No abstract

RF tomography employs geometric diversity to obtain high resolution images of targets, potentially with narrow band waveforms. In order to properly capitalize on this spatial diversity, precise knowledge of the sensor positions is required to sub-wavelength accuracy. While GPS can provide good estimates for these locations, remaining uncertainties distort the images produced and limit their utility. In this paper, an autofocusing approach is proposed to compensate for these uncertainties. A cost function is established for the image by evaluating the sharpness of image sub-regions in the neighborhood of known point-like targets. These targets can be identified either from a knowledge base or by examining preliminary imaging results. The cost function is then minimized by adjusting the estimated sensor positions using particle swarm optimization. The algorithm is shown to significantly improve the quality of RF tomographic images, allowing the separation of closely spaced targets that were severely distorted in uncorrected images. Results are provided for both simulated and measured data. A discussion of potential enhancements and wider applications of the algorithm is also included.

ABSTRACT:In this paper, we apply microwave holography to infrared (IR) thermal images of electromagnetic (EM) fields to measure near-field and far-field radiation patterns of antennas. The phase of the field is retrieved from IR thermograms (magnitude-only thermal images of the field) measured in the near field of the antenna using microwave holography. One method to extract the phase is to use an iterative plane-to-plane (PTP) two-dimensional (2D) holographic phase retrieval method. After the phase is recovered, the near-field thermographic/holographic data can be processed to determine the complex intensity (magnitude and phase) of the field at any distance in front of the antenna under test (AUT). Of particular interest is the far-field radiation pattern of the antenna or, especially for phased array antennas, the aperture source-plane distribution. Numerical simulations were performed to determine the feasibility, accuracy, and sensitivity of these IR thermographic/holographic phase-retrieval techniques. The advantages and disadvantages of the technique are also discussed. To demonstrate the feasibility of the technique, actual IR thermograms were obtained at the Air Force Research Laboratory/ Rome Research Site (AFRL/RRS) in Rome, (New York) and holograms were derived from a simple planar 6 ϫ 6 phased array patch antenna using the PTP technique. The phase-retrieval results are presented and compared with the known results for this antenna, as measured on the near-field range at the National Institute of Technology and Standards (NIST) using standard hard-wired probes. The agreement between the results for this antenna is very good.

An infrared detection technique is used to measure the electromagnetic fields near apertures of planar and cylindrical structures. Qualitative and quantitative results are presented and compared with theoretical solutions where applicable.