Development of A UAS-Based Ultra-Wideband Radar for Fine-Resolution Soil Moisture Measurements

Soil moisture plays a key role in land-atmosphere interactions and is an important variable in hydrological modeling. Satellite-based microwave sensors have been demonstrated for estimating surface soil moisture, but the spatial resolution is poor. Retrieval of soil moisture over inhomogeneous terrain is difficult. There is a need for fine-resolution soil moisture data 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 for airborne soil moisture measurements. A highly linear, highly configurable chirp is generated by a direct digital synthesizer (DDS) driven voltage-controlled oscillator (VCO) with phase-locked loop (PLL). This system enables us to generate an ultra-linear chirp of up to 10 GHz bandwidth. We have also developed a custom ultrawideband antenna to collect data over incidence angles between 0 and 50 degrees with an adequate signal-to-noise ratio and meet the bandwidth requirements. The radar performance is verified using simulated targets in the laboratory, and the measured impulse response is close to the ideal response.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. We have operated the radar and collected data at test sites near Tuscaloosa, AL. Radar measurements are being calibrated and validated against detailed in-situ measurements collected using soil moisture probes. In this paper, we will discuss the design and development of the radar, measurements, and field results.