3-D Near-Field Passive Radar Imaging Using Multiple Illumination Sources

Near-field (NF) passive radar imaging depends on the illumination of the imaging scene by a non-cooperative transmitter (Tx). It is demonstrated that combining imaging results obtained with Tx antennas at different positions can enhance the performance of passive radar imaging. On the one hand, multiple Tx antennas provide diverse illumination perspectives, reducing the likelihood of unilluminated regions on the targets of interest (TOIs). On the other hand, the coherent summation of imaging results obtained for different illuminations helps to suppress potential artifacts. This approach is in particular advantageous for imaging complex objects with concave structures such as dihedral arrangements, where the ghosts due to multiple reflections are highly configuration-dependent. For each illuminating configuration, a single-frequency inverse source solver is utilized to reconstruct the equivalent sources of the TOIs and the resulting single-frequency images are then superimposed coherently with corresponding phase and magnitude correction methods. The obtained multi-frequency images are finally coherently combined to enhance the imaging quality. Both simulation and measurement results are presented to validate the effectiveness of the approach.

Near-Field Focusing Operators for Planar Multi-Static Microwave Imaging Using Back-Projection in the Spatial Domain

Based on a plane-wave expansion of the observation data in quasi-planar multi-static scattering scenarios, an improved formalism for image creation utilizing back-projection in the spatial domain is derived. The underlying integral expressions for different focusing operators are derived analytically leading to magnitude correction factors, which are mostly relevant for reconstructing microwave images when the distance from the scattering object to the aperture plane is small. It is shown that the derived imaging procedure is superior to the traditional back-projection only compensating the phase delay of the measurement signals and validate our findings based on simulated as well as measured data. Since the derived focusing operators correspond to a low-pass filtering of the spatial images, the resulting modified multi-static back-projection algorithms effectively suppress imaging artifacts as well.