Effects of Tropical Rainfall to the Ku-Band Satellite Communications Links at the Equatorial Atmosphere Radar Observatory

摘要

This study investigates the effects of tropical rainfall on the Ku-band satellite communications links that connect Research Institute for Sustainable Humanosphere (RISH), Kyoto University in Japan to Equatorial Atmosphere Radar Observatory (EAR; 0.2°S, 100.3°E) in Indonesia, using the satellite Super bird C (144°E in orbit). Rain attenuation of the up- and down-link radio wave signals is, for the first time, obtained at the same time in the tropics, monitoring each signal level that has been received at both stations in Japan and Indonesia for the past three years from 2003 to 2005. The up-link attenuation at each station can be estimated from the down-link signal level measured at its opposite station, because SCPC (Single Channel Per Carrier) signals used in this experiment are linearly amplified without saturation of the satellite transponders. At EAR in Indonesia, a slightly larger attenuation ratio between up and down links is statistically presented for the attenuation range of higher that 10 dB, suggesting the effects of smaller raindrop size distributions (DSD) than observed at RISH in Japan. This tendency is more conspicuous in the rainfall events when the observed attenuation shows only one peak in its time series, indicating the effects of simple convective precipitating clouds with one single cell. At RISH in Japan, a larger difference between worst month and yearly average statistics is found, due to a larger variation of the ground temperature that affects the slant-path length during the seasons, although the yearly average time percentages as such are larger at EAR than RISH up to the up-link attenuation of 15 dB. Using time percentages of their local rainfall rates, fairly good agreement is found between the observations and the ITU-R (International Telecommunication Union—Radiocommunication Sector) predictions for both locations. At EAR in Indonesia, however, the time percentages of the attenuation of more than 10 dB become significantly smaller than those predicted by the ITU-R methods for both up and down links. This indicates the remarkable reduction of equivalent path lengths down to about 2 km, caused by a fairly localized structure of convective precipitating clouds. Simultaneous X band radar observations have revealed that intense echo cores of typical rain cells causing severe attenuation are confined to about 2 km along the propagation path. This result, which seems rather small even compared with those obtained in other tropical observation sites, may be attributed to unique features of the EAR site that is located in a highland basin 865 m above the mean sea level and frequently observes simple precipitating clouds with a single cell.