GNSS Transmissometry (GNSS-T): Modeling Propagation of GNSS Signals through Forest Canopy.

Mapping forest transmissivity on a large scale is needed for soil moisture and vegetation optical depth (VOD) calibration validation efforts led by passive microwave remote sensing missions. To this end, we recently introduced a Global Navigation Satellite System (GNSS) Transmissometry (GNSS-T) technique from a mobile platform to measure vegetation opacity directly using readily available GNSS signals, which assumes negligible ground multipath. In order to better assess the limitation of such an approach, our previously developed Signals of Opportunity (SoOp) Coherent Bistatic Scattering model (SCoBi) is modified to simulate first-order scattering contributions when the receiver is located above ground but below canopy. This paper describes the advancement of SCoBi from the case of a passive receiver overlooking vegetation to below-canopy upward receivers. This extension allows for fully polarimetric, complex simulations through evaluation of the coherent superposition of electric fields interacting within the canopy and with the forest floor. The simulation results shed light on errors associated with measurement configurations and site characteristics on the VOD measurements.