Radiometric observation of atmospheric influence on space-to-earth Ka-band propagation in Taiwan

This paper presents radiometric sensing of atmospheric influence on space-to-earth Ka-band propagation in Taiwan. This influence is quantified as attenuation inferred from the opacity of the sky, which, in turn, is derived from the brightness temperatures observed by a ground-based, dual-channel (23.8 and 31.4 GHz) radiometer through a simple conversion calculation. The sky brightness temperatures measured by the radiometer were obtained at Central Weather Bureau's Taipei Weather Station from March 18 to 24, 1998. The conversion calculation involves the use of mean radiating temperatures (T(mr)) at the frequencies of interest, which are approximated by means of multiple regression of surface pressure, temperature, and water vapor density in the current study. Radiosonde soundings collected at the Taipei Weather Station in each March from 1988 to 1997 provide dew point, temperature, and pressure profiles for the determination of regression. We first verify radiometric observations of zenith attenuation at 23.8 and 31.4 GHz by using concurrent radiosonde measurements. Subsequently, angular brightness temperatures of the sky obtained using the radiometer are utilized to characterize the spatial dynamics of space-to-earth Ka-band attenuation due to the presence of atmospheric constituents. During the one-week long campaign, averages of attenation obtained using the radiometer were 0.90 dB at 23.8 GHz and 0.41 dB at 31.8 GHz with biases of -0.28 and -0.29 dB, respectively, compared with radiosonde soundings. Root mean squared errors (RMSEs) between radiometer and radiosonde attenuation were 0.29 dB for 23.8 GHz and 0.43 dB for 31.4 GHz.