Long-term statistics of the worst month rain attenuation cumulative distribution in Ka and Ku band satellite communications links

Long-term rain attenuation statistics of the Ka and Ku band satellite signals obtained from 1986 to 2006 at Osaka Electro-Communication University in Neyagawa, Osaka, are investigated for the characteristics of their worst month cumulative time percentages. The long-term worst month statistics over more than 20 years are, on average, in good agreement with the ITU-R predictions, while their yearly values show fairly large year-to-year variations. The yearly variations are found to be caused by two different factors, attributed to the coefficients a and b of an exponential function, respectively, which connects the worst month statistics of each year with the long-term average cumulative time percentages. Specifically, the coefficient a means an overall increase of the yearly worst month statistics from the long-term average statistics. This coefficient is shown to be roughly determined by total rainfall in each year, and it can be more precisely estimated by the maximum monthly rainfall during each year. On the other hand, the coefficient b means the gradient of the worst month statistics against the long-term average statistics in logarithmic scale. This coefficient is found to be closely related to the occurrence of heavy rainfall due to convective clouds such as shower in summer, rather than the amount of rainfall during a year.