Towed Array Research Articles

Towed Acoustic Countermeasures for Defending Acoustic Homing Torpedoes

The growing torpedo threat to ships and submarines demands effective countermeasures for defence. Detection, classification and localisation of an attacking torpedo is the first step towards launching effective countermeasures. Studies are on the rise to use the existing sonar systems to detect torpedoes and subsequently use countermeasures. The use of towed array sonar systems for torpedo detection and acoustic counter measures are the most recent and not reported much in open literature. This paper presents a modular acoustic counter measure approach using towed decoy against acoustic homing torpedoes. Describes the technologies and algorithms required for different modes of towed decoy, which is effective against both active and passive acoustic homing torpedoes. Towed decoy signal generation schemes and their realisation using digital signal processing hardware are outlined in this paper.

Identification of species-specific delphinid echolocation click types

Delphinid echolocation clicks are often recorded in large, high-quality datasets collected by moored autonomous passive acoustic sensors, but the particular species present during these acoustic encounters is generally unknown. We combined two acoustic data types with visual observations to obtain species identity for some of these clicks. We assigned species identity to acoustic encounters in eight years of recordings at five western North Atlantic mooring sites using sighting data from contemporaneous shipboard and aerial visual surveys. We also assigned species identity to acoustic encounters in towed acoustic array data collected concurrent with visual surveys during four ship-based cetacean surveys in the western North Atlantic and the Gulf of Mexico. 114 mooring encounters with seven species and 181 towed array encounters with nine species were labeled this way. We identified recurring click types during all encounters using an unsupervised clustering algorithm to groups clicks based on pairwise spectral distances. Labeled click types thus identified in the moored data were compared to labeled click types identified in the towed array data to determine their consistency and utility for delphinid species discrimination in moored passive acoustic data. Results for bottlenose dolphin, Atlantic spotted dolphin, Risso’s dolphin, and pilot whales will be presented.

Real-time passive acoustic monitoring results of the 2018 Pacific Region International Survey of Marine Megafauna off Western Canada

Real-time passive acoustic monitoring (PAM) was conducted as part of the first large-scale systematic survey of marine mammal distribution and abundance in territorial outer-coastal waters off western Canada. The survey was conducted for approximately five weeks, from early July to mid-August, 2018 using a 69-m Canadian Coast Guard and Fisheries Vessel. A four-element, towed hydrophone array and sonobuoys (type AN/SSQ-53F and AN/SSQ-53G) were deployed and monitored in real-time 24/7 by experienced bio-acousticians. The towed array was deployed for approximately 640 h (78% of the time underway). Approximately 145 independent detections of marine mammals were made. Sperm whales (n = 53) and porpoises (n= 49) represented just over two-thirds of all detections. Ten near real-time localizations of sperm whales and 31 localization of porpoises were made, representing just over 60% of all (n = 67) all high-quality localizations. Sonobuoy effort consisted of semi-systematic deployments day and night and when sightings of large baleen whales occurred. A total of 112 sonobuoys were deployed successfully and monitored in directional (DIFAR) mode. Marine mammal calls were recorded on 32% (36) of sonobuoys. Sonobuoy detections included killer whales calls and clicks, sperm whale clicks, dolphin clicks, fin whale calls, and blue whale calls (both B and D type calls).

Achieving consensus and convergence on a towed array passive acoustic monitoring standard for marine mammal monitoring

Towed passive acoustic monitoring (PAM) arrays from surface vessels has become a mature technology for marine mammal monitoring during marine seismic surveys. Multiple U.S. federal agencies have expressed a desire for consistent standards when implementing towed PAM. Two workshops involving academics, commercial PAM operators, and regulators have led to a comprehensive outline for an ASA towed array draft standard that covers requirements for initial operation planning, hardware, real-time monitoring and data acquisition, localization procedures, and post-operation validation. It does not cover operational mitigation decision criteria (such as power-down and/or shutdown of seismic airgun arrays), sound source verification, or how to initially establish the required detection range of the system. The standard’s fundamental goal is to reduce situations where background noise levels (arising from a variety of mechanisms) prevent effective PAM, and the dominant strategy employed is to standardize how acoustic measurements are logged, reported, and evaluated. Standardized plots would permit a non-technical supervisor or regulator to easily determine whether the PAM operation was self-noise limited under multiple operational circumstances. The approach taken here can serve as a template for future PAM standards for other deployment platforms [Work sponsored by BSSE and ONR Living Marine Resources Program.]

Synthetic aperture towed-array processing: The Edmund J Sullivan Legacy

Dr. Edmund J. Sullivan developed synthetic aperture towed-array processing. Confronted by many nay-sayers Dr. Sullivan persisted with his work and developed the first approach as the signal processing group leader (SPGL) at the SACLANT ASW Center (now CMRE) in La Spezia, Italy. The first notional passive synthetic aperture processor using an overlapped correlation method was jointly developed [“Extended Towed Array Processing by Overlapped Correlator,” (JASA, 1989)]. This idea was to blossom even further in his collaborative works (Stergiopolous et al.) extending these ideas to a fully, recursive (in-time) model-based passive-synthetic aperture processor [“Space-time array processing: A model-based approach” (JASA, 1997)]. He also began collaborations with the Swedish Navy where he performed joint experiments in model-based passive synthetic aperture evaluating its performance in the ocean and demonstrating its effectiveness. Dr. Sullivan began mentoring more researchers and teaching at the University of Rhode Island where he advised students in underwater acoustics and processing leading researchers to the model-based approach (Cousins), synthetic aperture processing (Edelson) [“On the performance of the overlap-correlator synthetic aperture technique” (JASA, 1991)] and broadband processing (Holmes) [“Broadband passive synthetic aperture” (JASA, 2006) ]. His ideas were summarized in his recent text, [Model-Based Processing for Underwater Acoustic Arrays (Springer, 2015)].Dr. Edmund J. Sullivan developed synthetic aperture towed-array processing. Confronted by many nay-sayers Dr. Sullivan persisted with his work and developed the first approach as the signal processing group leader (SPGL) at the SACLANT ASW Center (now CMRE) in La Spezia, Italy. The first notional passive synthetic aperture processor using an overlapped correlation method was jointly developed [“Extended Towed Array Processing by Overlapped Correlator,” (JASA, 1989)]. This idea was to blossom even further in his collaborative works (Stergiopolous et al.) extending these ideas to a fully, recursive (in-time) model-based passive-synthetic aperture processor [“Space-time array processing: A model-based approach” (JASA, 1997)]. He also began collaborations with the Swedish Navy where he performed joint experiments in model-based passive synthetic aperture evaluating its performance in the ocean and demonstrating its effectiveness. Dr. Sullivan began mentoring more researchers and teaching at the University of ...

Variability of Doppler shift in the ocean bottom reverberation data from the East Sea of Korea

Sea trials measuring the ocean bottom reverberation using an active towed array sensor (ATAS) equipped with triplet receiver array were conducted in the East Sea of Korea during August, 2015. The continuous-wave waveforms with center frequency 3 kHz and pulse lengths of 0.1, 0.3, and 1 s are used as source signals, respectively. Ship velocity was between 4 and 5 knots and the water depth varies from 500 to 1250 m. Also, sound speed profiles were obtained by expendable bathythermographs (XBTs) during the ship maneuvering, where the depths of minimum sound speed appeared between 208 and 306 m. From the measurement results, we observed variability and spreading of reverberation Doppler shifts, which depends on the bottom bathymetry, platform motion, source depth, and receiver array depth. In particular, significant asymmetry of the Doppler spectrum occurred at the depths of the source and receiver below 120 m. The instantaneous frequency Doppler of reverberation signal in each beam direction provides information on the bathymetry and scattering characteristics of the bottom. This frequency shift is traced in time and analyzed.

Addressing challenges in automated whistle extraction

Automated extraction of dolphin whistles from field recordings presents a challenging problem, as typically these recordings are noisy, contain multiple overlapping whistles and interfering signals. This results in many spurious detections, partial extraction and fragmentation of the whistle contours, which could cause problems for applications such as whistle-based localization. This paper presents the probability hypothesis density (PHD) filter, a non-traditional whistle tracking method based on multi-target Bayesian framework. Two implementations of the PHD filter, one based on Gaussian mixtures and one based on particle methods, are adapted for this task and tested on a large real-world dataset consisting of six dolphin species. The proposed filters successfully track simultaneous whistles, and compare favourably to standard methods. Moreover, it is shown how the proposed methods can be used to enhance the localization efforts with linear towed arrays.

Optimal Operating Depth Search for Active Towed Array Sonar using Simulated Annealing

In an active towed array sonar, it is important to find the optimal operation depth. Generally, the optimal depth can be chosen via numerical simulations for all sonar depths and this imposes great burdens of time and cost.In this paper, an efficient approach is proposed to find the optimal depth using the optimisation technique. First, the sonar performance function is newly defined as a measure of how well the active sonar might perform. This function depends on the properties of the ocean environment and the positions of sonar and underwater target. Then, the simulated annealing to find an optimal solution for maximising sonar performance is used. The optimised depth agrees well with the depth obtained from direct searching for all depths of source and receiver combinations, but its computational time is largely reduced.

Effect of Element Failures on the Performance of Passive Towed Array Sonars

Linear towed arrays are one of the principal methods of underwater target detection, especially at low frequencies. A typical towed array contains many hydrophones in a flexible tube towed several hundred meters behind the ship. A practical problem in towed arrays is hydrophone or channel failures. Failed channels impact the side lobe levels and increase the leakage from tow-ship noise. This significantly affects the achievable figure of merit of the system. It is necessary to achieve quantitative assessments of the performance degradation to decide on the future course of action for maintenance or repair. In this paper, we propose a method to quantitatively assess the performance degradation of the sonar due to element failures. This method is capable of suggesting the operational envelopes of the platform which will still give acceptable performance under failed conditions. Simulation results and an online performance monitoring scheme are proposed, which will give in situ information on the performance degradation and sonar health. The experimental results with a linear array towed at different speeds and the corresponding degradation experienced due to element failures are also evaluated and found to match with the predicted results.

A novel estimation algorithm for torpedo tracking in undersea environment

A novel estimation algorithm is introduced to handle the popular undersea problem called torpedo tracking with angle-only measurements with a better approach compared to the existing filters. The new algorithm produces a better estimate from the outputs produced by the traditional nonlinear approaches with the assistance of simple noise minimizers like maximum likelihood filter or any other algorithm which belongs to their family. The introduced method is extended to the higher version in two ways. The first approach extracts a better estimate and covariance by enhancing the count of the intermediate filters, while the second approach accepts more inputs so as to attain improved performance without enhancement of the intermediate filter count. The ideal choice of the placement of towed array sensors to improve the performance of the proposed method further is suggested as the one where the line of sight and the towed array are perpendicular. The results could get even better by moving the ownship in the direction of reducing range. All the results are verified in the MATLAB environment.

Subarray Beam-space Adaptive Beamforming Combined with Array Shape Estimation based on Non-Acoustic Sensor

To address the issue of serious decline in performance of the array signal processing caused by the towed array shape distortion during maneuvering, this paper presents a new method of subarray beam-space adaptive beamforming combined with an array shape estimation method based on non-acoustic sensor. Firstly, the array shape through the approximate circular arc structure of the array segment between the adjacent sensor during maneuvering is preliminary calculated. Next, the final estimated array shape through a smooth processing method on the entire array shape by means of a calibration method using spline interpolation technique is achieved. This method is able to estimate the array shape in real time. The array steering vector based on the real-time estimation of the array shape is updated, using a subarray beam-space adaptive beamforming (SBABF) to reduce the demand of the number of the snapshots. And the beam-space covariance matrix converges fast within a few snapshots. The SBABF method combined with array shape estimation (AE-SBABF) was verified by simulation data and sea trial data processing results. During maneuvering, the AE-SBABF can not only improve the array processing gain of the target effectively, but also solve the left/right ambiguity problem well.

DOA estimation of multiple sources for a moving array in the presence of phase noise

The authors investigate the issue of direction of arrival (DOA) estimation in the presence of phase noise for multiple sources with a synthetic linear array, which is synthesised by a short moving array. The extended towed array measurements (ETAM) method can extend the array aperture greatly, but can only be applied to multiple coherent sources and requires the array to move with constant velocity. To tackle the problems, the authors generalise the ETAM method to multiple incoherent sources and array non-uniform motion case. The authors first formulate the manifold of the extended synthetic array (SA) moving with known velocity in a straight line. Then the initial DOA estimates and phase correction factors are estimated successively by two-dimensional multiple signal classification (2D-MUSIC) spectrum search using adjacent measurements sampled by the moving array. Moreover, to reduce the complexity, the authors also propose a reduced dimensional MUSIC (RD-MUSIC) method to turn the two-dimensional peak search to one-dimensional. As the array aperture is extended by proper compensation, the DOA estimation performance of the proposed SA method improves. Besides, the proposed method can resolve more number of sources than sensors since every two measurements are used for estimation in each process. Simulation results validate the effectiveness of the proposed method.

Failure Analysis of Metallic Armoured Electro-Optic Mechanical Cables in Underwater Towed Sonar Systems

In towed sonar systems generally, the array is towed behind a ship using a combination of electro-optic mechanical (EOM) tow cables - negatively buoyant metallic armoured Heavy Cable and neutrally buoyant fibre armoured Light Cable- the former for depth control and latter for depth maintenance. Any failure in these cables can affect the performance of the sensor array and can even result in loss of the towed system to sea. During a series of towed array trials, it was observed that the metallic armoured cable had lost its circularity and became flattened during deployment and retrieval from ship. This may be due to the extreme radial forces acting on the cable while passing through a) rollers located at winch spooler and b) main fairlead comprising of 5 rollers located at aft end of ship, which causes permanent changes in cable’s internal configuration. To verify the cause and study the effects of such flattening on functionality and life of the cable, cyclic load testing studies were carried out after simulating the actual deployment-retrieval conditions as that in the ship. Both indigenously developed and imported cable samples were chosen as test samples for performance comparison. During the tests, it was observed that severe flattening of armour and core cable resulting in failure of electrical & optical lines had occurred in indigenous cable samples. The imported cable samples showed no flatness or failure of lines. Based on the results, steps were taken to improve the radial load bearing capacity of the indigenous cable. Modifications were made in a) core cable sheath material b) core cable sheath thickness & c) armour wire material and an improved indigenous cable was developed, which has withstood all the above tests and is now ready to be inducted to the system for sea trials.

MVDR Beamformer with Subband Peak Energy Detector for Detection and Tracking of Fast Moving Underwater Targets Using Towed Array Sonars

MVDR Beamformer with Subband Peak Energy Detector for Detection and Tracking of Fast Moving Underwater Targets Using Towed Array Sonars

The Application of Magnetometers and Electromagnetic Induction Sensors in UXO Detection

This paper introduces the physical model of unexploded ordnance (UXO) in the magnetization process and the formula derivation of the magnetic dipole model. The application of magnetometers and electromagnetic induction sensors in the detection of UXO is introduced. The magnetometers introduce the total field measurements technology of the CS optical-pump marine magnetometer and the MagSTAR (Magnetic Scalar Triangulation and Ranging) gradient detection technology. The electromagnetic induction sensors introduce the working theories and the popular product models of Geophex company and Geonics Ltd. The method of detecting UXO by MTADS of United States Naval Research Laboratory (The multisensor towed array detection system) compares the difference between the United States Naval Research Laboratory for target recognition.

Development of vector hydrophone using thickness–shear mode piezoelectric single crystal accelerometer

In this study, a vector hydrophone was developed for a towed array sonar system using a shear type accelerometer. In general, inertia type vector hydrophones using accelerometers have higher sensitivity than multimode vector hydrophones. However, the sensitivity still needs to be enhanced as much as possible for practical application. For the purpose, the effect of the structural parameters of a shear type accelerometer on the receiving voltage sensitivity (RVS) of the vector hydrophone was rigorously analyzed, and the structure of the vector hydrophone was optimized to maximize the RVS. The developed vector hydrophone has an external diameter of 23 mm and the minimum RVS over the given frequency range of -201.4 dB. It was also verified that the designed vector hydrophone exhibits the characteristic of a dipole mode beam pattern. Furthermore, a prototype of the vector hydrophone was fabricated with the optimal dimensions, and its acoustic characteristics were measured to verify the validity of the design. The vector hydrophone developed in this work has the highest feasible RVS over the desired frequency range in comparison with other types of vector hydrophone like the multimode hydrophone.

Research on suppression for tow-ship interference

Towing line array underwater detection equipment detection performance will be tug of radiation noise interference, in order to enhance the performance of sonar detection, the key from the array spacing and follow-up signal processing to proceed. The purpose of using the towing array underwater probing device is to target the tail of the tug, but in actual use the towed array of half-wave lengths is affected by the strong tug noise The detection performance of the ship's tail direction is greatly restricted, and it is imperative to improve it. Otherwise, it will not be able to detect the threat target from the wake of the wake, and the target can not be found in time. This paper focuses on the reception directivity of the quarter-wave spacing matrix under the single-frequency signal and the wideband signal. By comparing the results of the two directional simulations, it is known that, unlike the half-wavelength interval matrix, the end-to-receive directionality of the quarter-wave spacing matrix is asymmetric, which can significantly suppress the interference in the direction of the drag Detection performance. The tandem noise can be suppressed by the combination of the received signal and the zeroing of the tampon, and the quarter-wave spacing matrix formed by the permutation-weighted combination array can further suppress the tug noise.

Clymene dolphin (Stenella clymene) whistles in the Southwest Atlantic Ocean

The Clymene dolphin despite being endemic for the Atlantic Ocean continues to be the least known species in the genus Stenella without available information on their vocal repertoire in Brazilian waters. Data were obtained during mitigation and monitoring work required by IBAMA under the federal environmental licensing as conditions of the license 108/16 for the 3D seismic survey in Para/Maranhao Sedimentary Basin process 02022.000015/2014. The species record was performed onboard the vessel Polarcus Alima with a Mseis (Night Hawk III) four-element towed array passing signals to a digital M-Audio, recording at 96 kHz/16bits. During visual and acoustic monitoring with the air-guns off, a group of approximate 80 dolphins were sighted and recorded on May 8th, 2016 (00˚38’00” N, 44˚45’25” O) at 3,425 m depth. The wav-files were analyzed through the spectrogram configured as DFT 2048 samples, 70% overlap and Hamming window of 1024 points generated by software Raven Pro 1.5. The results of 14 min recording allowed the extraction of 257 whistles. Minimum frequencies ranged from 5.20 kHz to 15.92 kHz (mean = 10.99 kHz); maximum frequencies ranged from 7.72 kHz to 28.04 kHz (mean = 16.16 kHz); whistle duration ranged from 0.01 s to 1.52 s (mean = 0.43 s). These results are important since they represent the first acoustic record of Clymene dolphins in Brazil.

Optimization of deployment depth for active towed array sonar (ATAS) using simulated annealing

The active towed array sonar (ATAS) is an active operating sonar towed behind the surface ship and deployed at various depths. The performance of ATAS may vary with the deployment depth. This study proposes a method which calculates the optimal deployment depth using simulated annealing (SA) in order to reduce the total computational time spent in the direct search of the deployment depth. We define a sonar performance function (SPF) with the probability of detection, which represents as a degree of how well the ATAS might perform at particular deployment depth. Then, the optimal depth is defined as the depth where the SPF is maximized. The SPF depends on the acoustic environment, target position and source-receiver depth. The optimization results are compared with the direct calculation of SPF at all deployment depth of source and receiver. Also, our study provides the optimal depths in the area of East Sea of Korea.

Approximately optimal distribution of depth sensors over towed array

Towed array shape estimation aided with non-acoustic sensors is widely used for its rather low computational complexity of solutions and rather explicit results. In addition, previous studies have emphasised that depth sensors' distribution has a dramatic influence on the accuracy of this kind of array shape estimation method. Established on the basic theory of towed array shape estimation using Kalman Filters, the approximately optimal distribution of a certain number of depth sensors over a certain number of discretised towed arrays yields the approximately best achievable performance in terms of minimum space average mean square error (AMSE), is addressed in this study. The effect of depth sensors' distribution has been discussed. Then an exact expression for the space AMSE is derived. The expression is simplified in a reasonable way considering the practical issues in order to calculate the minimum space AMSE rapidly and effectively. The performance assessments demonstrate the effectiveness of the newly proposed method.