| << | Profiling of Water Vapour and Temperature | >> |
Abstract
Gaseous attenuation of electromagnetic waves can influence the remote-sensing or telecommunication radio systems operating on slant propagation paths through the troposphere. Absorption by molecular oxygen and absorption by water vapour form the most significant parts of total gaseous attenuation for frequencies 1 – 350 GHz. Fluctuations of water vapour density profile in the troposphere cause the changes in attenuation and so determine the statistical characteristics of attenuation. The water vapour profiles as well as the profiles of other physical quantities of the air are retrieved from standard radiosonde measurements or they can be estimated from microwave radiometer measurements. Radiometric profiling has an advantage in greater temporal resolution, but it usually suffers from the lower spatial resolution of estimated profiles compared with radiosondes. In order to obtain the reliable estimates of gaseous attenuation, the errors inherent in radiometric profiling have to be assessed.
Specific gaseous attenuation along the slant propagation path is calculated using the procedure defined by ITU-R. The statistics (cumulative distribution function, CDF) of estimated total gaseous attenuation are studied on the base of vertical profiles obtained from both radiosonde and radiometer data measured during three months in summer 2008. The radiosondes are launched at Libus meteorological station in Prague, the dual band radiometer TP/WVP-3000 operates at Geodetic Observatory Pecny which is located about 30 km south-east from Prague. The differences between gaseous attenuation CDFs obtained from vertical profiles measured by radiosonde and from vertical profiles estimated from the radiometer measurements are analyzed. Statistic dependence on the frequency is revealed