An Automatic Focusing Technique for Inverse Source Reconstructions Based Near-Field Passive Radar Imaging

An autofocusing phase correction method for 3D near- field (NF) passive radar imaging is presented for scenarios where the direct signal is absent. Phase shifts caused by delays from measurement instrumentation and spatial propagation are compensated separately. A single-frequency inverse source solver is used to reconstruct spectral representations of equivalent surface sources for the targets of interest (TOIs) from the captured scattering fields in NF measurements. These equivalent sources are then used to generate single-frequency images, from which the phase information of the induced sources is extracted for phase correction and source localization. Normalizing the induced sources within the target region to a specific, though arbitrary reference source, compensates for the spatially invariant part of the phase shifts. Additionally, analyzing the time-domain signals of the normalized induced sources provides the time differences of arrival (TDoA) from the Tx to different anchors, enabling Tx source localization. Once the Tx position has been determined, spatially varying phase shifts are corrected. Simulation and measurement results are presented to validate the feasibility and effectiveness of this approach.