Underwater Gas Leakage Flow Detection and Classification Based on Multibeam Forward-Looking Sonar

The risk of gas leakage due to geological flaws in offshore carbon capture, utilization, and storage, as well as leakage from underwater oil or gas pipelines, highlights the need for underwater gas leakage monitoring technology. Remotely operated vehicles(ROVs) and autonomous underwater vehicles(AUVs) are equipped with high-resolution imaging sonar systems that have broad application potential in underwater gas and target detection tasks. However, some bubble clusters are relatively weak scatterers, so detecting and distinguishing them against the seabed reverberation in forward-looking sonar images are challenging. This study uses the dual-tree complex wavelet transform to extract the image features of multibeam forward-looking sonar. Underwater gas leakages with different flows are classified by combining deep learning theory. A pool experiment is designed to simulate gas leakage, where sonar images are obtained for further processing. Results demonstrate that this method can detect and classify underwater gas leakage streams with high classification accuracy. This performance indicates that the method can detect gas leakage from multibeam forward-looking sonar images and has the potential to predict gas leakage flow.

High-resolution forward-looking imaging algorithm for missile-borne detectors

Aiming at a novel missile-borne detector in the optional burst height proximity fuze, a self-adaptive high-resolution forward-looking imaging algorithm (SAHRFL-IA) is presented. The echo data are captured by the missile-borne detector in the target regions;thereby the azimuth angulation accuracy at the same distance dimension is improved dynamically. Thus, azimuth information of the targets in the detection area may be obtained accurately. The proposed imaging algorithm breaks through the conventional misconception of merely using azimuth discrimination curves under ideal conditions during monopulse angulation. The real-time echo data from the target region are used to perform error correction for this discrimination curve, and finally the accuracy of the azimuth angulation may reach the optimum at the same distance dimension. A series of experiments demonstrate the validity, reliability and high performance of the proposed imaging algorithm. Azimuth angulation accuracy may reach ten times that of the detection beam width. Meanwhile, the running time of this algorithm satisfies the requirements of missile-borne platforms.

RESEARCH ON THE BISTATIC FORWARD-LOOKING SAR IMAGING

The bistatic Synthetic Aperture Radar (SAR) systems with separate transmitter and receiver antennas provide a new potential to imaging in the forward-looking geometry. Analysis of the Doppler property in this paper indicates the feasibility of Bistatic Forward-Looking (BFL) SAR imaging. Considering the different Doppler property determined by the two platforms in BFL SAR, a new 2-D point target spectrum is derived in our study. Based on the spectrum, an imaging method is chosen for the configuration, and the point target simulation validates the analysis.

Nonlinear RCMC method for spaceborne/airborne forward-looking bistatic SAR

In the spaceborne/airborne forward-looking bistatic syn- thetic aperture radar （SA-FBSAR）, due to the system platforms＇ remarkable velocity difference and the forward-looking mode, the range cell migration （RCM） not only depends on the target＇s two- dimensional location, but also varies with the range location non- linearly. And the nonlinearity is not just the slight deviation from the linear part, but exhibits evident nonlinear departure in the RCM trajectory. If the RCM is not properly corrected, nonlinear image distortions would occur. Based on the RCM model, a modified two-step RCM compensation （RCMC） method for SA-FBSAR is proposed. In this method, firstly the azimuth-dependent RCM is compensated by the scaling Fourier transform and the phase multi- plication. And then the range-dependent RCM is removed through interpolation. The effectiveness of the proposed RCMC method is verified by the simulation results of both point scatterers and area targets.

Property analysis of bistatic forward-looking SAR with arbitrary geometry

Bistatic forward-looking synthetic aperture radar（SAR） has many advantages and applications owing to its twodimensional imaging capability.There could be various imaging configurations because of the geometric flexibility of bistatic platforms,resulting in kinds of models built independently among which there could be some similar even the same motion features.Comprehensive research on such systems in a more comprehensive and general point of view is required to address their difference and consistency.Property analysis of bistatic forwardlooking SAR with arbitrary geometry is achieved including stripmap and spotlight modes on airborne platform,missile-borne platform,and hybrid platform of both.Emphasis is placed on azimuth space variance of some key parameters significantly affecting the subsequent imaging processing,based on which the frequency spectra are further described and compared considering respective features of different platforms for frequency imaging algorithm developing.Simulation results confirm the effectiveness and correctness of our analysis.

Railway Track Verification Using a Forward-Looking SAR GPR

It is a challenge to evaluate the conditions of railway track without interruption of regular traffic. In this paper, the authors introduce the detection of cavities under the railway substructure by using forward-looking ground penetrating radar （FLGPR）. Main advantages of FLGPR are that such a system can illuminate a large area and can stand off a long distance over its down-looking counterpart. Two methods, frequency wave-number （F-W） synthetic aperture imaging （SAI） and beam-forming by delay and sum （DAS）, are applied to process the collected data. Analysis and measuring show that the distinct radar image of the cavity beneath the substructure 1.2 m deep can be formed by these two methods.

Monetary policy rules：Forward-looking and backward-looking in an overlapping generations model

In the framework of an overlapping generations model, forward-looking monetary policy roles and backward-looking monetary policy rules were investigated. It is shown that the monetary steady state is more likely to be indeterminate under an active forwardlooking rule than under the corresponding backward-looking rule. It is also shown that backward-looking roles can render the monetary steady state unstable.

Sparse three-dimensional imaging for forward-looking array SAR using spatial continuity

For forward-looking array synthetic aperture radar(FASAR),the scattering intensity of ground scatterers fluctuates greatly since there are kinds of vegetations and topography on the surface of the ground,and thus the signal-to-noise ratio(SNR)of its echo signals corresponding to different vegetations and topography also varies obviously.Owing to the reason known to all,the performance of the sparse reconstruction of compressed sensing(CS)becomes worse in the case of lower SNR,and the quality of the sparse three-dimensional imaging for FASAR would be affected significantly in the practical application.In this paper,the spatial continuity of the ground scatterers is introduced to the sparse recovery algorithm of CS in the threedimensional imaging for FASAR,in which the weighted least square method of the cubic interpolation is used to filter out the bad and isolated scatterer.The simulation results show that the proposed method can realize the sparse three-dimensional imaging of FASAR more effectively in the case of low SNR.

改良耳穴压豆联合局部按摩治疗儿童特发性异常瞬目症的临床研究

目的 观察改良耳穴压豆联合局部按摩治疗儿童特发性异常瞬目症的临床疗效。方法 纳入2019年10月—2022年4月在上海市松江区中心医院眼科门诊确诊的4~12岁特发性异常瞬目症患儿90例,随机分成观察组(A组)、中医组(B组)和对照组(C组)各30例(60只眼)。分别予改良耳穴压豆、经典耳穴压豆和常规心理—行为疗法配合滴眼液治疗。3组均观察21 d。分别于治疗前、治疗后测量患儿瞬目频率、学习能力的数据。结果(1)瞬目频率评分:3组治疗前瞬目频率评分比较,差异无统计学意义(P>0.05)。治疗后A组与B组瞬目频率评分均较治疗前降低,差异均有统计学意义(t_(A组)=19.313、t_(B组)=15.000,均P=0.000),C组较治疗前,差异无统计学意义(P>0.05)。治疗后3组间比较,差异有统计学意义(F=1093.886,P=0.000)。与C组比较,A组和B组瞬目频率评分均较低,差异均有统计学意义(t_(A组)=-11.655,t_(B组)=-11.171,均P=0.000);A组和B组比较,差异无统计学意义(P>0.05)。(2)阅读速度比较:治疗后A组与B组阅读速度均较治疗前增加,差异均有统计学意义(t_(A组)=-60.647,t_(B组)=-61.465,均P=0.000),C组较治疗前,差异无统计学意义(P>0.05)。治疗后3组间阅读速度比较,差异有统计学意义(F=19.150,P=0.000)。与C组比较,A组与B组阅读速度均较快,差异均有统计学意义(t_(A组)=4.744、t_(B组)=4.033,均P=0.000);A组与B组比较,差异无统计学意义(P>0.05)。(3)听写速度比较:治疗后A组与B组听写速度均较治疗前增加,差异均有统计学意义(t_(A组)=-115.348、t_(B组)=-109.133,均P=0.000),C组较治疗前,差异无统计学意义(P>0.05)。治疗后3组间比较,差异有统计学意义(F=91.933,P=0.000)。与C组比较,A组与B组听写速度均较快,差异均有统计学意义(t_(A组)=9.751、t_(B组)=8.203,均P=0.000);A组与B组听写速度比较,差异无统计学意义(P>0.05)。(4)中医症候积分:3组治疗前中医症候积分比较,差异无统计学意义(P>0.05)。治疗后A组与B组中医症候积分均较治疗前降低,差异均有统计学意义(t_(A组)=-4.365、t_(B组)=-4.062,均P=0.000),C组较治疗前,差异无统计学意义(P>0.05)。治疗后3组间比较,差异有统计学意义(F=17.259,P=0.000)。与C组比较,A组和B组中医症候积分均较低,差异均有统计学意义(t_(A组)=-4.979、t_(B组)=-4.625,均P=0.000);A组和B组比较,差异无统计学意义(P>0.05)。(5)临床疗效:A组(96.67%)与B组(93.33%)总有效率均优于C组(66.67%),差异均有统计学意义(χ^(2) A组=9.017,P=0.003;χ^(2) B组=6.667,P=0.010),而A组与B组比较,差异无统计学意义(P>0.05)。(6)复发率:A组(10.34%)与B组(14.29%)复发率均低于C组(80.00%),差异均有统计学意义(χ^(2) A组=24.191、χ^(2) B组=20.728,均P=0.000);而A组与B组比较,差异无统计学意义(P>0.05)。结论 改良耳穴压豆联合局部按摩治疗儿童特发性异常瞬目症临床疗效较好,具有操作简单、疗效可靠、复发率低等优势,值得临床推广。

基于改进混合坐标系的大斜视俯冲机动平台SAR快速时域成像算法

多级子图像融合是加速合成孔径雷达(Synthetic Aperture Radar,SAR)时域成像的关键手段.然而,机动平台SAR的大斜视采集增强了频谱支持区域的不规则特性,降低图像融合的效率和精度.针对这些问题,本文设计了一种改进的混合坐标系,在此基础上开发了大斜视俯冲机动平台SAR快速时域成像算法.得益于改进混合坐标系中建立的等效斜距模型,频谱对斜视角的敏感度降低,消除了频谱空变现象.因此,设计频谱预处理函数可以有效压缩和拼接频谱,提升图像融合的效率和精度.通过仿真和原始数据处理,验证了提出算法性能的优越性.

基于椭球模型的斜视星载SAR距离模糊仿真

距离模糊比是星载SAR图像的一项重要指标,与正侧视成像相比,大斜视成像星地几何关系更为复杂,传统距离模糊计算与仿真中将地球视为球体的假设会对计算结果造成较大偏差,不能准确评估SAR图像质量。本文采用更为准确的地球椭球模型代替球体模型,并提出了一种基于斜视天线坐标系的模糊区计算方法,利用此方法对距离模糊比沿距离向测绘带的变化进行仿真,同时对距离模糊比沿方位向的空变性也进行研究。本文研究成果对星载SAR系统设计具有重要参考价值。

一种适用于星载大斜视聚束SAR的改进距离徙动算法

星载大斜视聚束合成孔径雷达(Synthetic Aperture Radar,SAR)探测范围大、目标驻留时间长,有广泛的应用前景,但会带来较大的距离徙动,使得回波信号耦合严重。传统算法使用固定窗接收数据,为了采集完整,则需要较大的采样窗,但采得数据中存在大量的无效数据,采样效率很低。为解决时域采样效率低等问题,可使用滑窗接收模式,但该模式一般结合时域算法或极坐标格式算法使用,而在星载SAR大场景成像的情况下,时域算法计算效率低下,极坐标格式算法的聚焦半径较小,因此本文提出了一种改进的距离徙动算法来解决大斜视滑窗接收模式下的高效精确成像问题。本文建立了滑窗接收模式下的时域回波模型,利用傅里叶变换的性质推导出精确两维解析频谱,并基于上述两维解析频谱,提出雷达视线坐标系下的两维空间域解耦方法并给出对应的完整成像处理流程,最后通过仿真实验验证算法的有效性。本文提出的新方法既能解决大斜视两维频谱扭曲带来的频域采样效率低下问题,又可避免传统方法在大斜视模式下的残留距离徙动放大问题,为后续的自聚焦处理提供便利。

基于分数阶傅里叶变换和图像加权熵的chirp scaling算法

针对传统的基于傅里叶变换和匹配滤波实现的chirp scaling(CS)成像算法中多普勒参数随斜距变化以及成像分辨率低的问题,提出利用分数阶傅里叶变换(FRFT)对CS成像算法进行优化。首先建立斜视合成孔径雷达(SAR)回波信号模型,理论推导利用FRFT代替匹配滤波进行信号压缩。针对方位向最优旋转角的搜索问题,对得到的图像基于加权最小熵建立代价函数,利用动量法的梯度下降优化算法进行迭代计算,最终得到分辨率更高的SAR图像。为验证算法的有效性,分别在点目标仿真数据和实测SAR数据集上进行实验。结果表明,与传统CS成像算法相比,该算法的成像结果成像主瓣宽度更窄、旁瓣更低、成像更加清晰。

STUDY ON THE SIGNATURE OF GROUND MOVING TARGET FOR AIRBORNE SQUINT SAR IMAGING

Compared with the side-looking Synthetic Aperture Radar (SAR), the flexible beampointing of squint SAR makes great application value. This paper derives the image signature of the ground moving target after the processing of Range-Doppler (RD) algorithm, the SAR signatures of ground moving targets are analyzed, including the geometry correction, the offsets and defocusing in both range and azimuth direction. Finally, computer simulation results validate its effectiveness. The research results are especially significant for moving targets detection and parameters estimation in squint mode SAR.

Compact wideband microstrip array antenna with beam squint

A compact wideband microstrip array antenna with a squint beam is introduced without the matched load. Its measured beam is at 10° offset to the broadside with a measured gain of 12 dB at 10 GHz. The measured impedance bandwidth （voltage standing wave ratio （VSWR）≤2） is over 16%, which agrees well with the simulated one. It is a low-cost wideband design with compact simple structure, suited for military and commercial application.

An approach to wide-field imaging of linear rail ground-based SAR in high squint multi-angle mode

Ground-based synthetic aperture radar(GB-SAR) has been successfully applied to the ground deformation monitoring.However, due to the short length of the GB-SAR platform, the scope of observation is largely limited. The practical applications drive us to make improvements on the conventional linear rail GB-SAR system in order to achieve larger field imaging. First, a turntable is utilized to support the rotational movement of the radar.Next, a series of high-squint scanning is performed with multiple squint angles. Further, the high squint modulation phase of the echo data is eliminated. Then, a new multi-angle imaging method is performed in the wave number domain to expand the field of view. Simulation and real experiments verify the effectiveness of this method.

NOVEL APPROACH BASED ON DERAMPING TECHNIQUE FOR SQUINTED SLIDING SPOTLIGHT SAR IMAGING

This paper investigates a novel approach based on the deramping technique for squinted sliding spotlight Synthetic Aperture Radar (SAR) imaging to resolve the azimuth spectrum aliasing problem. First of all, the properties of the azimuth spectrum and the squint angle impacts on the azimuth spectrum aliasing problem are analyzed. Based on the analysis result, an operation of filtering is added to the azimuth preprocessing step of traditional Two-Step Focusing Approach (TSPA) to resolve the azimuth folding problem and remove the influence of the squint angle on the azimuth spectrum aliasing problem. Then, a modified Range Migration Algorithm (RMA) is performed to obtain the precise focused image. Furthermore, the focused SAR image folding problem of traditional TSPA is illuminated in this paper. An azimuth post-processing step is proposed to unfold the aliased SAR image. Simulation experiment results prove that the proposed approach can solve the spectrum aliasing problem and process squinted sliding spotlight data efficiently.

THE STRIP IMAGING OF AIRBORNE SQUINT SIDELOOKING MODE SAR

Airborne squint side-looking strip imaging mode SAR has the advantages of better flexibility and larger field in applications than the classic side-looking mode SAR. Because of the large range migration and serious range-azimuth coupling terms, the imaging processing of squint mode SAR is a full two-dimensional (2-D) phenomenon. In this paper, different algorithms, which can be used for the imaging processing of squint mode SAR, are compared with each other in terms of their focusing quality and their ability to handle the large range migration of the squint side-looking mode SAR. And their abilities of real-time imaging are also discussed. The algorithms contained here are 2-D FFT method, fast polynomial transform(FPT) method, and the direct correcting method based on range-Doppler focusing algorithms. Other new methods are also discussed here briefly.

Study of a millimeter-wave squint indirect holographic algorithm suitable for imaging with large field-of-view

In this paper a millimeter-wave （MMW） squint indirect holographic method is presented, which is suitable for imaging with a large field-of-view. The proposed system employs the squint operation mode to remove the background and twin- image interferences, which achieves a similar effect to off-axis holography but leaves out the large-aperture quasi-optical component. The translational scanning manner enables a large field of view and ensures the image uniformity, which is difficult to realize in off-axis holography. In addition, a corresponding imaging algorithm for the presented scheme is developed to reconstruct the image from the recorded hologram. Some imaging results on typical objects, obtained with electromagnetic simulation, demonstrate good performance of the imaging scheme and validate the effectiveness of the image reconstruction algorithm.

SIDE-LOOKING ECHO DATA TRANSFORM TO SQUINT MODE SAR IMAGING

Nowadays the side-looking SAR echo data can be obtained easily from the commercial channel, while that of other SAR imaging modes such as squint, spotlight are difficult to be acquired. This paper presents a new scheme to transform the side-looking returns to squint ones, in a direct and an indirect approach respectively. Direct transformation uses the data with a wide azimuth beam angle. The maximum of the required squint angle is limited under several degrees. Squint data under indirect transformation can be obtained by adding a platform velocity along slant range according to the required squint angles. Then the squint data is determined by the angle between the new forward velocity and line-of-sight direction. This method results in higher squint angle compared with the first one. Verification shows the feasibility of these approaches with illustration of side-looking E-SAR raw data processing. The future work will be on the precise Doppler centroid estimation and effective imaging algorithm development.