Shallow-water Low-frequency Synthetic Aperture Sonar: Signal Processing & System Implementation

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 Phaseâ…¡, 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.