Ultra-Wideband Radar for Drone-based Soil Moisture Measurements

Abstract:

Soil moisture is a key variable that controls land-atmosphere interactions in hydrological models. Soil moisture levels control evaporation, runoff and infiltration processes. Microwave sensors operating in 1-6 GHz range have been effective in measuring surface soil moisture. Global measurements of soil moisture with passive microwave sensors have also been demonstrated. However, the resolution of satellite-based passive microwave sensors is poor, and retrieval of soil moisture information over inhomogeneous terrain is difficult. There is a need for fine-resolution soil moisture measurements to support both scientific modeling research and operational applications.

The Remote Sensing Center at the University of Alabama has developed a compact, ultra-wideband microwave radar operating over the frequency range of 2-6 GHz drone-based soil moisture measurements. We designed and developed a flexible radar system that allows us to easily change the frequency of operation. A direct digital synthesizer (DDS) driven ultrawideband voltage-controlled oscillator (VCO) with phase-locked loop (PLL) enables us to generate an ultra-linear chirp of up to 10 GHz bandwidth. We have also developed a custom ultrawideband antenna to provide the desired beam pattern and meet the bandwidth requirements. We designed the system to collect data over incidence angles between 0 and 50 degrees with adequate signal-to-noise ratio. The radar performance is verified using simulated targets in the laboratory, and the measured impulse response is close to the theoretical. We have integrated the radar with a hybrid multirotor unmanned aircraft system (UAS) that can be operated within FAA limits. The long endurance and low speed capability of the hybrid multirotor aircraft minimizes motion-related errors. In August 2020, we will conduct a field campaign and collect data at test sites near Tuscaloosa, AL. Radar measurements will be calibrated and validated against detailed in-situ measurements collected using soil moisture probes. In this paper, we will discuss the design and development of radar, measurements, and the field results collected from bare and vegetation-covered soils.