This talk presents the design of a through-canopy radar sensing system for fine-grained environmental monitoring beneath forest canopies. Conventional optical remote sensing cannot observe understory fuels, while SAR-based systems can penetrate vegetation but often provide limited vertical resolution in complex, moisture-rich forest structures. To address this gap, we propose a UAS-mounted canopy-penetrating radar system that communicates with passive RF tags deployed below the canopy. These tags act as maintenance-free ground references: they retro-reflect and modulate the radar signal, enabling the radar to distinguish tag responses from natural canopy clutter and recover information about the intervening vegetation and understory medium. I will discuss the core system design, including passive retro-reflective tag architecture, lightweight C-band FMCW radar integration, tag detection and localization, and physics-guided interpretation of backscatter signals for understory fuel and moisture characterization.