microwave beam would not adversely affect the performance of telecommunication systems and that possibly the beam power density could be increased. Because of equipment limitations, these experiments deposited power in the lower ionosphere comparable to the microwave beam power density. Modified and expanded facilities would be required to stimulate heating of the upper ionosphere, verify the frequency-scaling theories, and establish the effects of the microwave beam on the upper atmosphere. The effects of a system consisting of many SPSs on telecommunication systems and those operating at frequencies higher than 30 MHz will have to be analyzed in more detail to establish the response to ionospheric heating and to indicate permissible peak power densities in the microwave beam. Although the SPS reference system was based on the use of microwave power transmission, some consideration was given to SPS laser systems. The most significant potential environmental effects for laser systems appear to be local meteorological changes and beam spreading as a result of tropospheric heating. The tropospheric heating will result from energy absorption by aerosols and molecules. Scattering from molecules and by absorption and scattering from aerosols will be greatest at short wavelengths and would be most significant for visible wavelength lasers. Aerosol effects will become important for infrared lasers only under hazy and overcast conditions. Laser wavelengths that have high atmospheric transmittance would be less likely to suffer from thermal blooming. The severity of the thermal blooming would be the function of several parameters, including the intensity of the laser, wind velocity, atmospheric density, absorption, and altitude. Thermal blooming could degrade and spread the beam, which would be less critical for the space-to-Earth SPS beam than for Earth-to-space transmission. It is unlikely that global climate changes could result from tropospheric heating since the absorption of laser energy would be less than the typical natural variations of the atmosphere. Ionospheric heating would also be negligible because the interactions would be confined to the laser beam volume. The findings on the effects on the atmosphere of SPS construction and operation are based upon transportation and power transmission characteristics associated with the SPS reference system. Since these characteristics were not defined precisely, and because the nature of the upper atmosphere is not completely understood, uncertainties remain. Many of these uncertainties are expected to be resolvable as information is obtained from other space missions. GEOSTATIONARY ORBIT ALLOCATION GEO and other orbits are accessible to all countries in accordance with the 1967 Space Treaty that states: "Outer space...is not subject to national appropriation by claims of sovereignty, by means of use, or occupation, or by any other means." The SPS along with other satellites and platforms would occupy positions in GEO. The spacing between adjacent SPSs in GEO will be determined by station-keeping requirements and capabilities and by electromagnetic compatibility. Spacing requirements will be influenced by microwave radiation frequency, its harmonics and noise, and off-axis antenna gain. If adjacent satellites are for telecommunications, spacing requirements will also be influenced by the susceptibility of such satellites to system noises and interferences that would be influenced by their off-axis antenna gain, filtering, and shielding. Reduction of the electromagnetic radiation from an SPS in the direction of an adjacent satellite and the ability of such a satellite to reject unwanted electromagnetic radiation will permit closer spacing. The U.N. Committee on the Peaceful Uses of Outer Space, the International Telecommunications Union, and the World Administrative Radio Conferences will have jurisdiction over the allocation of SPS frequencies and GEO positions to ensure that GEO will be of maximum benefit to all users. This requirement, operative under existing space laws, is imposed not only on the SPS, but on any other satellite or platform operating in GEO. EFFECTS ON ASTRONOMY The SPS and other large objects in Earth orbit have the potential to interfere with astronomical observations if light is reflected from their surfaces. Optical astronomy will be affected if an increase in night sky brightness results in a proportional reduction in the effective aperture of an optical telescope for the study of faint light sources. Suitable materials and surface finishes and design approaches could be selected to reduce reflected light from SPS surfaces such as solar arrays and transmitting antennas. Radio astronomy will be affected if undesirable microwave radiation from an SPS interferes with or damages sensitive radio astronomy receivers or if re-radiation from a receiving antenna on Earth interferes with radio astronomy observations. Emissions from man-made sources in allocated radio astronomy bands of the frequency spectrum are constrained by international treaty. To avoid interferences, microwave transmission equipment would have to be designed to comply with existing regulations. Potential interferences that could not be resolved by including such design features in the microwave transmission system or by satisfying receiving antenna siting criteria may be amenable to technical and functional solutions applied to radiotelescopes, including long baseline interferometry and signal cancellation techniques. By the time an SPS system is placed into operation, it may also be possible to construct radiotelescopes on the far side of the Moon, where they would be shielded from SPS and terrestrially produced electromagnetic radiation interferences.
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