Abstract
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° inclination 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 ground-based 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°, 60°, 30°, and 15° are 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.
Highlights
According to the national fifteen-year space program implemented by the National Space Program Office (NSPO), a low-earth orbit (LEO) experimental satellite, ROCSAT-1, is scheduled to be launched at the end of January 1999
The results indicate a latitudinal variation in the percentage of the times that the rainfall rate exceeds a certain level; it is larger in the southern part of Taiwan and smaller in the northern part of Taiwan
The data employed for the long-term statistical analysis are the rainfall rates at one-minute time resolution collected by more than 70 tipping bucket rain gauges distributed over Taiwan island from 1988 to 1995
Summary
According to the national fifteen-year space program implemented by the National Space Program Office (NSPO), a low-earth orbit (LEO) experimental satellite, ROCSAT-1, is scheduled to be launched at the end of January 1999. In order to understand the characteristics of the propagation channel at Ka band over the Taiwan area for the purpose of designing an optimal earth-satellite communication link, the atmospheric effects influencing the quality of satellite communication mentioned above should be investigated thoroughly. The results indicate a latitudinal variation in the percentage of the times that the rainfall rate exceeds a certain level; it is larger in the southern part of Taiwan and smaller in the northern part of Taiwan This feature implies that the rain attenuation effect on Ka band propagation is more severe in the southern part than in the northern part of the island. This article compares the rainfall rate measured by a high performance optical rain gauge and the sky noise temperature observed by a radiometer centered at 19.5 GHz
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