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 degradation 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 uplink. ROCSAT-1 is the low earth orbit (LEO) satellite with a circular orbit at the altitude of 600 km and 35¢X_ninclination angle and scheduled to be launched at the beginning of 1999. Given the extremely high frequency of the beacon, impairment of ROCSAT-1 communications due to rain attenuation should be seriously considered. In this paper, the groundbased instruments for the Ka band propagation experiments of ROCSAT- 1, including Chung-Li VHF radar, 19.5 GHz radiometer, optical rain gauge, automatic weather station, and disdrometer, are introduced. The spatial distribution of the long-term statistics of rainfall rate is analyzed in this paper on the basis of 8 years (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 more 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 frequency 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¢X, 60¢X, 30¢X, and 15¢X_nare respectively 50K, 80K, 100K, and 130K, corresponding to the attenuations of 0.7dB, 0.78dB, 1.2dB, and 2.7dB. Data analysis indicates that the observed clear-air sky noise temperature increasing exponentially with the decrease of the zenith angle is in perfect agreement with our theoretical prediction. The sky noise temperature in the case of precipitation is also investigated. A comparison between observed precipitation sky noise temperature and surface rainfall rate shows that a salient time shift in the two is seen, implying that great caution should be taken in establishing an empirical relationship between precipitation sky noise temperature and surface rainfall rate.

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