Electromagnetic modeling of scattered signals of opportunity: challenges and approaches

he possibility of exploiting signals of opportunity such as navigation signals for remote-sensing applications has been the object of extensive scientific research. Over the years, the potential of this technique has been mainly investigated for wind scatterometry and sea-surface altimetry, resulting in mission concepts and prototypes currently in orbit or under study. More recently, the potential of the Global Navigation Satellite System (GNSS) has attracted significant scientific and industrial interest, especially for land applications. The possibility of retrieving soil moisture and vegetation biomass by means of GNSS reflections, originally demonstrated through theoretical models and simulations, has been confirmed through the analysis of ground-based and airborne measurements. Even if these campaigns significantly contributed to consolidating the physics that is behind the interaction between navigation signals and some important geophysical parameters of the illuminated surface, a complete understanding of the experiments still needs further scientific efforts, especially for satellite observations. In this work, we review the main progress that has recently been made at two universities of Rome, Sapienza and Tor Vergata, as well as by some other research groups. Current challenges and modeling approaches are summarized, focusing the attention on the potential offered by GNSS reflectometry with respect to more established passive and active remote-sensing techniques. An electromagnetic model and the corresponding numerical simulator designed to characterize the field scattered under bistatic illumination and for the study of the signal at the receiver generated by sources of opportunity are reviewed and summarized.