We derive a multipath model for sensing through walls using radars. The model considers propagation through a front wall and specular reflections at interior walls in an enclosed room under surveillance. The model is derived such that additional eigenrays can be easily accommodated. A synthetic aperture radar (SAR) system is considered, and stationary or slowly moving targets are assumed. The focused downrange and crossrange locations of multipath ghosts are established and validated using numerical, as well as experimental data. The multipath model permits an implementation of a multipath exploitation algorithm, which associates, as well as maps, each target ghost back to its corresponding true target location. In doing so, the proposed algorithm improves the radar system performance by aiding in ameliorating the false positives in the original SAR image, as well as increasing the signal-to-clutter ratio at the target locations, culminating in enhanced behind the wall target detection and localization.

Multipath Model and Exploitation in Through-the-Wall and Urban Radar Sensing

One of the main challenges in through-the-wall radar imaging (TWRI) is the strong exterior wall returns, which tend to obscure indoor stationary targets, rendering target detection and classification difficult, if not impossible. In this paper, an effective wall clutter mitigation approach is proposed for TWRI that does not require knowledge of the background scene nor does it rely on accurate modeling and estimation of wall parameters. The proposed approach is based on the relative strength of the exterior wall returns compared to behind-wall targets. It applies singular value decomposition to the data matrix constructed from the space-frequency measurements to identify the wall subspace. Orthogonal subspace projection is performed to remove the wall electromagnetic signature from the radar signals. Furthermore, this paper provides an analysis of the wall and target subspace characteristics, demonstrating that both wall and target subspaces can be multidimensional. While the wall subspace depends on the wall type and building material, the target subspace depends on the location of the target, the number of targets in the scene, and the size of the target. Experimental results using simulated and real data demonstrate the effectiveness of the subspace projection method in mitigating wall clutter while preserving the target image. It is shown that the performance of the proposed approach, in terms of the improvement factor of the target-to-clutter ratio, is better than existing approaches and is comparable to that of background subtraction, which requires knowledge of a reference background scene.

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A Subspace Projection Approach for Wall Clutter Mitigation in Through-the-Wall Radar Imaging

Moving-target detection in ultrawideband (UWB) synthetic aperture radar (SAR) is associated with long integration time and must accommodate azimuth focusing for reliable detection. This paper presents the theory on detection of moving targets by focusing and experimental results on single-channel SAR data aimed at evaluating the detection performance. The results with respect to both simulated and real data show that the ability to detect moving targets increases significantly when applying the proposed detection technique. The improvement in signal-to-clutter noise ratio, which is a basic requisite for evaluating the performance, reaches approximately 20 dB, using only single-channel SAR data. This gain will be preserved for the case of multichannel SAR data. The reference system for this study is the airborne UWB low-frequency SAR Coherent All RAdio BAnd Sensing II.

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Detection of Moving Targets by Focusing in UWB SAR—Theory and Experimental Results

Detection of moving objects concealed behind a concrete wall corner has been demonstrated, using Doppler-based techniques with a stepped-frequency radar centered at 10 GHz, in a reduced-scale model of a street scenario. Micro-Doppler signatures have been traced in the return from a human target, both for walking and for breathing. Separate material measurements of the reflection and transmission of the concrete in the wall have showed that wall reflections are the dominating wave propagation mechanism for producing target detections, while wave components transmitted through the walls could be neglected. Weaker detections have been made of target returns via diffraction in the wall corner. A simple and fast algorithm for the detection and generation of detection tracks in down range has been developed, based on moving target indication technique.

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Radar Detection of Moving Targets Behind Corners

In urban sensing and through-the-wall (TTW) radar, the existence of targets inside buildings results in multipath returns. These multipath returns are exploited to achieve target localization with a single sensor. A time-of-arrival (TOA) wall association algorithm is derived to relate multipath returns to their respective walls and targets, followed by a nonlinear least squares (NLS) optimization to localize the targets. Simulations and experimental data are used to validate the proposed algorithms.

Target Localization with a Single Sensor via Multipath Exploitation

We are examining the feasibility of ground target detection and tracking in urban centers using ground moving target indicator (GMTI) radar when line-of-sight (LOS) coverage is intermittent, yet multipath responses from building walls are available. We enhance the standard ray-tracing propagation methods that assume specular wall reflections and point scattering target models. Our two-dimensional analysis includes near field reradiation from the illuminated wall to a target having a multistatic response. We also evaluate the degradation of power and coherency in the signal processing due to wall surface roughness. Results of this work provide insight for the development of processing techniques that may be required for multipath exploitation radar.

Multistatic scattering from moving targets in multipath environments

The present invention includes an application of a dynamic logic algorithm to detect slow moving targets. Show moving targets are going to be moving in the range from 0-5 mph. This could encompass troop movements and vehicles or convoys under rough terrain. The method can be defined as a seven step process of detecting slow moving targets using a synthetic aperture radar (SAR), said slow moving targets being objects of interest that are moving in the range from 0-5 mph, wherein this method is composed of the steps of receiving SAR signal history data having an SAR image; assuming a presence of slow moving target in a SAR image based-on range, cross-range position, and velocity; assuming a presence of clutter; assigning target and clutter models that are probability distribution functions (pdf) that are defined to account for every pixel in the SAR image, wherein the target is modeled using a sum of Gaussians fitted along the target shape model, while the clutter is modeled with a uniform distribution; computing a “target present” predetermined threshold value; converging the target model to a minimum variance value; and comparing the target model minimum variance value to the predetermined threshold to determine if a target is present or absent.

Dynamic logic algorithm used for detecting slow-moving or concealed targets in synthetic aperture radar (SAR) images

Identifying and localizing targets within buildings using exterior sensors will offer superior advantages to the military and law enforcement communities. Research on wall-penetrating radar has produced significant advances in recent years regarding this topic. However, wall parameter ambiguities, multiple reflections, clutter, and measurement noise pose significant challenges to developing robust detection and estimation methods. In the present work we demonstrate can be mitigated using dynamic logic (DL), an adaptive method for iterative maximum likelihood.

Dynamic Logic Applied to SAR Data for Parameter Estimation Behind Walls

This work proceeds from the paper we published in RadarCon 2009 titled “Multistatic scattering from moving targets in multipath environments”, where we explored the potential to track moving ground targets with radar as they enter urban areas and become obscured by buildings. An X-band radar data collection was performed which validates the predicted multipath response, and the received multipath power in relation to the line-of-sight (LOS) response. Results from a bistatic experiment are used to examine the spatial coherency of energy reflecting from a large, rough surface, and the power distribution in angle that illuminates a target as it traverses in front of a building. This experiment may inspire knowledge-based methods to coherently process multipath returns, beyond that of standard GMTI processing, i.e., free-space matched-filtering (FFT) and CFAR detection.

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Validating multipath responses of moving targets through urban environments

The Cramer-Rao inequality is applied to the likelihood function of the synthetic aperture radar (SAR) scatterer parameter vector to relate the choice of flight path to estimation performance. Estimation error bounds for the scatterer parameter vector (including height) are developed for multi-dimensional synthetic apertures, and quantify the performance enhancement over a limited sector of the image plane relative to standard-aperture single-pass SAR missions. An efficient means for the design and analysis of SAR waveforms and flight paths is proposed using simulated scattering models that are limited in size. Comparison of the error bounds to those for standard-aperture SAR show that estimates of scatterer range and cross-range positions are accurate for multi-dimensional aperture SAR, even with the additional estimator for height. Furthermore, multi-dimensional SAR is shown to address the layover problem

Robert Linnehan

Papers

Multistatic scattering from moving targets in multipath environments

Dynamic logic algorithm used for detecting slow-moving or concealed targets in synthetic aperture radar (SAR) images

Dynamic Logic Applied to SAR Data for Parameter Estimation Behind Walls

Validating multipath responses of moving targets through urban environments

On the design of SAR apertures using the Cramer-Rao bound