Airborne Ultra-Wideband Microwave Radar for the Remote Sensing of Soil Moisture

Abstract

Soil moisture content is a parameter critical to the accuracy of hydrological models. Small Uninhabited Aircraft Systems (sUAS) can bridge the gap between crewed aerial missions and in situ measurements, providing higher spatial resolution than can be achieved using a crewed aircraft while still covering a large area. They can also offer fine-resolution data for operational applications, including forecasting floods and precision agriculture.A compact, ultra-linear microwave FMCW radar is designed and integrated with an sUAS to make precise, repeatable soil moisture measurements over a wide area. A multirotor aircraft enables flights at relatively low speeds, providing a stable platform that minimizes motion-related errors. A direct digital synthesizer (DDS) driven ultrawideband (UWB) voltage-controlled oscillator (VCO) in a phase-locked loop (PLL) enables the generation of a customizable chirp of up to 10 GHz bandwidth. The synthesizer generates a 2.5-6.5 GHz ultra-linear chirp for soil moisture measurements. An ultrawideband antenna provides the desired beam pattern and meets the bandwidth requirements.Laboratory tests show a near-ideal impulse response and ultra-linear chirp, improving the performance of the radar and enabling faster data processing. A field program conducted at two sites near Tuscaloosa, Alabama includes measurements made over several targets of interest, including bare and vegetation-covered areas. An automated flight control unit enables the aircraft to fly parallel lines and measure the same ground track at multiple angles of incidence.Trihedral corner reflectors are fabricated and characterized in an anechoic chamber and placed in the field to provide targets for estimating backscatter-response for different areas. Collected field data are range-compressed, motion compensated, and focused using a synthetic aperture radar algorithm. Backscatter estimates are generated from the data and compared with in situ measurements. The effects of vegetation over a target are investigated, and a method of estimating and correcting vegetation effects is proposed.The design and implementation of a mission operation and navigation interface for crewed remote sensing missions are discussed. The future of remotely-piloted remote sensing operations is discussed, with a trade study analysis to help select a platform for a given scenario.