Depth Camera Motion Tracking for Motion Compensation and ISAR Imaging in Walk-Through Security Scanners

This paper investigates the fusion of radar and depth camera for maintaining high-quality radar images in walk-through security scanners with significantly reduced array geometries. Therefore, after sensor calibration, the reconstruction volume is initially divided into sub-volumes and linked to the closest body part of a walking person being scanned. The depth camera's motion tracking provides the location and orientation of each body joint during the gait, allowing us to determine their trajectory and velocity. This information is then used to compensate for movement in the image reconstruction of each recorded radar frame. Afterwards, the motion compensated images are then superimposed to one ISAR image, considering the complete trajectory of the person. We conducted measurements with a $3.6 \,\mathrm{G}\mathrm{Hz}$-to-$10.6 \,\mathrm{G}\mathrm{Hz}$ walk-through security scanner and a real person carrying various objects. Motion tracking with a depth camera shows promising results for both imaging approaches. In the motion-compensated images, small details became visible and focused compared to reconstruction results without applied motion compensation. The ISAR approach demonstrated effective alignment of individual frames into one superimposed image, producing a full-body image even with a reduced array dimension. While these results prove the benefit of the fusion of radar and depth camera, further investigations into array design are necessary, as object visibility is strongly affected by the selected array geometry for ISAR imaging. This fact could complicate the reliable detection of potential threat objects with a downsized imaging array.