【作者】 陈庆；

【导师】 徐文；

【作者基本信息】 浙江大学 ， 信号与信息处理， 2010， 博士

【摘要】 水下小目标声探测的核心问题之一在于解决探测距离与分辨力的矛盾。传统基于物理阵的高分辨力通常采用高频来实现,而长距离探测要求使用低频。合成孔径声纳(SAS)作为一项实现高分辨力成像的新兴手段,其成像分辨力具有与距离、频率无关的显著特点,使得低频小孔径声纳获取高的空间分辨力成为可能。同时自主水下航行器(AUV)技术愈趋成熟,为实现合成孔径处理提供了崭新的机会。本论文研究围绕浙江大学985二期建设项目“水下自主航行器平台浅水低频合成孔径声纳系统”进行,以在浅水低频条件下获取高质量的水底高分辨力成像为主要研究目标。研究内容包括信号处理算法与SAS系统实现两部分,其中浅水低频SAS信号处理算法研究是本论文的主体内容,包括宽带多径抵消处理(WMR)、分解反投影(FBP)快速合成孔径成像以及相关运动补偿等。在浅水低频条件下,声信号的多径传播会导致合成孔径声纳成像质量的下降,具体表现为出现重影伪目标及图像对比度下降。采用具有一定垂直孔径的声纳接收阵,进行垂直维阵处理,可以实现对多径传播信号的方位分辨及抵消。为了在低频段实现高的时延分辨力,需采用宽带信号,然而目前多径抵消算法大多基于窄带条件。同时由于受小型航行器平台搭载空间限制,声纳只能采用小垂直孔径接收阵,要获取垂直维高的方向分辨力需采用自适应阵处理算法,并要求其对环境扰动有一定的宽容性。本论文基于宽带波束形成算法,提出了一种宽带多径抵消处理结构,并在已有窄带宽容Capon波束形成(RCB)基础上,提出了一种基于宽带预导向协方差矩阵(SCM)、具有快速收敛能力的宽带宽容Capon波束形成算法,称为宽带预导向宽容Capon波束形成(SRCB)。仿真和湖试数据处理结果表明,基于SRCB实现的宽带多径抵消处理相对常规时延求和波束形成(DSB)和常规宽带宽容Capon波束形成(WRCB)具有更好的方向分辨性能和干扰抵消能力。合成孔径成像算法是SAS信号处理的核心算法之一,为运算量最集中的部分。SAS成像算法研究主要考虑减少处理运算量和对任意运动轨迹适用的灵活性两方面。FBP成像算法由于其时域实现的灵活性以及相对常规时延求和(TDS)成像较高的计算效率,近几年来得到了广泛的关注。本论文提出了一种改进的FBP算法,并进行了距离误差理论分析。与已有算法相比,该FBP算法不局限于SAS匀速直线运动假设,适用于任意运动轨迹。直线SAS成像和圆弧SAS成像仿真以及湖试数据处理结果都验证了该FBP算法的有效性。由于SAS成像对运动失配很灵敏,需进行运动补偿。论文SAS系统配备了DVL、电子罗盘及深度传感器,基于其导航数据即可对三维运动轨迹及姿态进行较准确的估计。SAS的运动失配分为轨迹失配与Doppler失配两类。由于论文SAS成像算法采用了具有任意轨迹适用性的FBP算法,运动轨迹失配补偿只需用估计的真实运动轨迹替代预先假定轨迹即可。对于运动Doppler失配,目前多数研究认为影响较小,且只能实现部分补偿。通过SAS信号到达时延分析可以发现在运动轨迹失配较小的条件下,Doppler失配对SAS输出成像结果影响很小。这与已有研究结果一致。然而当同时存在严重运动轨迹失配时,运动Doppler失配会导致SAS成像质量大幅下降,需进行有效补偿。本论文提出了一种能够实现对SAS运动Doppler失配进行精确补偿的算法。该算法可以与FBP成像算法有效结合,实现快速运动补偿及合成孔径成像。论文研究过程中设计并实现了一套AUV平台浅水低频SAS原型系统,包括声纳整体硬件和实时控制与数据采集软件两部分。通过一次水池测试和两次湖上试验实际使用,该声纳原型系统正常稳定工作并采集了大量实验数据。湖试数据的处理分析结果验证了论文研究的各种SAS信号处理算法的有效性及性能。同时作为国内第一台AUV平台合成孔径声纳,各项设计指标都已达到,为水下小目标探测提供了新的技术途径。更多还原

【Abstract】 For acoustic detection of small targets in underwater environments, one of the core issues is to solve a requirement conflict between long operating range and high spatial reso-lution. Traditional physical aperture-based sonar system employs high-frequency signaling to achieve high resolution, while low-frequency is required for long operating range. As an emerging underwater imaging technology, synthetic aperture sonar (SAS) processing can achieve an extremely high spatial resolution, which is range- and frequency-independent, thus making low-frequency small-aperture high resolution sonar imaging possible. In the meantime, the autonomous underwater vehicle (AUV) technology is growing mature, pro-viding a novel platform to implement synthetic aperture processing.This thesis concerns development of an AUV platform synthetic aperture sonar sys-tem, supported by Zhejiang University 985 Project PhaseⅡ, targeting to obtaining high resolution and superior quality seafloor acoustic images in shallow-water low-frequency operating environments. The research work includes study of signal processing algorithms and design/implementation of the relevant system hardware. The main part of this the-sis focuses on SAS-related signal processing development, including wideband multipath rejection (WMR), factorized back-projection (FBP) fast synthetic aperture imaging, and related motion compensation.In shallow water environments, multipath propagation arising from acoustic bound-ary interactions could degrades SAS imaging performance, leading to ghost targets and reduced image contrast. Using a vertically-displaced sonar receiving array can resolve and reject those multipath interferences by applying array signal processing in the vertical di-mension. Wideband signals are commonly utilized in low-frequency SAS systems in order to achieve a high range resolution; however, most of the current multipath rejection al-gorithms is only applicable to narrowband signals. Besides, due to space constraints of a small vehicle platform, only a small-size vertical receiving array can be mounted, demand- ing an adaptive algorithm for high vertical resolution. Thus some robust processing has to be developed in the presence of environmental disturbances. In this thesis, a novel WMR processing approach is proposed based on wideband beamforming. The narrowband robust Capon beamforming (RCB) is extended to wideband cases based on a steered covariance matrix (SCM), called steered RCB (SRCB), with property of near-instantaneous conver-gence. Numerical simulations and lake experimental results have verified the performance improvement of the SRCB-based WMR over the conventional delay and sum beamform-ing (DSB)-based and conventional wideband RCB (WRCB)-based approaches, in regard to both direction resolution and multipath rejection capabilities.Synthetic aperture imaging algorithm is one of the most critical parts in SAS signal processing, being most computationally intensive. Development of an imaging algorithm often focuses on two aspects:efficiency in computation and applicability to an arbitrary motion track. FBP algorithm has been extensively investigated for its appealing properties of being more flexible in accommodating motion mismatch and consuming less compu-tation time compared to conventional time-delay and sum (TDS) algorithm. An improved FBP algorithm is developed in this thesis, along with its theoretical range error analysis. Different from the existing FBP versions, the new development is not limited to a straight-line track of SAS; instead, it can handle arbitrary SAS moving tracks. Numerical simu-lations for both straight-line and circular SAS imaging and experimental data processing results have verified the effectiveness of the improved FBP algorithm.As a tradeoff to its superior resolution performance, SAS imaging is rather sensitive to motion errors, thus requiring proper motion compensation. SAS developed in this thesis has installed a Doppler velocity log (DVL), a compass and a depth sensor; using data from all those navigation devices, three-dimensional moving track and altitude of the SAS can be estimated to some accuracy. Typical SAS motion mismatch can be classified into two types:track mismatch and Doppler mismatch. The moving track mismatch is usually more important and can be corrected using the TDS and FBP imaging algorithms, simply by sub-stituting the estimated track for the assumed track. On the other hand, the moving Doppler mismatch is often regarded as having minor effect upon SAS imaging, and can only be compensated partially. However, it is observed in this thesis that the Doppler mismatch can seriously degrade SAS imaging performance when a large moving track mismatch co-exists. A new SAS Doppler compensation method is thus derived, which can accurately correct the Doppler mismatch. In addition, the compensation method can be implemented jointly and effectively with the FBP imaging algorithm, achieving both fast motion com-pensation and synthetic aperture imaging.During the thesis research, a prototype AUV platform shallow-water low-frequency SAS system has been designed and built, including sonar system hardware, real-time con-trol and data acquisition software. Through a tank test and two lake experiments, the proto-type SAS system has shown working reliably while acquiring a large amount of real data. The processing results and analyses of the experiment data have verified the perfonnance of relevant signal processing algorithms proposed in this thesis. Besides, as the first pro-totype SAS on an AUV platform in China, all design specifications have been met. thus rendering a novel technical platform for underwater small target detection.更多还原