Microwave radio meteorology: Seasonal fading distributions

Abstract

Refractive fading can significantly impair service on terrestrial line‐of‐sight microwave transmission systems. Digital radio systems are particularly vulnerable to this fading which is produced by vertical gradients in the microwave refractive index. A characterization of the seasonal variation in fading and its dependence upon meteorological parameters provides the potential to improve transmission performance by better tailoring of transmission equipment design and usage to the amount of fading expected at a given location and time of year. In this work we study the seasonal variation in fading occurrence at Palmetto, Georgia, over a 28‐month period. The changes in the shapes of seasonal fading distributions as a function of fade threshold, path length, and antenna height are examined and interpreted meteorologically. The Palmetto distributions are compared with those measured at other worldwide locations and the year‐to‐year fading variability for Palmetto is estimated. The Palmetto studies show that mean monthly wind speed V is the best meteorological predictor of monthly accumulated time that signal is below −20 dB, t20, at Palmetto. The predictor is t20 = 17600 (12/V ‐ 1)2 and has a correlation coefficient of 0.88. Year‐to‐year variability in observed fading is greatest in early autumn when monthly accumulated 20 dB fading time differs by about 6000 s between years. Seasonal distribution shapes depend strongly on geographical location. The studies comparing different worldwide distributions show that 95% of the total annual fading occurs during the season experiencing monthly water vapor pressures > 10 mbars and that the peak in the seasonal fading distributions, outside of low latitude coastal regions, is associated with the months in the humid season when there is the least precipitation.