drifting anomalies, large-area events, or numerous smaller-area anomalies in the power beam and the pilot beam cause spatial and temporal modulations that vary the illumination of the ground-based rectenna, and under adverse geometry, can cause large variations in pilot signal parameters. The latter have a strong potential for causing serious ambiguities in the control decisions and system stability considerations. An issue that has received only cursory attention is the effect on atmospheric electricity of the power beam and the rectenna. Direct interactions between the power beam and the atmospheric electric fields are not thought to be crucial at the 2.45 GHz frequency. However, the mere physical presence of the rectenna may have some modifying influence on the occurrence and electrical behavior of thunderstorms over and around the rectenna. Further study is needed in this area. Finally, electric and magnetic fields in the vicinity of the rectenna and the transmission lines may have biological as well as meteorological significance, especially if direct-current power is used. The anticipated field strengths will have to be determined based upon SPS design parameters and previous experience with existing direct-current, power-transmission systems. 3.2 NONMICROWAVE ATMOSPHERIC EFFECTS ABOVE 60 KM Above 60 km the population of ionized atomic and molecular species becomes significant. Consequently, deposition of rocket effluents above 60 km can affect natural processes involving neutral and ionic species as well as the magnetosphere that is coupled to the ions by electromagnetic interactions. Between 60 and 500 km (low earth orbit is at approximately 500 km) the atmosphere is subject to modification from rocket thruster effluents. The main source of SPS-related disturbance comes from chemical rocket exhaust products, including water, carbon dioxide and smaller quantities of various other effluents, including carbon monoxide, oxides of nitrogen, and unburned fuel (hydrogen and methane). Space vehicle reentry will also contribute some effluents such as ablated materials and oxides of nitrogen to the lowest layer of this region.
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