Surface Soil Moisture Retrieval using Reflectometry of S-band Signals of Opportunities

Abstract

Surface soil moisture is one of the few direct hydrological variables which can be measured. It plays a crucial part in the water cycle, agriculture, drought development, runoff generation, and many other phenomena. Satellite observations from active and passive microwave radiometers are best suited for the retrieval of soil moisture. The relationship between soil dielectric constant and water content is direct and is used to determine the surface soil moisture levels. Active microwave remote sensing techniques measure the energy reflected from target surfaces (ocean, soil, biomass) after transmitting a pulse of microwave energy, whereas passive microwave sensors measure the self-emissions of the target surfaces. The passive missions by ESA's SMOS and NASA's SMAP have demonstrated this technology for remote sensing on a global scale. Global Navigation Satellite System-Reflectometry (GNSS-R) is an alternative approach to the remote sensing of soil moisture, as demonstrated through several airborne and ground-based experiments. The new technique of Signals of Opportunity (SoOp) uses a bistatic radar configuration in which the non-cooperative transmitter already transmits signals designed for communication or navigation. The receiver reuses the reflected energy from the target surface (ocean, soil, biomass), thereby making the digital communication and navigation signal spectrum useful to the remote sensing science community. Several airborne and ground-based experiments have been conducted on the use of digital communication signals, a range of frequencies from P-band to Ku-band, for measurement of ocean surface roughness, wind speed, and soil moisture. This thesis presents the preliminary results obtained for reflectivity retrievals for the 2017 and 2018 S-band tower-based SoOp field experiment conducted at Purdue's Agronomy Center for Research and Education (ACRE). XM signals were recorded by a sky-facing antenna and an Earth-facing antenna mounted atop a tower. The line-of-sight (direct) signal is captured by the sky-facing antenna and reflected signal from the soil captured by the Earth-facing antenna was used for the ambiguity function of XM transmission. A link budget was used to determine the received signal to noise ratio (SNR). The cross-correlation between the direct and the reflected XM signals was used to estimate reflectivity values. The reflectivity retrievals were compared with the in-situ soil moisture content at 5 cm depth obtained by the HydraProbes. The reflectivity values were also verified by a Signals of Opportunity (SoOp) Coherent Bistatic (SCoBi) simulated model.