Estimating Vegetation Optical Depth With Mobile GNSS Transmissiometry in Temperate Forests During SMAPVEX22

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 in 2022 (SMAPVEX22). 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. We assessed the stability of direct signal measurements over multiple days, demonstrating the reliability of the GNSS-T measurements. We reported the VOD measurement results for various sites across different forest regions during intensive observation periods and evaluated their correlation with respect to in situ vegetation parameters such as basal area, biomass, canopy height, and diameter breast height, finding a strong correlation with the basal area ($R^{2}=0.73$). In addition, with a predictive regression model, we demonstrated a strong dependence of the measured VOD on combination of such forest parameters. An evaluation of the VOD values at different satellite elevation angles highlighted an increasing trend in VOD with the incidence angle. The results showed the potential utility of mobile GNSS-T for generating large-scale VOD observations. Although spatially averaged VOD maps might not be directly comparable to spaceborne observations, combining mobile GNSS-T data with other sensors such as LiDAR can provide a reliable reference for airborne or spaceborne VOD estimates.