Security screening system for large crowded venues such as stadium events and air and rail transportation hubs have been limited to slow, single person scanning. We present a 3D holographic imaging system that operates in the unlicensed 5.8 GHz ISM band and can screen crowds of people for large hidden threat objects. We further present a unique divide-and-conquer algorithm capable of efficiently processing the system’s voluminous data sets to search for suspicious objects. Current RF holographic screening systems have utilized expensive millimeter wave technology that required prolonged (multi-second) single-person scans, making them unsuitable for high throughput scenarios. While moving from millimeter waves to centimeter waves reduces the imaging resolution, it greatly reduces system cost, increases screening range, and increases screening penetration through obscurants (such as clothing) to better detect hidden threats in dense, unstructured crowds. Furthermore, our ad-hoc sensing array elements can be integrated into ceilings, floors, walls, and structural columns to facilitate screening anywhere it is needed. The imaging algorithm quickly locates scattering objects with a large volume by casting the imaging problem as a multi-resolution octree.
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