exception is the southwestern United States). Hole boring, as defined in Ref. (1), will increase the annual power availability by 9% to 33% depending upon the cloudform frequencies of the individual sites and can be particularly beneficial in extending the persistence time for an acceptable transmission efficiency. If an 80% frequency for the transmission efficiency to equal or exceed 80% is defined as the minimum requirement for commercial viability of the laser-SPS concept, then three receptor sites separated by a centroid radius of from 200 to 300 miles must be available for each transmitted laser beam for most regions of the United States; for the Southwest, however, only two sites separated by as little as 100 miles will be sufficient. The average persistence time, during which the prevailing meteorological conditions allow a high transmission efficiency, is considerably shorter than 8 h at many sites, so that any viable laser-SPS concept must be capable of frequent beam switching between sites with a minimum of downtime. Obviously, this operational scenario is considerably different from that envisioned for the microwave-SPS concept, and the economic and engineering viability of the multiple-site concept must be further evaluated. Superficially, it seems that the smaller land area required for each laser-receptor site will outweigh the additional cost of three times the number of sites and their associated hardware when compared with the microwave-SPS concept. An evaluation of the effects of frequent beam switching will require an analysis of the dynamic response of the electric-power grid. Additional research is needed to develop a joint power-availability model for multiple sites including more sophisticated statistical cloud-cover models and the statistical effects of frontal passage over multiple-site clusters. REFERENCES 1. R.E. Beverly Ill, Meteorological Effects on Laser Propagation for Power Transmission, Space Solar Power Review 3, 9-29 (1982). 2. R.E. Beverly 111, Laser-SPS Systems Analysis and Environmental Impact Assessment. Space Solar Power Review I, 317, 1980. 3. R. Nybro. Private Communication. National Climatic Center, NOAA, Ashville, NC, 1980. 4. P.J. O'Reilly, Point Comparisons of Total Cloud Cover from Satellites and from Surface Observations, USAF Air Weather Service Technical Report No. 246, 1973. 5. D.B. Miller and R.G. Feddes, Global Atlas of Relative Cloud Cover 1967-1970 Based on Data from Operational Satellites, National Environmental Satellite Service, Washington. DC. 1971. 6. A.R. Coburn. Improved Three-Dimensional Nephanalysis Model, Air Force Global Weather Center Tech. Memo. AFGWC-TM-71-2. 1971. 7. R.G. Feddes and K.-N. Liou. Cloud Composition Determination by Satellite Sensing Using the Nimbus VI High Resolution Infrared Sounder, Air Force Geophysics Laboratory Report No. AFGL-TR-77-0123, 1977, and references therein. 8. LA. Lund and M.D. Shanklin, Universal Methods for Estimating Probabilities of Cloud-Free Lines- of-Sight Through the Atmosphere, J. Appl. Meteorol. 12, 28, 1973. 9. LA. Lund, Persistence and Recurrence Probabilities of Cloud-Free and Cloudy Lines-of-Sight Through the Atmosphere. J. Appl. Meteorol. 12, 1222, 1973. 10. R.E. Beverly III, Meteorological Effects on Laser Beam Propagation, Satellite Power Systems Laser Studies, Vol. II, Rockwell International Technical Report No. SSD 80-0119-2, published in NASA CR-3347, 1980. 11. LA. Lund, A Model for Estimating Joint Probabilities of Cloud-Free Lines-of-Sight Through the Atmosphere,./. Appl. Meteorol. 12, 1040, 1973.
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