Estimating vegetation optical depth with mobile GNSS transmissiometry in temperate forests during SMAPVEX’22

Authors: GHOSH, A - University Of Georgia; FARHADI, M - University Of Georgia; HOQUE, M - University Of Georgia; BOYD, D - Mississippi State University; BOURGEAU-CHAVEZ, L - Michigan Technological University; Cosh, Michael; COLLIANDER, A - California Institute Of Technology; KURUM, M - University Of Georgia

Submitted to: IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/3/2025
Publication Date: 2/13/2025
Citation: Ghosh, A., Farhadi, M., Hoque, M.E., Boyd, D., Bourgeau-Chavez, L., Cosh, M.H., Colliander, A., Kurum, M. 2025. Estimating vegetation optical depth with mobile GNSS transmissiometry in temperate forests during SMAPVEX’22. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing. 18:6451-6463. https://doi.org/10.1109/JSTARS.2025.3541182.
DOI: https://doi.org/10.1109/JSTARS.2025.3541182

Interpretive Summary: Navigation systems which are based on satellite signals operate at a scientifically interesting frequency. This L-band frequency is sensitive to water in liquid state, which include soil moisture vegetation water content. The attenuation or loss of strength of the signal for navigation through a forest canopy can be directly related to the amount of water in the canopy. A study was conducted to deploy a navigation system through a variety of forested domains in the northeastern U.S. and compare the canopy water content estimates to physically measured canopy water contents and demonstrated the feasibility of this approach. This is valuable to foresters and climate scientists studying water storage in forested domains.

Technical Abstract: This study investigates the potential of mobile Global Navigation Satellite System (GNSS) Transmissometry (GNSS-T) measurements for estimating Vegetation Optical Depth (VOD) in temperate forests, focusing on the Soil Moisture Active Passive (SMAP) Validation Experiment 2022 (SMAPVEX22). Two intense observation periods (IOPs) were conducted in the northeastern United States, targeting fully wooded and partially forested areas in Massachusetts and New York. Our methodology employed a dual-GNSS receiver setup, with one receiver positioned in open terrain to serve as a reference for direct signals, and another deployed on a mobile unit (helmet-based or robotic system) to spatially sample vegetation across expansive forested regions during SMAPVEX22. First, the stability of direct signal measurements over multiple days was assessed, demonstrating the reliability of the GNSS measurements. Subsequently, we validated the VOD measurements against in situ vegetation parameters, and found a strong correlation between basal area and VOD estimates. In addition, with a predictive regression model, a strong dependency of measured VOD data on the comprehensive structure of the forest is demonstrated. We also evaluated VOD values for different satellite elevation angles, highlighting the impact of elevation angle on the measurements. Finally, we reported VOD measurement results for various stations across different forest regions at SMAPVEX22 field campaign. Our study showed the feasibility of utilizing mobile GNSS-T for generating large-scale VOD maps. Such spatially averaged VOD maps can serve as the ground truth for airborne or spaceborne VOD estimates.