What is GNSS-T VOD?
The understanding of electromagnetic wave interaction with vegetation canopies is a fundamental research question of biosphere remote sensing until today. The remote sensing community is still waiting for a well representative calibration and validation network for this interaction.
VODnet is a potential answer. VODnet is a collection of in situ sites that operate ground-based GNSS Transmissometry (GNSS-T) set ups to measure in hyper-temporal resolution the vegetation optical depth (VOD). VOD is a wave-medium interaction parameter and contains information on the biomass, water (internal & external) and structure of canopies. It is also directly retrievable from radiative transfer model (RTM)-based analyses of active or passive microwave remote sensing data.
Hence, for the first time VODnet enables the direct comparison of VOD from in situ sites across a diversity of biomes with collocated remote sensing data. This will foster algorithm development for estimation of biomass (dry & wet), water content / potential, interception & dew as well as structure and shape of canopies. This means VODnet creates real-world field laboratories on assessing the canopy information (states & dynamics) contained in VOD.
Instrumentation
Across the network, a range of multi-frequency (L1/L2/L5), multi-constellation (GPS, GLONASS, Galileo, Beidou) GNSS receiver chips are in use. They range from low-cost (ublox ZED-F9P) to survey-grade (Septentrio mosaic-X5). Typically, survey-grade antennas are used.
Recommended Reads
- Feldman, A. F. (2024). Emerging Methods to Validate Remotely Sensed Vegetation Water Content. Geophysical Research Letters, 51(14), e2024GL110505. https://doi.org/10.1029/2024GL110505
- Jagdhuber, T., Schmidt, A. S., Fluhrer, A., Chaparro, D., Jonard, F., Piles, M., ... & Kunstmann, H. (2025). Estimation of Forest Water Potential from Ground-Based L-band Radiometry. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing. https://doi.org/10.1109/JSTARS.2025.3533567
- Hellwig, F. M., Jagdhuber, T., Jonard, F., Fluhrer, A., Schellenberg, K., Zehner, M., ... & Kunstmann, H. (2025). Assessing C-band SAR-based VOD in forest ecosystems using physical scattering models (No. EGU25-12323). Copernicus Meetings.
- Humphrey, V., & Frankenberg, C. (2023). Continuous ground monitoring of vegetation optical depth and water content with GPS signals. Biogeosciences, 20(9), 1789–1811. https://doi.org/10.5194/bg-20-1789-2023
- Humphrey, V. (2025): GNSSVOD toolkit. https://github.com/vincenthumphrey/gnssvod
- Schellenberg, K., Jagdhuber, T., Chaparro, D., Binks, O., Hellwig, F., Dubois, C., ... & Schmullius, C. (2024, July). Estimating canopy interception water storage with GNSS-transmissometry. In IGARSS 2024-2024 IEEE International Geoscience and Remote Sensing Symposium (pp. 4507-4510). IEEE.