Synthetic Aperture Measurements Research Articles

Image analysis and resolution for detection-based synthetic-aperture passive source localization

This paper follows a detection-theoretic approach for using synthetic-aperture measurements, made at multiple moving passive receivers, in order to form an image showing the locations of stationary sources that are radiating unknown electromagnetic or acoustic waves. The paper starts with a physics-based model for the propagating fields, and, following the general approach of McWhorter et al (2023 arXiv:2302.06816, IEEE Open J. Signal Process. 4 437–51), derives a detection statistic that is used for the image formation. This detection statistic is a quadratic function of the data. Each point in the scene is tested as a possible hypothesized location for a source, and the detection statistic is plotted as a function of location. Because this image formation process is nonlinear, the standard linear methods for determining resolution cannot be applied. This paper shows how to analyze the detection image by first writing the noiseless image as a coherent sum of shifted complex ambiguity functions of the source waveform. The paper then develops a technique for calculating image resolution; resolution is found to depend on the sensor-source geometry and also on the properties (bandwidth and temporal duration) of the source waveform. Optimal filtering of the image is given, but a simple example suggests that optimal filtering may have little effect. Analysis is also given for the case in which multiple sources are present.

Experimental Evaluation of the Statistical Isotropy of a Reverberation Chamber’s Plane-Wave Spectrum

Synthetic-aperture measurements in a loaded reverberation chamber are used to calculate power-angle profiles describing the instantaneous distribution of received power versus azimuth angle-of-arrival. Averaging multiple power-angle profiles leads to estimates of the reverberation chamber's power-angle spectrum. A comparison to simulated power-angle spectra for an ideal reverberation chamber indicates that the loaded reverberation chamber tends to provide a statistically anisotropic plane-wave spectrum. Analysis of the measurements' power-delay-angle profiles suggest that this anisotropy due to both the presence of unstirred multipath components and strong early-time stirred multipath components arising from reflections off of the mode-stirring paddles' planar surfaces. Guidelines are provided for minimizing the effect of a statistically anisotropic plane-wave spectrum, such as the use of additional stirring mechanisms and minimizing the contributions of unstirred multipath components.

Nondestructive testing potential evaluation of a terahertz frequency-modulated continuous-wave imager for composite materials inspection

The sub-terahertz (THz) frequency band has proved to be a noteworthy option for nondestructive testing (NDT) of nonmetal aeronautics materials. Composite structures or laminates can be inspected for foreign objects (water or debris), delaminations, debonds, etc., using sub-THz sensors during the manufacturing process or maintenance. Given the harmless radiation to the human body of this frequency band, no special security measures are needed for operation. Moreover, the frequency-modulated continuous-wave sensor used in this study offers a very light, compact, inexpensive, and high-performing solution. An automated two-dimensional scanner carrying three sensors partially covering the 70- to 320-GHz band is operated, using two complementary measurement approaches: conventional focused imaging, where focusing lenses are used; and synthetic aperture (SA) or unfocused wide-beam imaging, for which lenses are no longer needed. Conventional focused imagery offers finer spatial resolutions but imagery is depth-limited due to the beam waist effect, whereas SA measurements allow imaging of thicker samples with depth-independent but coarser spatial resolutions. The present work is a compendium of a much larger study and describes the key technical aspects of the proposed imaging techniques and reports on results obtained from human-made samples (A-sandwich, C-sandwich, solid laminates) which include diverse defects and damages typically encountered in aeronautics multilayered structures. We conclude with a grading of the achieved results in comparison with measurements performed by other NDT techniques on the same samples.