In the radio wave network systems for frequencies above 10GHz, it could be important to take into account for the attenuation due to rainfall. In order to obtain the attenuation by rain in the specific area, the estimation of rainfall rates is required for the time ratio of annual accuaulated time distributions of one minute rain rates. In this report. Morita and Karasawa-Matsudo methods for the estimation of accumulated time distributions of one minute rain rates have been evaluated on the basis of the experimental data for 20 years. As a result, it appears that accuracies in the methods could be improved by making minor corrections for conversion ratios and conversion coefficients. Furthermore, we discussed errors of the accumulated time distribution of rain rates.
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