Near-field Target Research Articles

The Focusing Properties of a Modular All-Metal Lens in the Near-Field Region.

This article proposes a modular and passive all-metal lens to achieve near-field focusing with adjustable focus. The proposed lens consists of all-metal units with the phase coverage range exceeding 360°, and the arrangement of units is guided by the phase compensation method. Specifically, using the strategy of module unit synthesis, the arrangement of lens units under different focuses can be assembled arbitrarily, which reduces the production costs by 39.3% and improves the freedom of lens design. The simulation and experimental results show that the lens exhibits excellent focusing properties and freely changes the position of the expected focus (0.30 m-0.75 m). Therefore, the modular all-metal lens designed in this article has the characteristics of high transparency and a high degree of freedom, which can provide low-cost and lightweight solutions for various applications in the field of antennas, such as near-field target detection, microwave imaging, biomedicine, and so on.

Near-Field Target Localization for EMVS-MIMO Radar With Arbitrary Configuration

Near-Field Target Localization for EMVS-MIMO Radar With Arbitrary Configuration

Assessment of ultrasound image quality in a reference phantom using gel and liquid standoff pads

Backgroundand Methods: This paper presents a review of image quality in ultrasound imaging when using gel and liquid standoff pads, compared to imaging without a standoff pad. Imaging metrics assessed are lateral resolution, nearfield target intensity, and contrast to noise ratio of echogenicity targets. Measurements were made with a Canon Aplio i800 ultrasound system on a standardized ultrasound phantom (Computerized Imaging Reference Systems, Inc.), and custom-made agarose phantoms. Two cases are examined: when the ultrasound images are taken with the probe at different heights above the phantom (the normal clinical use-case for standoff pads); and when the ultrasound probe is artificially always placed at the same height above the phantom (allowing for direct image comparison). Results and conclusionThe main differences in the standard clinical use-case were observed in nearfield image intensity, with a mean difference of 22.4 ± 11.1 % between highest and lowest positions. Investigation of probes at the same height above the phantom shows that differences appear to be due primarily due to the height of the probe above the phantom, rather than the pads themselves. Elevational beamwidth was found to be a minimum of 2.2 mm at 25 mm axial depth, and adding a standoff pad can change vessel and cyst visualization due to the elevational beamwidth. This is a useful result to be aware of for clinicians using standoff pads on modern ultrasound systems.

Design of large-angle distortion-free and spot adjustable LED spot array projector based on microlens array and eyepiece

The spot array projector has long been a challenging and intriguing research focus in the fields of projection and lighting applications. In the current literature, there is a lack of a comprehensive and detailed presentation on the design and analysis method for a spot array generator based on the structure of imaging eyepiece and MLA. We present a novel design and optimization method for a large-angle, distortion-free and spot adjustable LED spot array projector that is composed of an eyepiece, two microlens arrays (MLAs), and a micro-LED array (MLEDA). The eyepiece system is optimized using imaging optical methods to project sub-beams to the target plane with a large angle. The sub-lens of condenser MLA is also optimized using imaging optical methods to refocus the collimating beam and match the numerical aperture (NA) with the eyepiece, and the sub-lens of the collimating MLA is acquired by using simulated annealing (SA) global illumination optics optimization method to achieve collimation and far-field homogenization. The predistortion MLEDA and the MLAs are proposed and implemented by the radial basis function (RBF) interpolation method, which correct the large-angle distortion introduced by the eyepiece. Both near-field and far-field applications can be realized by the proposed system. In the near-field applications, different spot geometries at the near-field target plane can be achieved. In the far-field applications, the power matching of the MLEDA is used to improve far-field uniformity of spot array. Moreover, the predefined-geometry arrangement spot array can be realized in both near and far fields. Two design examples with full field of view (FOV) projection of 80° and 100° are provided to validate the proposed method. Overall, the proposed system offers a promising solution for various applications requiring target identification or 3D calibration.

On the estimation of near-field air blast peak overpressure from cylindrical charges

In the near field, the air blast peak overpressure from cylindrical charges may vary by several times with angles at the same scaled distance. We propose an estimation equation for the peak overpressure from cylindrical charges, which can predict the peak overpressure from cylindrical charges at any positions in the near field. The estimation equation is in the form of piecewise function which contains three variables of length-to-diameter ratio, angle and scaled distance, and the applicability of our estimation equation is verified by experiment and simulation. After preliminary verification, the application range of length-to-diameter ratio of cylindrical charge is [Formula: see text] and that of scaled distance is [Formula: see text]. The estimation equation can be rapidly applied to the assessment of near-field target damage and protection against air blast from cylindrical charges, which is a significant supplement to the traditional empirical equations for spherical charge.

Aberration compensation in two-dimensional reflective optical systems

We present a novel approach to minimize aberrations in imaging systems. The energy distributions at the source and target of an optical system play a crucial role in designing freeform surfaces through illumination optics methodologies. We quantify the on-axis and off-axis aberrations using a merit function that depends on the energy distributions. The minimization of the merit function yields optimal energy distributions, which subsequently enable us to design freeform reflector surfaces that cause the least aberrations. We validate our method by testing it for two configurations, a single-reflector system with a parallel source to a near-field target, and a double-reflector system with a parallel source to a point target.

Near-field target localisation based on the distributed acoustic sensing optical fibre in shallow water

The distributed acoustic sensing optical fibre (DASF) is widely used for target detection and localisation on land as well as underwater optical fibre safety monitoring. To extend it to near-field target localisation in shallow water, DASF based on phase-sensitive optical time-domain reflectometry (Φ-OTDR) is used to receive near-field target signals. Given that the acoustic wavefront in the near field is spherical, the localisation method is based on the near-field localisation principle. The feasibility and accuracy of Φ-OTDR based on DASF for effectively locating near-field targets in shallow water are validated through simulations and practical experiments. Furthermore, the first experiment for applying DASF based on Φ-OTDR to near-field target localisation in shallow water is conducted, laying the theoretical and experimental groundwork for its improved application to underwater target detection.

Near-Field Multiple Target Localization in Frequency Diverse Array Based on Tensor Decomposition

Target localization is a fundamental problem in array signal processing. The problem of locating near-field targets with multiple-input multiple-output (MIMO) radar has been studied extensively; however, most of the conventional matrix-based approaches suffer from limitations in terms of the representation and exploitation of the multidimensional nature of MIMO radar signals. In this paper, we addressed the problem of localizing multiple targets in the near-field region, aiming at pursuing a solution applicable for multidimensional signal that is able to achieve sufficient accuracy. A tensor-based signal model impinging on a monostatic frequency diverse array multiple-input multiple-output (FDA-MIMO) radar was formulated, and a corresponding tensor decomposition-based localization algorithm (TenDLA) that showcases the connection between the tensor-based analysis and the localization problem was developed. Additionally, a correction procedure to mitigate the estimation deviations on the range and angle was presented, yielding significant improvements in estimation accuracy. Numerical examples demonstrated the validity and effectiveness of the proposed approach, and it was shown that this approach is superior to conventional methods due to its high-resolution estimation accuracy as well as its relatively low computational costs.

A Near-Field Area Object Detection Method for Intelligent Vehicles Based on Multi-Sensor Information Fusion

In order to solve the difficulty for intelligent vehicles in detecting near-field targets, this paper proposes a near-field object detection method based on multi-sensor information fusion. Firstly, the F-CenterFusion method is proposed to fuse the information from LiDAR, millimeter wave (mmWave) radar, and camera to fully obtain target state information in the near-field area. Secondly, multi-attention modules are constructed in the image and point cloud feature extraction networks, respectively, to locate the targets’ class-dependent features and suppress the expression of useless information. Then, the dynamic connection mechanism is used to fuse image and point cloud information to enhance feature expression capabilities. The fusion results are input into the predictive inference head network to obtain target attributes, locations, and other data. This method is verified by the nuScenes dataset. Compared with the CenterFusion method using mmWave radar and camera fusion information, the NDS and mAP values of our method are improved by 5.1% and 10.9%, respectively, and the average accuracy score of multi-class detection is improved by 22.7%. The experimental results show that the proposed method can enable intelligent vehicles to realize near-field target detection with high accuracy and strong robustness.

Detection of the near-field targets by non-coaxial underwater single-photon counting lidar

The strong backscattering of near-field water severely limits the detection ability of the coaxial single-photon counting lidar, especially when the target signal peak overlaps with the background(water) signal peak. Based on the important influence of scattering angle on the distribution of echo photons in the volume scattering function (VSF) of water and the BRDF model of target, a detection efficiency model of non-coaxial underwater single-photon counting lidar is established in this paper. It reveals that when the water attenuation coefficient is a fixed value, the maximum detection efficiency and range walk error (RWE) decrease exponentially with the increase of target distance. The variation of the center distance of the transceiver system has the best detection efficiency, which has little effect on the RWE. Compared with the coaxial system, the target detection efficiency of the non-coaxial underwater single-photon counting lidar remains unchanged while suppressing the strong backscattering of near-field water. Monte Carlo simulation results show that the non-coaxial system has faster searching ability for target signals with high attenuation coefficient and overlapping echo peaks. This research lays a theoretical foundation for the fast target search of single-photon counting lidar in complex underwater environment.

High-resolution surface plasmon resonance holographic microscopy based on symmetrical excitation

Surface plasmon resonance holographic microscopy (SPRHM) is able to simultaneously obtain the amplitude- and phase-contrast surface plasmon resonance (SPR) images, showing great potentials in imaging near-field targets with high sensitivity. However, suffered by the decaying length of surface plasmon wave which can be as long as tens of microns, the spatial resolution of SPRHM is lower than that of traditional holographic microscopy. In this work, we propose to enhance the spatial resolution in SPRHM by exciting surface plasmon resonance in two symmetrical directions and detecting the complex amplitudes of the reflected light symmetrically. Through the Fourier analysis of the recorded composite hologram, the reconstruction schemes for high-resolution amplitude- and phase-contrast SPR images are established, respectively. The feasibility and advantages of the proposed method is verified by numerical simulations and experimental demonstrations of small-size particles and micro-structures.

Phase Design and Near-Field Target Localization for RIS-Assisted Regional Localization System

This paper explores the near-field regional target localization problem with the reconfigurable intelligent surface (RIS) assisted system. Traditional near-field localization strategies typically estimate the position of the target through line-of-sight (LOS) signals transmitted from the anchor node. In practice, accurate location estimation is a challenging issue when the LOS link between the anchor node and target may be unavailable due to the obstacle. To this end, this paper investigates the possibility to confirm the position of the target node consisted in an area of interest (AOI) by retrieving information from the RIS reflection signals. Specifically, we establish a general framework for RIS-assisted regional localization, which consists of RIS phase design and position determination. By defining the average localization accuracy (ALA) of the AOI, we first present a discretization method to design the RIS phase. Then, a robust phase design problem is formulated via transforming the AOI into an uncertainty model of position parameters, and an efficient iterative entropy regularization (IER)-based algorithm is proposed to solve it. Using the designed RIS phase, we develop a near-field target localization algorithm and discuss the power optimization problem for the RIS-assisted localization system (RALS). Numerical results demonstrate the effectiveness of the proposed framework, in which both phase design strategies almost coincide with the optimal RIS phase, and the proposed localization method can attain the near-optimal localization performance by applying the designed RIS phase schemes.

Near-Field Target Localization Based on the Distributed Acoustic Sensing Optical Fiber in Shallow Water

Near-Field Target Localization Based on the Distributed Acoustic Sensing Optical Fiber in Shallow Water

A series of SNR-based speech intelligibility models in the Auditory Modeling Toolbox

This technical paper presents a series of speech intelligibility models that have been developed since the original version proposed by Lavandier and Culling [(2010). Journal of the Acoustical Society of America 127, 387–399]. This binaural model accounts for better-ear listening and binaural unmasking to predict the intelligibility of a near-field target speech among multiple stationary noise sources in rooms for normal-hearing listeners. Subsequent model versions allowed to consider a reverberated speech target in the far-field, envelope-modulated noise sources, and hearing-impaired listeners. As an intermediate step before considering speech maskers, a monaural version incorporating a harmonic-cancellation mechanism was recently developed to account for the effect of a stationary harmonic masker. This technical review is oriented towards model users and explains when and how each model should be used, points at its advantages and limitations, and provides an example of predictions using a data set from the literature. All these models along with the data, signals and code used to prepare the presented figures are made available within the Auditory Modeling Toolbox (AMT 1.1).

Indoor near-field target detection characteristics under radio and radar joint operation at 2.4 GHz ISM band

This paper presents an experimental research work on detecting an indoor near-field target at 2.4-GHz industrial, scientific, and medical (ISM) band. The research is based on hypotheses that if the detection target is in proximity to antennae, then the dominant electromagnetic (EM) wave couplings would happen between the target and antennas. Any other EM wave interaction between the antennas and indoor background objects would be secondary to the target-to-antennas EM-wave interaction. This unique coupling may allow us to distinguish and measure the EM wave energy scattered by the target to the receive antenna in the form of signal power using a spectrum analyzer. The detection power is a strong function of the distance between the target and receiving antenna, which could be characterized by radar equations. In such scenario, the hardware module configured to test the hypotheses plays the role of a radar sensor that detects indoor targets at near-field region under the coexistence and cooperation of WiFi radio link at the ISM band. In the experiment, at the presence of a flat round metal target with radius 5 cm, the measured detection signal power ranged from 10 nW to 1 mW (-30 dBm) when the sensing distance (r) decreased from the edge of a radiative near-field (r = λ = 12.5 cm) to the reactive near-field region (r = λ/2π = 2 cm). These were well-agreeable with theoretical estimations by radar equations, validating the hypotheses.

A Novel Imaging Method Using Fractional Fourier Transform for Near-Field Synthetic Aperture Radiometer Systems

In the domain of synthetic aperture radiometry, a Fourier transform (FT) relationship can be established between the brightness temperature of the target in the far-field region and the visibility function output by the system. However, when the target is in the near-field range of the imaging system, the existing far-field imaging methods cannot be used directly for the inversion of the near-field visibility function, because the target radiation signal cannot simply be regarded as a plane wave signal. In this letter, we propose a novel imaging method for synthetic aperture radiometer systems on the basis of the characteristics of the near-field target radiation signal. We introduce the near-field error term to reformulate the relationship between visibility function and brightness temperature in near field. Based on the reestablished signal model, we present an image reconstruction algorithm via fractional Fourier transformation, named near-field fractional FT (NF-FRFT), to estimate the near-field brightness temperature. Compared with the conventional near-field imaging method, the proposed NF-FRFT method can achieve better image reconstruction quality without extra hardware consumption. Furthermore, another advantage of this approach is that no additional array layout design is required. The validity of this method is demonstrated by a series of simulations and experiments.

Estimation of the rotation velocity and scaling for ISAR imaging of near-field targets based on the integrated generalized cubic phase function and image interpolation

ABSTRACT Inverse synthetic aperture radar (ISAR) imaging of the near-field target is potentially significant with the development of radar technology. The near-field target cannot be mapped to the ISAR image correctly because it does not satisfy the far-field distance approximation. To efficiently solve the distortion problem, it is more desirable to estimate the rotation velocity for near-field ISAR imaging. In this article, we propose a method for estimating the rotation velocity of the near-field target based on integrated generalized cubic phase function. Different from the far-field target, the received signal of the near-field target in a range bin is considered as the cubic phase signal, and the rotation velocity can be determined by the coefficients of the cubic phase signal. Afterwards, we utilize the cubic convolution image interpolation to rescale the image on the homogeneous Cartesian domain. Experimental results of simulated data demonstrate the effectiveness of the proposed method on rotation velocity estimation and scaling of the near-field target.

Near-field target strength of finite cylindrical shell in water

In this study, near-field acoustic scattering from a finite elastic cylindrical shell is investigated based on a previous cylinder method. An analytic expression for scattered pressure is derived in the form of a Kirchhoff–Helmholtz integral with the assumption that the field induced on the cylinder surface is determined by the canonical solution of an infinite elastic cylindrical shell. A useful approximate solution of the integral is obtained in the near-field zone of the receiver and then applied to formulate the near-field target strength. The results of the approximate solution are compared with the full numerical solution and experimental data measured using a linear frequency-modulated continuous-wave pulse at a frequency range of 5 to 25 kHz in a water tank in a laboratory. The results indicate consistency, except for some discrepancies due to experimental restrictions. The approximate solution is validated via finite element modeling using COMSOL Multiphysics. Moreover, the range dependency of the near-field formula and the effect of the end-caps in the finite cylindrical shell are investigated through simulations.

3-D Large-Pitch Synthetic Transmit Aperture Imaging With a Reduced Number of Measurement Channels: A Feasibility Study.

A 3-D synthetic transmit aperture ultrasound imaging system with a fully addressed array usually leads to high hardware complexity and cost since each element in the array is individually controlled. To reduce the hardware complexity, we had presented the large-pitch synthetic transmit aperture (LPSTA) ultrasound imaging for 2-D imaging using a 1-D phased array to reduce the number of measurement channels M (the product of number of transmissions, [Formula: see text], and the number of receiving channels in each transmission, [Formula: see text]). In this article, we extend this method to a 2-D matrix array for 3-D imaging. We present both numerical simulations and experimental measurements. We combined L × L adjacent elements into transmission subapertures (SAP) and K × K adjacent elements into receive SAPs in synthetic transmit aperture (STA) imaging. In the image reconstruction, we conducted the first attempt to apply and integrate Gaussian-approximated spatial response function (G-SRF) with delay and sum (DAS) to improve the image contrast, especially for the near-field targets. The imaging performance obtained from G-SRF was also evaluated numerically and compared with the previously presented frequency-domain SRF (Freq-domain SRF). The 3-D large-pitch synthetic transmit aperture (3-D-LPSTA) with G-SRF can provide a computationally efficient solution compared with the standard 3-D-STA method. With approximately 1900-fold reduction in the number of measurement channels, 3-D-LPSTA can provide image contrast comparable with the standard 3-D-STA with a full array and significantly better than using a periodically sparse array with similar complexity. In addition to reducing the system complexity, the 3-D-LPSTA achieves 700-fold reduction in computational complexity and 523-fold reduction in data storage. Finally, we evaluated and implemented the G-SRF using phantom data, which were consistent with the simulation results showing that the G-SRF can improve the image contrast. The results demonstrate that the proposed 3-D-LPSTA shows the great potential for designing an inexpensive ultrasound system to ensure the real-time 3-D clinical ultrasound imaging using large arrays. The limits of the proposed method were also discussed.

Approach for ISAR imaging of near-field targets based on coordinate conversion and image interpolation

Inverse synthetic aperture radar (ISAR) imaging of near-field targets is potentially useful in some specific applications, which makes it very important to efficiently produce high-quality image of the near-field target. In this paper, the simplified target model with uniform linear motion is applied to the near-field target imaging, which overcomes the complexity of the traditional near-field imaging algorithm. According to this signal model, the method based on coordinate conversion and image interpolation combined with the range-Doppler (R-D) algorithm is proposed to correct the near-field distortion problem. Compared with the back-projection (BP) algorithm, the proposed method produces better focused ISAR images of the near-field target, and decreases the computation complexity significantly. Experimental results of the simulated data have demonstrated the effectiveness and robustness of the proposed method.