Underwater Sonar Systems Research Articles

Effects of a Subsurface Eddy on Acoustic Propagation in the Northwestern Pacific Ocean

Using a ray tracing model, acoustic propagation through a subsurface eddy is investigated. The eddy with a 460 km diameter lies at approximately 23° N, 126° E. The presence of the subsurface eddy is found to result in significant alterations to the received acoustic field. Affected by the subsurface eddy, the location of a given convergence zone can be different from usual with optimum propagation conditions. For a source deployed outside of the eddy, the subsurface eddy creates a surface duct by modifying the sonic layer depth. A maximum of 15 km difference in the range of a given convergence zone is found between the propagations with the presence and absence of the eddy when the source is placed around 100 m depth. However, for a source within the eddy, the intense shift in convergence zone patterns occurs only when the source is deployed around 300 m depth. These ocean-acoustic results can provide essential information for operating technologies such as communication devices, underwater sonar, and navigation systems.

Designing a Geodesic Faceted Acoustical Volumetric Array Using a Novel Analytical Method

We present a novel analytical method as an efficient approach to design a geodesic-faceted array (GFA) for achieving a beam performance equivalent to that of a typical spherical array (SA). GFA is a triangle-based quasi-spherical configuration, which is conventionally created using the icosahedron method imitated from the geodesic dome roof construction process. In this conventional approach, the geodesic triangles have nonuniform geometries due to some distortions that occur during the random icosahedron division process. In this study, we took a paradigm shift from this approach and adopt a new technique to design a GFA that is based on uniform triangles. The characteristic equations that relate the geodesic triangle with a spherical platform were first developed as functions of the operating frequency and geometric parameters of the array. Then, the directional factor was derived to calculate the beam pattern associated with the array. A sample design of GFA for a given underwater sonar imaging system was synthesized through an optimization process. The GFA design was compared with that of a typical SA, and a reduction of 16.5% in the number of array elements was recorded in the GFA at a nearly equivalent performance. Both arrays were modeled, simulated, and analyzed using the finite element method (FEM) to validate the theoretical designs. Comparison of the results showed a high degree of compliance between the FEM and the theoretical method for both arrays. The proposed novel approach is faster and requires fewer computer resources than the FEM. Moreover, this approach is more flexible than the traditional icosahedron method in adjusting geometrical parameters in response to desired performance outputs.

Machine learning approach to prediction of real-time ocean sound speed profile

Sound speed profile (SSP) determines sound propagation in the ocean. Variation of sound speed profiles with time and location heavily impacts underwater sonar systems used in ocean science and engineering. However, mapping sound speed profile over a vast area of the oceans in real-time is impossible by the traditional way, where sound speed is calculated from insitu profiling measurements of temperature, salinity, and pressure, which is expensive and, therefore, limited in sparse locations and time. Here, we aim to develop a machine learning model to predict real-time sound speed profiles anywhere in the global oceans. Our model takes advantage of the big data of long-term global temperature, salinity, and pressure measurements and further accounts for the variability of the surface ocean by inputting sea surface temperature and sea surface salinity data, which can be updated by satellite remote sensing in real-time. Here, SSPs are predicted for Pacific, Atlantic, and Indian ocean regions. The results show that the estimated SSPs had a root-mean-square error of 0.26 m/s and a coefficient of determination of 0.99. About 99% of the estimates lie within ±0.4 m/s of the SSPs obtained from in situ temperature and salinity profiles.

Bionic target localization method based on range-azimuth joint dictionary

The traditional underwater sonar system usually achieve high angle resolution by increasing array aperture and the number of array elements, but this method will inevitably lead to complex system and high cost. Given that big brown bats have obtained surprisingly high resolution using a simple system, this paper proposes a bionic target localization method. First, a range-azimuth joint dictionary was constructed based on the bionic system of multi-harmonic emission and double random array reception. Then, the coherence characteristic of the dictionary was analyzed and the range and azimuth of the target were estimated, and at last the experimental verification was completed. The results show that the bionic range-azimuth joint estimation based on sparse signal representation can achieve high-precision target localization under the condition of echo high aliasing.

Movement tracing and analysis of benthic sting ray (Dasyatis akajei) and electric ray (Narke japonica) toward seabed exploration

Creation of a seabed map is a significant task for various activities including safe navigation of vessels, commercial fishing and securing sea-mined resources. Conventionally, search machines including autonomous underwater vehicles or sonar systems have been used for this purpose. Here, we propose a completely different approach to improve the seabed map by using benthic (sting and electric) rays as agents which may explore the seabed by their autonomous behavior without precise control and possibly add extra information such as biota. For the first step to realize this concept, the detail behavior of the benthic rays must be analyzed. In this study, we used a system with a large water tank (10 m × 5 m × 6 m height) to measure the movement patterns of the benthic rays. We confirmed that it was feasible to optically trace the 2D and 3D movement of a sting and an electric ray and that the speed of the rays indicated whether they were skimming slowly over the bottom surface or swimming. Then, we investigated feasibility for measuring the sea bottom features using two electric rays equipped with small pingers (acoustic transmitters) and receivers on a boat. We confirmed tracing of the movements of the rays over the sea bottom for more than 90 min at 1 s time resolution. Since we can know whether rays are skimming slowly over the bottom surface or swimming in water from the speed, this would be applicable to mapping the sea bottom depth. This is the first step to investigate the feasibility of mapping the seabed using a benthic creature.

An Optimized Planar MIMO Array Approach to In-Air Synthetic Aperture Sonar

Modern three-dimensional (3-D) imaging sonar sensors are perfectly capable of creating awareness of the complete nearby environment with great accuracy. However, as range increases, these systems start to struggle with object detection as the poor attenuation of sound through air causes the signal-to-noise ratio to become too small. When looking at other sensing modalities, such as radar, this problem is tackled by using systems that utilize more than one element in the sending and/or receiving stage resulting in a greater effective detection range. At this further distance, the resolution of the obtained image will tend to decrease. Both radar and underwater sonar systems make use of synthetic aperture systems to increase the spatial imaging resolution. In this letter, we combine both techniques to create an in-air (simultaneous transmit) synthetic aperture beamforming (STSTAB) sensor, based on the eRTIS 3-D sonar sensor, that will increase the system's range and angular resolution without sacrificing the measurement rate. Different aspects of the emitter array will be addressed, such as the optimal placement of the elements of the emitter array, along with the coded signals that will be emitted simultaneously. The result will be a fast-measuring imaging sonar sensor with a narrow point-spread-function and improved range.

Reference Phase Stabilizer for Distributed Underwater Sonar Systems.

An optical fiber is a promising approach for data and clock transmission in distributed underwater sonar systems. However, synchronization is a critical challenge in distributed sonar systems, which mandates accurate clock synchronization down to a sub degree. Potential phase misalignment is caused by fiber length variations. In this paper, we propose a fiber-based phase stabilizer method to achieve accurate clock synchronization among sensor nodes. We use fiber-based feedback loop between sensor nodes and central station unit to monitor phase variations. Subsequently, we leverage phase shifters symmetrically arranged on the forward lane and feedback lane to compensate real-time phase variation and maintain high-precision synchronization. Besides, an ambiguity eliminator circuit is designed to remove the clock’s cyclic ambiguity. Both analysis and experimental results suggest that the proposed phase stabilizer can achieve 10 MHz reference clock synchronization within 0.4 degree. We also analyze the impact of the reference clock’s phase error on the system range detection accuracy, which indicates that the proposed phase stabilizer can greatly improve detection accuracy of sonar systems.

Локальное позиционирование подводных аппаратов гидроакустической системой с ЛЧМ зондирующими сигналами.

Локальное позиционирование подводных аппаратов гидроакустической системой с ЛЧМ зондирующими сигналами.

Underwater Sonar Systems: A Methodology to Generate Bathymetry and Sub-strata Data

Underwater Sonar Systems: A Methodology to Generate Bathymetry and Sub-strata Data

Highly sensitive fiber laser ultrasound hydrophones for sensing and imaging applications.

We have developed highly sensitive photonic ultrasound hydrophones based on polymer-packaged dual-polarization-mode fiber lasers. The incident ultrasound wave is scattered by the polymer cylinder due to the difference in elastic property. The scattered wave can drive harmonic vibration of the cylinder and result in optical response in terms of beat-frequency variation of the laser output. Experimental results exhibit a broadband ultrasound response at frequencies below 1 MHz. The individual vibration modes are excited by the ultrasound waves with different efficiencies, yielding a frequency-dependent response. A hydrophone with a diameter of 5 mm presents a detection limit of 2 mPa/Hz1/2 at 200 kHz. We further demonstrate its capability of ultrasound imaging for applications in underwater acoustics and sonar systems.

Detection of Underwater Carrier-Free Pulse based on Time-Frequency Analysis

Carrier-free short pulse widely employed in UWB radar is brought into high-resolution sonar system, which has unique advantages: attaining more target information, restraining fluctuation of reverberation envelop efficiently in short-range detection and achieving accurate estimation. In essence such pulse is transiently short in time domain and wide in frequency domain, and as such it is difficult to separate signal to noise based on Fourier Transform spectrum. So as to seek for detection methods of short pulse, minor differences of energy distribution of time-frequency characteristics are presented on three time-frequency methods such as Short Time Fourier Transform, Wavelet Transform and Hilbert-Huang Transform. With these results, a tri-channel detector is established for such underwater short pulse in noise environment, which is generally suitable not only for detection module of underwater sonar system but also that of radar system.

Acoustic filter design for precise measurement of parametric array in near-field region

A parametric array is a nonlinear conversion process that generates a narrow beam of low-frequency sound using a small aperture and that is used in active underwater SONAR and communication systems. A parametric array generates a highly directional difference frequency wave (DFW) by nonlinear interaction of bi-frequency primary waves at higher frequencies. However, it is difficult to measure the parametric array effect in the near distance of the transducer, because the high-pressure level of the primary waves may cause the receiving transducer to provide with pseudo-sound signals of difference frequency by its nonlinearity. The pseudo-sound signals make it difficult to measure the real DFW due to the parametric array process. In this study, we confirmed the existence of pseudo-sound and its effects on measurement of the near-distance DFW by the parametric array using numerical simulations based on the KZK equation. Also, a newly designed acoustic filter was proposed to eliminate pseudo-sound signals. This new acoustic filter uses the resonance of a thickness extensional mode. With the acoustic filter, low frequencies (DFW) were passed and high frequencies (primary frequencies) were effectively reduced. Using this acoustic filter, the precise characteristics of the difference frequencies could be measured. [Work supported by ADD(UD100002KD).]

Liquid Crystal-Based Hydrophone Arrays

We describe a fiber optic hydrophone array system that could be used for underwater acoustic surveillance applications (e.g. military, counter terrorist, and customs authorities in protecting ports and harbors), offshore production facilities or coastal approaches as well as various marine applications. In this paper, we propose a new approach to underwater sonar systems using the voltage-controlled liquid crystals and simple multiplexing method. The proposed method permits measurement of sound under water at multiple points along an optical fiber using the low cost components and standard single mode fiber, without complex interferometric measurement techniques, electronics or demodulation software.

Bistatic detection and imaging of proud targets in shallow water using coherent space‐time‐frequency processing.

An ongoing challenge for underwater sonar systems is to discriminate a man made target (typically having an elastic shell) from surrounding clutter returns especially in the presence of multipath. Network of autonomous systems of unmanned vehicles provides a practical means for bistatic measurements and thus potentially bistatic enhancement for target detection. One popular technique for automatic target detection is synthetic aperture imaging. An aspect that complicates this technique is the evolutional, time dependent aspect of the bistatic echo spectrum from elastic shells especially for the acoustic guided waves echo components at low to midfrequency. To this end, time‐frequency analysis, and, in particular, Wigner‐Ville analysis have been shown to provide a robust processing tool for interpreting the evolutional time dependent aspect of the scattered acoustic wave field especially around the midfrequency enhancement. Time‐frequency methods will be used to enhance the sonar images created from monostatic or bistatic low‐frequency measurements. The proposed approach will be tested using numerical simulations and experimental at‐sea data. [Work supported by the ONR.]

Experimental Investigation of Underwater Noise Produced by Ships by Means of Sound Intensity Method

Ship noise is a one of most important part of the field of the acoustic environment. Its reduction and monitoring is a very important aspect in the performance of underwater acoustic systems and in the habitability of the vessel for the crew and passengers. In naval operations the noise radiated by a ship is also dominant source of information for underwater sonar weapons systems. Sound intensity is the vector quantity which informs us about the sound power flux, the direction of the sound source. The probe tracks the position of source and at the same time, measures the component of sound-intensity vector. Generally only one component of vector sound intensity was measured, perpendicular to a given measurement surface. The sound intensity of a moving source includes eects such as flow noise that are not present when the source is stationary. The accuracy of measuring acoustic intensity using two closely spaced hydrophones by correcting the phase mismatch error by measuring the transfer function between the two detector systems at the same time was examined. The two measuring hydrophone instrumentation systems were earlier carefully calibrated. Our results have been obtained using digital methods, where was used PULSE B&K measuring system. PACS numbers: 43.30.Nb 1. Intorduction Very little data is available on sound intensity and even sound pressure level (SPL) measurements and noise spectra of small boats or big vessels. Noise from warships and merchant vessels with keel aspect source levels ranging between 140 and 192 dB re 1 „Pa at 1 m. Hostile naval forces also use the underwater noise signatures radiated by surface ships and submarines for detection and classification of ships as targets. At low speeds up to 8 kn, the underwater noise signature of the ship is dominated almost totally by tones from the ship’s Diesel generator set. Above this speed, the ship’s signature is dominated by three sources: main engine diesel firing rate harmonics, blade harmonics, and than wideband cavitation noise. The eect of the noise aspect of maritime modern civilization on sea ecosystems is frequently ignored, but it is beginning to be recognized as a trouble worthy of further research. A small number of available measurements have been made of the full sound intensity vector, and little has been done so far to explore its use. The sound intensity hydrophone probe was designed to measure sound intensity together with the unit direction of flow as a vector quantity. This was achieved by setting up two hydrophone probe to measure the sound energy flow. The distance between them was 120 mm, it allows to determine the sound intensity with accuracy §1 dB in the frequency

Space-time-frequency analysis of the scattered acoustic wave field from elastic shells recorded by bistatic sonar systems.

An ongoing challenge for underwater sonar systems is to discriminate a man made target (shell) from surrounding clutter returns and to provide robust classification features for the estimation of the physical target characteristics (e.g., shell thickness and material properties). To this end, time-frequency analysis, and, in particular, Wigner–Ville analysis, has been shown to provide a robust processing tool for interpreting the evolutional time dependent aspect of the scattered acoustic wave field from elastic shells. The design of a robust space-time-frequency bistatic sonar system to enhance the target detection of shells with the use of a sensor array will be presented. Practical implications for bistatic mine countermeasure sonar systems, using a network of autonomous underwater vehicles, will be discussed.

Analysis of the Effect of Reflected Wave from Sonar Dome on Radiation Impedance for Cylindrical Array

Radiation impedance is one of the very important factors in designing an underwater sonar system. In such a system, transducer array is surrounded by a dome to protect its elements from the underwater environment. In such a case, the effect of a reflected wave on radiation impedance cannot be ignored. To analyze the effect in a cylindrical array, a simplified model was introduced and radiation impedance was calculated by considering the following variables: the separation distance from the reflector (z), the interval between elements (d), wave number and element size (ka), and reflection coefficient (Γ). It is clarified that the effect of a reflected wave increases with ka and Γ, with decreasing z and d.

Design of an Absolutely Aplanatic Acoustic Lens

The underwater sonar system using an acoustic lens is receiving attention again because it does not need a complex electrical circuit and exhibits reduced volume and cost. However, in our former experiments, singlet spherical or aspherical lenses could not concentrate sound at a focal point completely because of aberrations. In this study, we tried to design a lens that can remove spherical and coma aberrations in a paraxial area using a ray tracing method, and two types of lens were designed. One is a biconcave acoustic lens with the refractive index of n=0.56, which corresponds to an acrylic resin, and the other is a biconvex acoustic lens with n=1.5, which corresponds to a silicon rubber. These lenses are evaluated by comparing of their sound pressure fields with those of a spherical lens and a plano-concave elliptic lens as calculated by a two-dimensional finite difference time domain (2D-FDTD) method. As a result, the biconcave aplanatic lens shows the best performance among these four lenses.

Pressure Dependence of Cymbal Transducers

The hydrostatic pressure limit that a receiver can withstand without failure is of major importance in underwater sonar systems. In this paper, the hydrostatic pressure tolerance and sensitivity of cymbal receivers were investigated. The failure mode in cymbal transducers under hydrostatic pressure is described. Effects of cavity geometry and material selection on hydrostatic piezoelectric coefficients and pressure limits were evaluated using both experimental data and finite-element analysis (FEA). It was found that cavity depth has a very strong effect on the stability of underwater sensitivity and pressure tolerance of these devices. Cymbals made with soft piezoelectric transducers (PZTs) possess higher figures of merit and better pressure tolerance than those made with hard PZTs. Alternatively, the cymbal sensitivity and pressure tolerance can be improved by changing the cap material.

Pressure Limitations of Miniature Hollow Sphere Hydrophones

Ultrasonic probes that can withstand high pressures and temperatures are needed for deep oil wells, underwater sonar systems, the processing of chemicals, and as automotive actuators. Pressure limits of hollow spherical lead zirconate titanate hydrophones with diameters in the range of 0.5–3.0 mm and wall thicknesses of 50–300 μm have been evaluated using both experimental data and finite element analysis. The spheres were tested at hydrostatic pressures up to 70 MPa (10,000 psi, 7000 meters) and temperatures of 25°C–200°C. The results of the tests and analyses indicate that, at room temperature, unelectroded spheres of all sizes within this size range can withstand pressures up to 7 MPa (1000 psi) and that smaller spheres can survive up to 70 MPa (10,000 psi). Combined temperature-pressure tests on 2 mm spheres indicate survival at temperatures and pressures as high as 200°C and 28 MPa (4,000 psi). Preliminary poling studies indicate that depoling may become a problem at high pressures and elevated temperatures.