First insights from a Europe-wide GNSS transmissometry network: improved retrieval of sub-daily canopy water status

The resilience of terrestrial ecosystems to drought and heat stress is a key control on the future terrestrial carbon sink. Plant hydraulic traits such as conductivity and capacitance provide useful indicators of this resilience. These traits can be inferred from sub-daily vegetation water dynamics, yet rapid changes in vegetation water content remain poorly observed and are therefore weakly represented in land-surface models due to the lack of suitable measurements at temporal and spatial scales.

 Microwave remote sensing is a promising tool to monitor diurnal variations in vegetation water content at scale, but no existing or planned satellite missions can currently resolve these dynamics at the sub-daily resolution. To address this critical observation and knowledge gap, SLAINTE has been proposed as an ESA New Earth Observation Mission Idea, comprising a constellation of identical, decametric, monostatic SAR instruments designed to capture sub-daily variations in vegetation water storage, plant water potential, and surface soil moisture (Steele-Dunne et al., 2024; Matar et al., 2024).

One of the challenges in mission development is the lack of sub-daily microwave data across ecosystems. To answer this need of characterizing variability, a GNSS transmissometry (GNSS-T) network has been set up across Europe. GNSS-T compares the signal to noise ratio (SNR) at two GNSS receivers, one above and one below the vegetation canopy, to estimate L-band attenuation, also known as vegetation optical depth (VOD) (Humphrey et al., 2023).

This GNSS-T network, consisting of 21 GNSS receivers across 9 different forest research sites in Europe, has been installed between 05/2025 and 09/2025, building on a pilot installation set up at our Dutch site one year earlier. Data are collected automatically, processed in near real-time, and prepared for dissemination through a central data server, which enables continuous monitoring for data gaps and basic quality checks.

We will discuss lessons learned and methodological advances from installation and operation of the network in sites with variable conditions. The insights gained from constructing an hourly VOD timeseries from high-frequency SNR signals will be presented. Building on the original methodology by Humphrey et al. (2023), we explore sensitivity to GNSS-related artefacts in order to yield robust diurnal signals across sites. These first results demonstrate the feasibility of GNSS-T for monitoring sub-daily vegetation water dynamics.

The established processing workflow, together with the extensive network, paves the way to linking GNSS-derived vegetation water content with plant hydraulic models to infer ecosystem-scale hydraulic traits from a microwave observable.

 

References:

Humphrey, V., & Frankenberg, C. (2023). Continuous ground monitoring of vegetation optical depth and water content with GPS signals. Biogeosciences, 20(1), 1789–1811. https://doi.org/10.5194/bg-20-1789-2023

Matar, J., Sanjuan-Ferrer, M. J., Rodriguez-Cassola M., Steele-Dunne, S. & De Zan, F. (2024). A Concept for an Interferometric SAR Mission with Sub-daily Revisit. EUSAR 2024; 15th European Conference on Synthetic Aperture Radar, pp. 18-22. IEEE, 2024.

Steele-Dunne, S., Basto, A., De Zan, F., Dorigo, W., Lhermitte, S., Massari, C., Matar J. et al. (2024) SLAINTE: A SAR mission concept for sub-daily microwave remote sensing of vegetation. EUSAR 2024; 15th European Conference on Synthetic Aperture Radar, pp. 870-872. VDE, 2024.