An Investigation of 19.5 GHz Sky Noise Temperature over Taiwan Area


A number of ground-based instruments, including 19.5 GHz radiometer, optical raingauge, portable weather station, and high resolution disdrometer, were set up to conduct the Ka band propagation experiment of the Experimental Communication Payload (ECP) for ROCSAT-1. In this article, 19.5 GHz background sky noise temperatures measured at Chung-Li and Tainan sites are presented and investigated. Long-term statistics of the 19.5 GHz background sky noice temperature observed by a vertically pointed radiometer in precipitation-free condition over the Taiwan area shows that the percentages of time that the sky noise temperature exceeds 20K, 30K, 40K and 50K are, respectively, 98%, 85%, 53%, and 27%. However, in precipitating environments, statistics shows that the percentage of time that the sky noise temperature exceeds 55K, 100K, 150K, and 200K are, respectively, 22%, 13%, 4.5%, and 2%. The statistics of sky noise temperatures observed at different zenith angles under environments without precipitation is also made. The results show that 80% of the observed sky noise temperatures at zenith angles of 10°, 30°, 50° and 70° are, respectively, in the ranges of 92 – 180K, 39 – 52K, 26 – 33K, and 21 – 27K. In addition, a comparison between surface rainfall rate recorded by the optical raingauge and sky noise temperature measured by 19.5 GHz radiometer shows that the former Jags behind the latter by about 5 minutes, implying non-uniform and inhomogeneous distribution of precipitation in the air. In order to measure the precipitation aloft. the Chung-Li VHF radar was operated simultaneously. Champaign observation shows that there is no latency between sky noise temperature and VHF backscatter from precipitation. This result implies that the VHF backscatter from precipitation can be employed to validate the observed sky noise temperature. In addition, we also find that the sky noise temperature may be as high as 155 K (corresponding to 3.6 dB attenuation) under an environment without surface precipitation. This feature is attributed to the dense water vapor and heavy cloud.


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