of theoretical prediction. Efforts directed toward a full application of the selffocusing instability in an underdense ionosphere need to be undertaken. Since a critical portion of the ionospheric heating assessment relies upon frequency-scaling laws for ohmic heating and self-focusing instabilities, these laws must be verified with experimental observation. Ground-based transmitters operating at higher powers and higher frequencies than those presently at Platteville need to be constructed. Such facilities would provide the means to corroborate frequency-scaling laws as well as permit further study of telecommunications impacts in the upper portions of the frequency spectrum. The results of studies thus far obtained, are applicable to the operation of a single satellite power system operating with a microwave beam having the power density of 23 mW/cm2 at the beam center. Studies need to be directed toward assessing the effect of multiple satellites on the ionosphere. In addition, it is desirable to know the upper power density limit to which the SPS can operate before inducing adverse telecommunication effects. Studies undertaken indicate that 23 mW/cm2 may be below the threshold for the onset of ionospheric changes. If higher power flux densities can be tolerated, major system design criteria will have to be reviewed, and possibly revised. These await the future. Acknowledgements — This work was funded by the United States Department of Energy Satellite Power System Project Office. Mr. F. Koomanoff, Director. Satellite Power System Project Office, has been a constant help throughout these studies. Technical discussions with Dr. S. Basu, Dr. Sunanda Basu, Dr. L. Duncan, Prof. M. Goldman, Dr. G. Meltz, Dr. F. Perkins, and Dr. W. Utlaut have been extremely helpful. REFERENCES 1. U.S. Department of Energy, Satellite Power System Concept Development and Evaluation Program Reference System Report, DOE/ER-0023. 1978. 2. A.V. Gurevich, Nonlinear Phenomena in the Ionosphere, Springer-Verlag, New York, 1978. 3. F.W. Perkins and M.V. Goldman, Self-Focusing of Radio Waves in an Underdense Ionosphere, J. Geophys. Res. 86, 600-608, 1981. 4. W.F. Utlaut, Ionosphere Modification Induced by High-Power H.F. Transmitters — A Potential for Extended Range VHF-UHF Communication and Plasma Physics Research, Proc. IEEE 63, 1022— 1043, 1975. 5. J.A. Fejer, Ionospheric Modification and Parametric Instabilities, Rev. Geophys. Space Sci. 17, 135-153, 1979. 6. G. Meltz and W.L. Nighan, Microwave Heating of the Lower Ionosphere, Proc. DOE/NASA Satellite Power System Program Review, April 22-25, 1980. Obtainable from Johnson Environmental and Energy Center, University of Alabama, Huntsville. 7. W.E. Gordon and L.M. Duncan, lonosphere/Microwave Beam Interaction Study, William Marsh Rice University Report DRL T01349, Houston, Texas, 1978. 8. F.W. Perkins and E.J. Valeo, Thermal Self-Focusing of Electromagnetic Waves in Plasmas, Phys. Rev. Lett. 32, 1234-1237, 1974. 9. J.C. Carroll, E.J. Violette, and W.F. Utlaut, The Platteville High-Power Facility, Radio Sci. 9, 889-894, 1974. 10. C.M. Rush, E.J. Violette, R.H. Espeland, J.C. Carroll, and K.C. Allen, Performance of the VLF, LF, and MF Telecommunication Systems in Simulated Satellite Power System Environment Ascertained by Experimental Means, Radio Sci. 16, 219-234, 1981. 11. S. Basu, Sunanda Basu, A.L. Johnson, J.A. Klobuchar, and C.M. Rush, Preliminary Results of Scintillation Measurements Associated with Ionospheric Heating in Overdense and Underdense Modes, Geophys. Res. Lett. 7, 609-612, 1980. 12. A.L. Johnson, Private Communication, 1980. 13. S.A. Bowhill, Satellite Transmission Studies of Spread-F Produced by Artificial Heating of Ionosphere, Radio Sci. 9, 974-986, 1974.
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