops. Within the scattering zone, absorption along photon paths is determined both from drop and gaseous contributions. In order to complete the absorption profile, photon trajectories and absorptions are computed from the point of cloud exit either to the Earth's surface or to the top of the Earth's atmosphere. The scattering model closely resembles that developed by McKee and Cox (12). A rectangular parallelepiped cloud region is subdivided by mutually perpendicular intersecting planes into smaller parallelepiped boxes. By assigning absorption and scattering parameters within each box, inhomogeneous clouds with the desired attenuation characteristics are constructed. Photon entry points on the top of the cloud may take on the desired spatial distribution, from uniform to Gaussian. A photon enters the cloud region with a specified nadir and azimuth angle and travels a distance s, at which point scattering occurs. Transmittance T is the probability that a photon travels a distance 5 between scattering events, where r is the optical depth, and ^3, is the volume extinction coefficient for the rth box. A random number is chosen for T, and 5 is determined by summing along the photon path until the right-hand-side of Eq. 1 equals the given random number. At
RkJQdWJsaXNoZXIy MTU5NjU0Mg==