Ka band propagation experiments of experimental communication payload (ECP) on ROCSAT-1 - Preliminary results

It is recognized that rain attenuation is the primary factor in the degrada tion of Earth-satellite communication at the Ka band frequency. The beacon signal of the ROCSAT-1 is set at 19.5 GHz for downlink and 28.5 GHz for upl ink. ROCSAT-1 is the low earth orbit (LEO) satellite with a circular orbit at the altitude of 600 km and 35 degrees inclination angle and scheduled to be launched at the beginning of 1999. Given the extremely high frequency o f the beacon, impairment of ROCSAT-1 communications due to pain attenuation should be seriously considered. In this paper, the groundbased instruments for the Ka-band propagation experiments of ROCSAT-1, including Chung-Li VH F radar, 19.5 GHz radiometer, optical rain gauge, automatic weather station , and disdrometer, are introduced. The spatial distribution of the long-ter m statistics of rainfall rate is analyzed in this paper on the basis of 8 y ears (1988-1995) rainfall rate data at one-minute time resolution, recorded by more than 70 tipping bucket rain gauges distributed over Taiwan island. It shows a pronounced latitudinal variation in the percentage of time that the rainfall rate exceeds a specified level, indicating that snore severe rain attenuation will be encountered in the southern part than that in the northern part of Taiwan. In addition, the sky noise temperature at the freq uency of 19.5 GHz is measured by using a radiometer, both in the conditions of clear-air and precipitation. The observed sky noise temperature in the case of clear-air at the elevation angles of 90 degrees, 60 degrees, 30 deg rees, and 15 degrees are respectively 50K, 80K, 100K, and 130K, correspondi ng to the attenuations of 0.7dB, 0.78dB, 1.2dB, and 2.7dB. Data analysis in dicates that the observed clear-air sky noise temperature increasing expone ntially with the decrease of the zenith angle is in perfect agreement with our theoretical prediction. The sky noise temperature in the case of precip itation is also investigated. A comparison between observed precipitation s ky noise temperature and surface rainfall rate shows that a salient time sh ift in the two is seen, implying that great caution should be taken in esta blishing an empirical relationship between precipitation sky noise temperat ure and surface rainfall rate.