Rain attenuation characteristics due to typhoon passage are discussed using the Ku-band BS satellite signal observations conducted by Osaka Electro-Communication University in Neayagawa from 1988 to 2019. The degree of hourly rain attenuation due to rainfall rate is largely enhanced as typhoon passes the east side of the station, while it becomes smaller in the case of west side passage. Compared to hourly ground wind velocities of nearby AMeDAS, the equivalent path lengths of rain attenuation become larger as the wind directions approach the same angle to the satellite, while they become smaller as the wind directions approach the opposite angle to the satellite. The increase and decrease of the equivalent path lengths are confirmed in other Ku-band and Ka-band satellite paths with different azimuth angles, such as CS, SKP, and SBC. Modified equivalent path lengths calculated by a simple propagation path model including horizontal wind speeds along the same direction to the satellite agree well with the equivalent path lengths observed by each satellite. The equivalent path lengths are, for the first time, proved to be largely affected by the direction of typhoon passage and the horizontal wind velocities.

Directions and speeds of the motion of rain areas are estimated for each type of rain fronts, using time differences detected in the rain attenuation of the Ku-band satellite radio wave signals that have been measured at Osaka Electro-Communication University (OECU) in Neyagawa, Osaka, Research Institute of Sustainable Humanosphere (RISH) in Uji, Kyoto, and MU Observatory (MU) of Kyoto University in Shigaraki, Shiga, for the past five years since September 2002. These directions and speeds are shown to agree well with those directly obtained from the motion of rain fronts in the weather charts published by Japan Meteorological Agency. The rain area motion is found to have characteristic directions according to each rain type, such as cold and warm fronts or typhoon. A numerical estimate of the effects of site diversity techniques indicates that between two sites among the three locations (OECU, RISH, MU) separated by 20-50 km, the joint cumulative time percentages of rain attenuation become lower as the two sites are aligned along the directions of rain area motion. In such a case, compared with the ITU-R recommendations, the distance required between the two sites may be, on an average, reduced down to about 60-70% of the conventional predictions.