The SWAMM project: implementation of a system for estimating the tropospheric water vapor content along a transmitter-receiver link

Measuring water vapor (WV) in the troposphere, where almost the totality of WV is found, is a key point to understand atmospheric composition and dynamics.In particular, a critical problem is to perform systematic WV measurements in the lowest part of the troposphere (~5-6 km) on a global scale,which helps to improve both climate modeling and numerical weather prediction capabilities at short time scale. Recent studies proposed an innovative approach (the Normalized Differential Spectral Attenuation-NDSA) capable to retrieve the integrated water vapour (IWV) from attenuation measurements made in the Ku/K bands along microwave links crossing the troposphere, such as those connecting two low Earth orbiting (LEO) satellites. The 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 then directly converted into IWV.Two ESA studies provided an exhaustive insight into all theoretical aspects of the NDSA technique applied to the case of two counter-rotating LEO satellites. The purpose of the 2-years long SWAMM project, started in 2016 and funded by Tuscany Region,is the implementation of a NDSA demonstrator. An instrument able to perform NDSA measurements at 19 GHz between a transmitter at ground and a receiver onboard a stratospheric platform will be designed and developed. The solutions devised for the instrument, as well as the retrieval algorithms and the expected results in weather prediction chains will be discussed. Possible configurations for deployment of the NDSA receiver (or transmitter) onboard HAPS in combination with a transmitter at ground, high altitude platform or satellite will be also discussed.Specific focus will be given on the synergy with satellites, not only with regard to the combined use of data, but also, and with special emphasis, on the possibility of joint operations.