Microwave remote sensing of soil moisture, above ground biomass and freeze-thaw dynamic: Modeling and empirical approaches

Human actions have accelerated changes in global temperature, precipitation patterns, and other crit-ical Earth systems. Key markers of these changes can be linked to the dynamic of Essential Climate Variables (ECVs)and related measures, such as Soil Moisture (SM), Above Ground Biomass (AGB), and Freeze-Thaw (FT) Dynamics.ECVs are crucial for understanding global climate changes, including hydrological and carbon cycles. Moreover, mon-itoring ECVs helps to validate climate models and inform policy decisions. Monitoring activities can be carried out at aglobal scale by using technologies like microwave remote sensing. However, other than proper technological develop-ments, the study of ECVs requires suitable theoretical retrieval tools, which leads to the solutions of inverse problems.In this survey, we analyze and summarize the main retrieval techniques available in the literature for SM, AGB, andFT, performed on data collected with microwave remote sensing sensors. Furthermore, we present the projectRETINA(REmote sensing daTa INversion with multivariate functional modeling for essential climAte variables characterization), recentlyfunded by the European Union under the Italian National Recovery and Resilience Plan of NextGenerationEU, underthe Italian Ministry of University and Research. The main goal ofRETINA, is to create innovative techniques foranalyzing data generated by the interaction of electromagnetic waves with the Earth’s surface, applying theoreticalinsights to address real-world challenges