Illustration of GNSS-T Satellites transmitting to a GNSS Antenna above a forest canopy

The microwave-derived parameter Vegetation Optical Depth (VOD) captures the attenuation of microwaves that propagate through vegetation and serves as a proxy for both vegetation water content (VWC) and vegetation structure. Satellite-based VOD products from passive radiometers have proven valuable for monitoring large-scale vegetation dynamics, yet their coarse spatial and temporal resolution, potential retrieval gaps, and remaining uncertainties in physical interpretation limit their use for understanding forest resilience to drought and environmental change. Additionally, ground-based infrastructure for in-situ VOD observation remains scarce, limiting our ability to validate radiometer-derived products, to improve the fundamental understanding of microwave-vegetation interactions, and to resolve the sub-daily hydraulic dynamics that drive plant responses to environmental stress. At CLIMERS, we pursue GNSS Transmissometry as a tool to close this gap, providing continuous, directionally resolved in-situ VOD observations that serve both as independent ground truth for satellite products and as a means to advance the fundamental principles underlying VOD retrieval.

GNSS-T quantifies vegetation-induced attenuation of direct line-of-sight signals from the Global Navigation Satellite System (GNSS), including the US-American GPS , European Galileo, Russian GLONASS, and Chinese BeiDou. These signals are transmitted continuously at carrier frequencies within the L-band, the same microwave frequency range employed by spaceborne radiometers for VOD retrieval. A stationary GNSS-T setup consists of at least two receivers: a reference station with an unobstructed sky view, which provides the unattenuated baseline signal and accounts for factors such as satellite transmission power and slant-range path loss, and a below-canopy station where signals have been attenuated by the intervening vegetation. By comparing simultaneous Signal-to-Noise Ratio, or Carrier-to-Noise Density Ratio, measurements from the same satellite at both locations at the same time, canopy transmittance can be isolated and converted to VOD through a radiative transfer model. Because ground-based receivers simultaneously track dozens of satellites distributed across the sky hemisphere, each signal traverses the canopy along a distinct path, enabling GNSS-T to resolve both the temporal evolution and the directional structure of canopy attenuation.

Our current research employs multi-constellation, multi-frequency GNSS-T deployments across different forest biomes. We work towards resolving the influence of GNSS signal characteristics and their implications for VOD retrieval, advancing the fundamental understanding of microwave-vegetation interactions, developing radiative transfer models applicable to both in-situ and spaceborne retrievals, and enabling robust validation of satellite-derived VOD products.

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