exceeds twice the expected highest ionospheric plasma frequency. The ionospheric plasma frequencies are well known for a large number of stations and the data are catalogued in the World Data Center. Fifteen or twenty MHz would probably be safe values to use as the expected highest plasma frequency, although the values should be checked against the data. Therefore, the sidebands on the uplink should be 30 to 40 MHz away from the downlink frequency for a sideband separation of 60 to 80 MHz and this requirement should be incorporated into the system design. 6. CONCLUSION Although we have presented a review of a number of distinct ionospheric phenomena, it is clear that for a complete evaluation of the solar power satellite impacts we must investigate a complex, coupled set of problems. The propellant emissions from an HLLV may produce nearly complete depletions in the ionospheric electron density over scales up to 2000 km; collisional damping of the SPS microwave beam will significantly enhance electron temperatures in the lower ionosphere; beam self-focusing can generate large-scale density irregularities in the upper ionosphere. In addition to their independently assessed effects, the interactive nature of these phenomena must be investigated. A comprehensive research program to perform the SPS environmental impact assessment has been organized and is being administered by the Department of Energy. In the study of ionospheric physics, theory has historically been relatively poor at predicting experimental results, although quick to explain observations. It is therefore premature to form conclusions based on the ionospheric studies to date. Theoretical investigations have identified several potentially serious SPS ionospheric impacts; limited experimental studies have in general supported these results and helped to define the effects. However, no significant telecommunications or climate effects have yet been experimentally demonstrated. Furthermore, to extrapolate the results of current ionospheric research to the SPS studies, we must also verify the scaling assumptions on which those extrapolations are based. All these considerations are incorporated into the ongoing research program to determine ionospheric effects of microwave beams. The originally proposed limit of 23 mW/cm2 for the power density in the microwave beam should be at least doubled pending further tests. REFERENCES 1. See, for example, Radio Sci. 9, 1974. 2. K. A. Brueckner, Laser Driven Fusion, IEEE Trans. Plasma Sci. 1, 13, 1973; J. Nuckolls, J. Emmett, and L. Wood, Laser-Induced Thermonuclear Fusion, Phys. Today 26, 46, 1973; L. C. Johnson and T. K. Chu, Measurements of Electron Density Evolution and Beam Self-Focusing in a Laser-Produced Plasma, Phys. Rev. Lett. 32, 517, 1974. 3. D. R. Nicholson, M. V. Goldman, P. Hoyng, and J. C. Weatherall, Nonlinear Langmuir Waves During Type III Solar Radio Bursts, Astrophys. J. 223, 605, 1978; M. V. Goldman and D. R. Nicholson, Vital Theory of Direct Langmuir Collapse, Phys. Rev. Lett. 41, 406, 1978. 4. W. C. Brown, Satellite Power Stations: A New Source of Energy?, IEEE Spectrum 10, 38, 1973; P. E. Glaser, Solar Power from Satellites, Phys. Today 30, 30, 1977. 5. L. H. Holway and G. Meltz, Heating of the Lower Ionosphere by Powerful Radio Waves, J. Geophys. Res. 78, 8402, 1973. 6. P. M. Banks and G. Kockarts, Aeronomy. Academic, New York, 1973; M. H. Rees and R. G. Robie, Observations and Theory of the Formation of Stable Auroral Red Arcs, Rev. Geophys. Space Phys. 13, 201, 1975; R. W. Schunk and A. F. Nagy, Electron Temperatures in the F-Region of the Ionosphere: Theory and Observations, Rev. Geophys. Space Phys. 16, 355, 1978.
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