The SATCROSS Measurement Campaign and its results: Application of the NDSA technique to a Ground-to-Ground Radio Link

The measure of Water Vapor (WV) in the lower troposphere is a critical issue, which still leaves important margins for improvements of observational data quality, as necessary for instance to enhance the forecast performance of Numerical Weather Prediction systems. To address this problem, a novel measurement technique for obtaining the Integrated Water Vapor (IWV) along a microwave link and based on a pair of attenuation measurements has been proposed years ago. This method is called NDSA (Normalized Differential Spectral Attenuation), and is based on measuring the "spectral sensitivity", namely the normalized incremental ratio of the spectral attenuation, that was found to be linearly related to the IWV along the radio-link path operating in the Ku and K frequency band. Some ESA studies have shown the NDSA capability to effectively estimate the IWV along the path between two counter-rotating Low Earth Orbit (LEO) satellites - one carrying a transmitter, the other a receiver - in a limb measurement geometry. The resulting link passes through the atmospheric limb crossing the lower troposphere with tangent heights smaller than 10 km, but involves higher tropospheric layers also. Currently, the the Italian Space Agency is supporting a project named SATCROSS, whose purpose is to provide a pre-feasibility study for a different measurement concept, as the number of LEO satellites is higher than two and they are orbiting in the same plane and along the same direction In fact, the SATCROSS project investigates the possibility to retrieve two-dimensional (2D) water vapor fields using a train of such co-rotating LEO satellites , displaced so that the links connecting the transmitting satellites to the receiving ones scan an annular sector of the troposphere, where appropriate tomographic inversion techniques can be applied to retrieve the 2D fields from the available set of IWV measurements obtained through the NDSA technique . SATCROSS comprised several activities, from the simulation of signals and of two-dimensional algorithms to the characterization of a real Cubesat mission payload. This presentation focuses on one particular activity: an instrument prototype operating in a ground-to-ground link configuration and the development of a measurement campaign. The prototype is an upgrade of a previous one and is designed and implemented to demonstrate the effectiveness of the NDSA measurements at 19 GHz. We will report and discuss the instrument's architecture, the road-map of this experimental activity, and the insights brought by the the analysis of the data obtained from the measurement campaign. In particular, we will focus on the sensitivity of the NDSA measurements to the IWV by comparing it with existing datasets of Relative Humidity (RH) and IWV measurements obtained from other consolidated measurement techniques and instruments.