WEIGHTED MEAN TROPOSPHERIC TEMPERATURE AND TRANSMITTANCE DETERMINATION AT MILLIMETER-WAVE FREQUENCIES FOR GROUND-BASED APPLICATIONS

A simple model for the radiometric determination of tropospheric trans mittance is based on an isothermal troposphere. In this model the key parameter is the weighted mean tropospheric temperature T-m, which cha racterizes the radiation and temperature properties of the troposphere . Statistical approaches in modeling this parameter are presented here by using ground temperature, ground relative humidity, and radiometer data. In order to determine the statistical coefficients for T-m mode ling and the parameters used in the transmittance retrieval algorithm, radiosonde data were used in a millimeter-wave propagation model for a site in the Swiss central plane and an Alpine site. Various observin g geometries at different millimeter-wave frequencies were considered. A determination of T-m from ground temperature was achieved with a rm s error between 4-5 K for the low-altitude site and 3-4 K for the high -altitude site. By incorporating relative humidity or radiometer data, an improvement of up to 25% relative to these values results, dependi ng on frequency and site. The zenith transmittance estimations far the low-altitude site with our best model have a rms error of 0.5% at 38 GHz, 1% at 94, 110, and 142 GHz, 1.5% at 115 GHz, 2% at 204 GHz, and 3 .5% at 279 GHz, whereas for the high-altitude site all rms errors are below 1%. The inclusion of radiometric information at 20 and 31 GHz di d not provide any additional improvement, which was confirmed by actua l measurements at 142 GHz.