each total sky cover ./. The second term on the right-hand side of Eq. 1 gives the probability of a cloudy line-of-sight multiplied by the weighted cloud transmissivity through that line-of-sight. This term accounts for those instances where some penetration through cloud cover is possible. The definition of^ and three different models for calculating r} are given in the sections below. The cumulative frequency, i.e., the frequency that the transmission efficiency equals or exceeds Tj, is where Kj is the frequency of occurrence of each total sky cover. By definition, Observational values of Kj were obtained for each receptor site from Part D (Sky Cover) of the U.S. Air Force/U.S. Navy Revised Uniform Summary of Surface Weather Observations, which is available from the National Climatic Center (NOAA) in Ashville, North Carolina. This summary is prepared from hourly observations and is a frequency distribution of total sky cover by tenths, plus mean sky cover, and total number of observations. Statistical data are tabulated by month for standard 3-hour groups and by month and annually for all hours and all years combined. The number of observations ranged from about 1000 to over 15,000 spanning from 6 to over 30 years of data gathering. The power availability for a given receptor site is then The power availability is simply the temporally averaged transmission efficiency assuming that power is constantly beamed from space to the receptor site. Cj probabilities were determined by Lund and Shanklin (8) for all the cloud-form categories listed in Table 2. Because total sky cover observations (K/s) are not reported with regard to cloud form, we must use the weighted Cs probabilities for all cloud forms given in tabular form by Lund and Shanklin as functions of the elevation angle </>, where 6 = 90 - <£• According to comparisons made by these authors, the total probability of a cflos when cloud forms are considered and when weighted Cj probabilities are employed are almost identical, differing by s 1% which is statistically insignificant. The use of weighted Cj probabilities, independent of cloud form, should be valid for all U.S. mid-latitude sites chosen for this study. The most important statistical parameter in laser power transmission is not the frequency that the transmission efficiency exceeds some useful value, but rather the frequency that the transmission efficiency exceeds some useful value and persists uninterrupted in this manner for a reasonable time. It is senseless to continue power beaming to a site when the transmission efficiency falls below some acceptable value and remains low; instead, the beam can be switched to an alternate receptor site. If we consider the persistence time, the average transmission efficiency expected during this time for each total sky cover j is then where Ej is the probability that once a cflos is established it will persist for at least a
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