A UGV-Based Forest Vegetation Optical Depth Mapping Using GNSS Signals

Abstract

In this study, we conducted an empirical investigation on mobile Global Navigation Satellite System (GNSS) Transmissometry (GNSS-T) measurements to explore vegetation optical depth (VOD). Our approach involved using a dualreceiver setup, with one receiver located in open terrain to capture direct signals as a reference and another deployed on an unmanned ground vehicle (UGV) to sample vegetation across expansive forested regions. Noteworthy findings reveal the negligible influence of ground multipath effects within these forested terrains, effectively resulting in sampling the forest canopy rather than the ground itself as the receiver moves. The UGV-based method also uncovers VOD fluctuations inside the forest, offering insights into spatial distribution and the influence of satellite position on VOD measurements. The study further examines the effect of tree heterogeneity and seasonal dynamics on the VOD estimates. This empirical study contributes to our understanding of the VOD mapping capabilities of the mobile GNSS-T approach and can potentially lead to non-intrusive quantification of Vegetation Water Content (VWC) at a landscape scale in forest terrains. These results are significant for advancing our knowledge of forest ecosystem dynamics and sustainable resource management.

Emerging Methods to Validate Remotely Sensed Vegetation Water Content

“…These GNSS VOD dynamics have been shown to correlate well seasonally with leaf water potential in a forest in Ozark, MO, USA (Yao et al, 2024) and diurnally with relative leaf water content in a forest in Pasadena, CA, USA (Humphrey & Frankenberg, 2023). Similarly, a GNSS sensor was mounted on an unmanned ground vehicle which traversed underneath pine and oak tree canopies, and the spatial distribution of the computed VOD matched that of LiDAR vegetation height retrievals from an aircraft (Ghosh et al, 2024). In a non-forested example, the GNSS VOD of two-meter high sunflowers in France were highly correlated with in-situ VWC measurements (Zribi et al, 2017).…”

“…First, GNSS can be used to directly measure VOD by positioning a GNSS sensor under a vegetation canopy and another above the canopy (Ghosh et al, 2024;Humphrey & Frankenberg, 2023;Yao et al, 2024). GNSS sensors take advantage of receiving L-band (1-2 GHz, like SMAP and SMOS satellites) signals from four main GNSS satellite constellations (e.g., one is global positioning system).…”