In recent decades, imaging sonar has been the most widely employed remote sensing instruments in the field of underwater detection. The multibeam echo sounder (MBES) plays an important role in obtaining high-accuracy seabed topography. However, the resolution of the MBES substantially decreases with the increasing distance. Synthetic aperture sonar (SAS) achieves constant resolution on the along-track, improving the fineness of the image. However, conventional side-scan SAS usually only achieves 2D images, and gaps always exist. In this modeling and experimental research paper, we propose a novel underwater acoustic imaging scheme to improve the imaging performance of MBES, based on the complementarity of MBES and SAS systems. We design a 2D transducer array to increase the detection efficiency and obtain spatial gain. Moreover, the processing scheme is analyzed to design the working parameters in actual engineering applications. We exploit a target echo simulation approach to establish the research basics of the imaging algorithms, which also reflects the shapes and shadows of targets to match actual situations as realistically as possible. The proposed imaging algorithm synthesizes a virtual aperture receiving array on the along-track and reserves the multi-element manifold on the across-track. This helps to improve the imaging quality of the MBES and achieves high-resolution 3D detection with no gaps. Simulation and tank experimental results demonstrate that the proposed scheme can significantly improve the detection ability of the MBES, especially for small 3D target detection, thus making it suitable for 3D high-resolution underwater detection applications.

High-resolution imaging method is one of the researching focuses of underwater acoustic detection. Underwater small-target detection also requires detailed imaging technology. Multibeam echo sounders (MBESs) and synthetic aperture sonar (SAS) are the effective instruments widely researched to obtain underwater acoustic images. Constrained by the theory, the along-track resolution of MBES decreases with distance and the gaps problem of SAS always exists and both inevitably limit the quality of acoustic imaging. In this paper, a two dimensional multibeam synthetic aperture sonar (MBSAS) model is designed to overcome the shortcomings of conventional underwater imaging instruments. MBSAS can provide a three dimensional (3D) high-resolution acoustic image without a gap problem. An echo model and transducer array manifold are designed to meet the requirements of engineering applications. Imaging theory and target simulations prove the feasibility of the MBSAS model. The performance of the proposed model is demonstrated with a tank experiment. A detailed image is obtained through an experiment that can indicate the shapes of targets and has the ability to separate adjacent targets. The simulations and experimental results indicate that MBSAS can obtain a more detailed 3D full-scan image than conventional MBES and SAS system with a better energy focusing ability.

The integrated observation of seabed topography, sediment geomorphology and sub-bottom profile information is very important for seabed remote sensing and mapping. To improve the efficiency of seabed detection and meet the needs of portable development of detection equipment, we developed a portable seabed feature integrated detection sonar (PSIDS) with whcih a single sonar device can simultaneously detect the above three types of seabed information. The underwater transducer is mainly composed of the following three components: a parametric emission array as the sound source, a high frequency receiving linear array for multibeam echo signal collection, and a two-dimensional vector hydrophone for receiving the low-frequency sediment echo signal. Field experiments were conducted to validate the performance of the PSIDS on 11–17 January 2018 in Jiaozhou Bay, China. (1) PSIDS could perform the functions of both multibeam sonar and sub-bottom profiler; (2) The synchronously and integrated measurement of various seabed information was achieved by alternately emitting multibeam echo-sounding and sub-bottom profiling signal using parametric source. The detection results proved the feasibility and practicability of PSIDS to achieve multiple seafloor characteristics. PSIDS provides a new idea for developing integrated seabed detection sonar. In terms of convenience and data fusion, it is a good option to use this equipment for integrated seabed detection.

The multipath spread in underwater acoustic channels is severe and, therefore, when the symbol rate of the time reversal (TR) acoustic communication using single-carrier (SC) modulation is high, the large intersymbol interference (ISI) span caused by multipath reduces the performance of the TR process and needs to be removed using the long adaptive equalizer as the post-processor. In this paper, a TR acoustic communication method using filtered multitone (FMT) modulation is proposed in order to reduce the residual ISI in the processed signal using TR. In the proposed method, FMT modulation is exploited to modulate information symbols onto separate subcarriers with high spectral containment and TR technique, as well as adaptive equalization is adopted at the receiver to suppress ISI and noise. The performance of the proposed method is assessed through simulation and real data from a trial in an experimental pool. The proposed method was compared with the TR acoustic communication using SC modulation with the same spectral efficiency. Results demonstrate that the proposed method can improve the performance of the TR process and reduce the computational complexity of adaptive equalization for post-process.

This paper presents a Shallow water High Resolution Multi-Beam Echo Sounder (SHMBES), which will be used widely in modern ocean exploration and underwater engineering. It is highly synthesized and developed on the basis of many new techniques such as theories of underwater acoustic and modern signal processing, Very Large Scale Integration (VLSI), DSP array processing, etc. It is mainly composed of sonar deck unit (including multi-channels receiver and A/D conversion, real-time digital signal processing system, host computer and interface, etc.) and sonar watertight housing (including transducer, signal generation and control phased transmitter, etc.), as well as real-time and postprocessing software. Five parts are discussed in the paper: Section I is system structure and technical specification. Section II is the theory analysis of split sub-array phase detection and multiple sub- array detection methods. In this section the improved multiple sub- array detection method is presented. Section III is precision evaluation experiment. Three sub-array configurations are chose to process the experimental data obtained at sea based on theoretical analysis. It has shown that, with optimized parameters, the multiple sub-array detection method not only realizes the wide coverage, but also improves the detection performance. Section IV lists Songhua Lake and Mogan Lake measurement results of underwater terrain. The instrument validity of hardware and software is tested by these lake experiments. Section V are the conclusions of the paper.

Filtered multitone (FMT) modulation divides the communication band into several subbands to shorten the span of symbols affected by multipath in underwater acoustic (UWA) communications. However, there is still intersymbol interference (ISI) in each subband of FMT modulation degrading communication performance. Therefore, ISI suppression techniques must be applied to FMT modulation UWA communications. The suppression performance of traditional adaptive equalization methods often exploited in FMT modulation UWA communications is limited when the effect of ISI spans tens of symbols or large constellation sizes are used. Turbo equalization consisting of adaptive equalization and channel decoding can improve equalization performance through information exchanging and iterative processes. To overcome the shortcoming of traditional minimum mean square error (MMSE) equalization and effectively suppress the ISI with relatively low computation complexity, an FMT modulation UWA communication using low-complexity channel-estimation-based (CE-based) MMSE turbo equalization is proposed in this paper. In the proposed method, turbo equalization is first exploited to suppress the ISI in FMT modulation UWA communications, and the equalizer coefficients of turbo equalization are adjusted using the low-complexity CE-based MMSE algorithm. The proposed method is analyzed in theory and verified by simulation analysis and real data collected in the experiment carried out in a pool with multipath propagation. The results demonstrate that the proposed method can achieve better communication performance with a higher bit rate than the FMT modulation UWA communication using traditional MMSE adaptive equalization.