A Novel Passive Millimeter Imager for Broad-Area Search - Final Report on Project PL09-NPMI-PD07 (PNNL-55180)

Abstract

SummaryThis report describes research and development efforts toward a novel passive millimeter-wave (mmwave) electromagnetic imaging device for broad-area search. It addresses the technical challenge of detecting anomalies that occupy a small fraction of a pixel. The purpose of the imager is to pinpoint suspicious locations for cuing subsequent higher-resolution imaging. The technical basis for the approach is to exploit thermal and polarization anomalies that distinguish man-made features from natural features. The scale of applicability ranges from several meters to hundreds of kilometers.This project began by building two research-grade passive mm-wave polarimeters for studying anomalies in ambient scene thermal emissions. The first system consisted of a full polarimeter to take single-channel imaging data using a 2D elevation and azimuthal steerable scanner. It was conservatively designed as a balanced Dicke switching radiometer to obtain reliable phenomenological data from the field. It was later adapted into a differential unbalanced Dicke radiometer that performed comparative radiance and polarimetry measurements between two closely aligned but slightly different views of a common scene. This differential radiometer afforded new insights into the small but real polarimetric changes that complex scenes can display as a result of man-made disturbances or, from reflective or angular materials that reflect/scatter efficiently the cold overhead skyshine into the field-of-view (FOV) of a down-looking imaging radiometer. The results were surprisingly devoid of scene clutter from natural scattering and reflective surfaces, presumably because of the random raking, fractal-like textures of natural surfaces. In contrast, surfaces or materials disturbed by non-random actions, such as human foot traffic over grass, produced notable radiometric anomalies. Subsequently, we designed and built a larger differential imager using a two-aperture binocular system to give better angular resolution. This system worked remarkably well and has provided surprisingly crisp differential scene data that appears to substantiate an important thesis of this proposed work: that differential thermal imagery using a large baseline offset between two imaging telescopes could generate sub-Rayleigh limit spatial resolution of small anisotropies induced into their emitted or scattered radiation. Demonstrated capabilities during the project include: We were able to provide a complete and sensitive Stokes analysis of the mm-wave radiation. All radiation has two vector quantities that completely describe the amplitude and phase of the E-and B-fields, and the changes of these field quantities can be quantified as a four-component vector known as the Stokes vector. Rapid analysis of this vector (I, Q, U, V) can be made with full polarimeters, and we carried out this work near to the theoretical radiometric sensitivities predicted by classical formulae. From an analysis of the Stokes vectors for sundry different materials and scenarios of observa...