System concepts for multistatic UAV-borne synthetic aperture radar

Abstract

Climate change and environmental monitoring present some of the most urgent scientific and societal challenges of our time. Remote sensing systems, particularly those deployed on UAVs, play a crucial role in addressing these challenges by enabling high-resolution observations of dynamic and often inaccessible regions. SAR is especially valuable for such applications due to its ability to penetrate cloud cover and, in certain cases, subsurface structures like glacial ice. UAV-borne SAR systems have shown promise in fields ranging from glacier monitoring and snow depth estimation to disaster response, archaeology, and infrastructure inspection. However, the full potential of these systems — especially in terms of spatial diversity and imaging flexibility — can only be unlocked by moving beyond conventional monostatic configurations to multistatic geometries. This thesis presents novel system concepts for multistatic SAR imaging using UAVs, addressing the limitations of current monostatic UAV-borne radar systems. While monostatic SAR has proven effective for high-resolution imaging in various remote sensing applications, its extension to multistatic configurations remains challenging due to increased complexity in synchronization, signal processing, and formation design. To overcome these challenges, three scalable and coherent multistatic SAR concepts are developed and investigated: a radar repeater network, a digital receive-only system, and a fully digital OFDM-based configuration. These concepts aim to enable flexible SAR imaging with multiple distributed UAVs while maintaining coherency in time, frequency, and phase — critical requirements for successful SAR image formation. Each concept is experimentally validated through a thorough coherency analysis and UAV-based measurements. The radar repeater network ensures coherency by retransmitting the radar signal from a repeater node. This configuration enables three-dimensional object localization by leveraging the triangular propagation path formed between the transmitter, target, and repeater. While effective, the approach is limited by maximum amplification and reduced scalability. To address these limitations, a digital receive-only system is introduced. It relies on receiving radar and sidelink signals and utilizes a novel digital demodulation scheme to separate and coherently process the multistatic radar returns. FMCW waveforms are employed, and the system is validated through experimental measurements demonstrating coherent image reconstruction and accurate object localization in a bistatic setup. The third concept employ the advantages of a digital OFDM waveform in a fully digital architecture. Independent radar nodes are triggered digitally and operate without shared analog references. Coherency across nodes is achieved through precise digital signal design and calibration. Experimental results confirm that the approach enables coherent monostatic, bistatic, and multistatic SAR imaging, offering the highest flexibility among the proposed concepts. Beyond the system development, the thesis analyzes the impact of multistatic geometry on spatial resolution, PSF characteristics, and sensitivity to localization errors. Various bistatic formations — such as tandem, tomographic, and forward scattering — are evaluated for their imaging capabilities. Results show that multistatic configurations offer enhanced imaging performance, geometric diversity, and application potential, especially in environmental monitoring and climate research. Overall, this work demonstrates the feasibility of flexible, scalable, and coherent multistatic SAR imaging using UAVs. The developed system concepts and experimental validations lay the groundwork for next-generation radar networks capable of operating in complex environments and contributing to essential sensing applications.