Estimation of tropospheric water vapor using differential attenuation at microwaves and comparison with other measurements

The measure of water vapor (WV) in the lowest part of the troposphere is a critical issue. Up to now, WV cannot be measured from space with sufficient accuracy. A novel technique at microwaves, called NDSA (Normalized Differential Spectral Attenuation) has been recently proposed to measure the Integrated Water Vapor (IWV) content along a path. The NDSA technique is based on the measure of differential attenuation undergone by two tone signals transmitted at closely spaced frequencies in the Ku-K band. To prove the potential of the NDSA technique, a prototype instrument has been designed and built. In 2024, a measurement campaign has been carried out with the NDSA prototype in a ground-to-ground configuration, with the objective to compare the IWV estimates provided by the NDSA prototype with those obtained using hygrometers, GNSS and Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) pointed along the same path as the NDSA prototype. The main results are shown below.