Achieving optical imaging beyond the diffraction limit imposed by a single finite aperture remains an important objective in astronomy, remote sensing, and long-range observation. In the optical regime, however, synthetic aperture imaging has remained difficult because phase information is not directly accessible to standard detectors. Macroscopic Fourier ptychography has provided a promising computational route toward this goal, yet its reliance on mechanical scanning or engineered illumination has largely confined it to cooperative and static scenes.
Inverse synthetic aperture macroscopic Fourier ptychography (ISAMFP) addresses this limitation by rethinking the role of motion within the Fourier ptychographic framework. Rather than treating target motion as a source of blur, the method exploits natural motion as a source of angular diversity for synthetic aperture reconstruction. For rough diffusely reflecting targets, motion also introduces phase modulation that varies with viewing angle, redistributes spectral content, and creates additional diversity for recovery. Through this mechanism, ISAMFP avoids mechanical scanning and illumination modulation while enabling non-interferometric reconstruction of high-frequency information from sub-aperture intensity measurements.
"What makes this framework distinctive is that diffuse reflection from rough target surfaces does not merely produce speckle; it also introduces motion-dependent phase modulation," said Professor Chao Zuo, the corresponding author of the study. "As the target moves, that modulation changes with viewing angle and generates additional spectral diversity, which allows fine structural details to be recovered from intensity-only measurements without mechanical scanning."
The team validated ISAMFP with a prototype system across three representative scenarios: a structured-motion USAF resolution target, a flying UAV, and complex multi-target scenes. In the structured-motion experiments, ISAMFP achieved up to a four-fold resolution enhancement beyond the diffraction limit of a single aperture. In the UAV experiment, image contrast increased from 0.086 in a single frame to 0.415 after multi-frame synthesis, whereas a simple average of 160 frames reached only 0.115. These results indicate that ISAMFP can recover structural details that remain unresolved in conventional single-frame imaging and are still absent after straightforward frame averaging.
"As a non-interferometric and intensity-based method, ISAMFP relaxes the stringent requirements of heterodyne detection and long-term phase stability that usually constrain inverse synthetic aperture optical imaging," Zuo said. "We believe this framework provides a practical route toward long-range, high-resolution imaging of dynamic, non-cooperative targets."
More broadly, ISAMFP extends inverse synthetic aperture imaging to moving, non-cooperative targets within the Fourier ptychographic framework, expanding the scope of synthetic aperture imaging in the optical domain. The method may prove valuable in long-range surveillance, target tracking, and high-resolution remote sensing. Future advances in high-frame-rate sensors, synchronized camera arrays, and motion estimation may further improve spectral coverage, reconstruction speed, and robustness, supporting its practical use in dynamic imaging scenarios.
