A novel method to estimate the tropospheric water vapor along a transmitter-receiver link: theory and first results

Measuring water vapor (WV) in the lowest troposphere (below 10 km) from space with sufficient accuracy is a still critical issue. However, it is well recognized WV measurements would help improving both climate modeling and numerical weather prediction capabilities at short time scale. To solve this problem, a novel approach (Normalized Differential Spectral Attenuation-NDSA) has been proposed, based on attenuation measurements made at Ku/K bands and capable to estimate the integrated water vapour (IWV) along microwave links crossing the troposphere. NDSA is based on the estimate of a parameter (called spectral sensitivity, S) related to the differential attenuation undergone by a pair of tone signals separated by a fractional band of less than 2%. It was demonstrated that S can be directly converted into IWV through linear relationships. Research studies, funded by ESA, provided an exhaustive insight into all theoretical aspects of the NDSA technique from space, but it still needs to be demonstrated with real measurements before moving toward space applications. The main goal of a recent project (SWAMM-Sounding Water Vapour by Attenuation Microwave Measurements), funded by Regione Toscana - Italy is to develop a low-cost radio link to perform the first NDSA measurements. Within the project the instrument is designed and built and preliminary tests based on a ground-to-ground link have been recently conducted to validate the concept. In this tests the transmitter was places on a mountain (around 200 m a.s.l.) and the receiver at about sea level for a link of around 80 Km. Project results, first measurements, retrieval tests and perspectives for moving toward a space configuration will be presented at the conference.