A measurement campaign to compare the normalized differential spectral attenuation (NDSA) method for integrated water vapor estimation with other techniques

Accurate measurement of water vapor (WV) in the troposphere is essential for improving climate models and numerical weather prediction. However, systematic observations, particularly in the lower troposphere (below ≃ 6km), remain challenging. To address this issue, an innovative technique called Normalized Differential Spectral Attenuation (NDSA) has been developed. NDSA retrieves integrated water vapor (IWV) by analyzing spectral sensitivity (S) from attenuation measurements in the 17 to 21GHz band, which can be directly linked to IWV. Previous studies funded by the European Space Agency demonstrated the feasibility of this method using Low Earth Orbit satellite links. More recently, a prototype NDSA instrument operating at 19GHz developed during the SATCROSS project and supported by the Italian Space Agency, was deployed along a terrestrial link. In 2024, a four-month campaign was carried out to compare the IWV estimates with those retrieved from Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) in the 430 to 490nm range, GNSS meteorological stations, and hygrometers. The measurements were conducted over a 91km link between St. Pietro Capofiume (10m a.s.l.) and Mt. Cimone (2165m a.s.l.). The results highlight the potential of NDSA retrieval, strengthening the case for future space-based missions.