Fig. 2. Annual power availability for the various U.S. geographical regions. The number in parentheses is the number of sites analyzed in that region. while the high end corresponds to the improved conditions affected by hole boring (model 3). Average results for model 2 usually fall close to the low end of the range and are only slightly better than for model 1. Seasonal variations for all models are highly region-dependent, as would be expected intuitively, and are often pronounced. Hole boring affords an improvement in the annual power availability of from 9% to 33% compared with model 2; significantly larger improvements are not possible without utilizing weapon-quality beams. Seasonal improvements can exceed 50%. Differences in the prevailing meteorological conditions between the regions are more readily apparent when transmission frequencies are compared, as in Fig. 3. The poor performance of sites in the Northwest, Atlantic, Midwest, and New England regions is particularly noticeable. Only sites in the Southwest offer a power availability in excess of 80% and a frequency for acceptable transmission efficiency (T > 0.80) suitable for commercial interest. To remedy this situation, it is obvious that the laser-SPS concept must rely upon multiple receptor sites for each transmitted beam. To establish receptor placement criteria and beam-switching scenarios, a knowledge of the persistence times for an acceptable transmission efficiency is needed. Persistence frequencies for a persistence time of 8 h are tabulated in Ref. (10) for all sites. To illustrate the wide range of behavior, the seasonal persistence frequency for T > 0.8 is plotted as a function of persistence time in Fig. 4 for sites having the best and worst meteorological conditions. Again, the pessimistic lower curves correspond to model 1 and the optimistic upper curves correspond to model 3. Attempted hole boring is of no use for sites plagued by frequent and heavy overcasts. For the
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