First Demonstration of Human-Portable Millimeter Wave SAR Imaging With Motion-Sensor-Free Autofocus for Search and Rescue Applications

Human-portable high-resolution millimeter-wave synthetic aperture radar (HR-mmW-SAR) enables effective imaging in confined or dynamic environments, with promising applications in search and rescue, security screening, and infrastructure inspection. However, the imaging performance of such systems is highly susceptible to motion-induced phase errors, as even slight manual perturbations can severely degrade image quality. Conventional motion compensation approaches typically rely on inertial navigation systems or inertial measurement units, which increase hardware complexity and cost. To address these limitations while preserving system simplicity and portability, this study presents the first demonstration of motion-sensor-free autofocus imaging in a human-portable HR-mmW-SAR system, enabled by a minimum entropy autofocus back-projection (MEA-BP) algorithm. The method estimates and corrects motion trajectory errors without the need for external motion sensors, by minimising image entropy through a coordinate descent strategy combined with efficient image-difference-guided updates. In contrast to existing autofocus methods developed for airborne SAR or those dependent on auxiliary sensors, MEA-BP directly addresses the unpredictable motion patterns associated with manually operated SAR platforms. Extensive simulations and real-world hardware experiments demonstrate that MEA-BP significantly improves image sharpness and suppresses motion-induced artefacts under practical use cases involving mild hand-induced perturbations. Furthermore, the algorithm maintains partial effectiveness under more challenging conditions, including abrupt motion stops in low signal-to-noise environments and short-duration rapid movements. These results confirm the robustness of the proposed method and establish its feasibility for compact, cost-sensitive, mobile SAR systems operating in real-world scenarios.