Sonar Operations Research Articles

A method to predict active sonar detection range in uncertain shallow water environments

Knowledge of maximum detection range is important for the appropriate operation of search sonar. For example, effective spacing of multistatic sonar sources and receivers will be influenced by good estimates of achievable detection range. But predicting detection range for low-frequency active sonar in shallow water can be problematic in the face of inadequate environmental characterization. Thus, incomplete knowledge of bottom sediment geoacoustics and bottom scattering strength, and absence of a good understanding of bottom reverberation mechanisms, will greatly hamper the accurate prediction of dependable detection ranges. Given a bistatic sonar configuration in shallow water, a method is proposed and described that exploits the observed transition time from reverberation to noise-limited conditions to simply and robustly predict maximum detection range for specified target strength. Modeling and analysis suggest that estimates of maximum detection range within 25% accuracy can be readily achieved.

The application of acoustic standards to sound sources and the subsequent levels in the ocean

The American National Standards on Acoustics are applied to common sound sources used in both acoustics research and in naval sonar systems operation. Metrics are quantified for both continuous and transient sources of sound. The standard definitions are reviewed with theoretical sound source models and applied to examples of energy sources of sound such as transients from a small omni explosive, an air gun, a light bulb, and a dolphin. A qualitative model of a typical surface ship sonar system is discussed and active sonar transmissions are analyzed with the requisite quantitative metrics required to characterize these emissions. The relative role of peak pressure, time spread, intensity, and energy flux is discussed for deep- and shallow-water environments. These results should be useful in environmental assessments, biological experiments, and in the system design.

Active sonar classification using perceptual signal features from musical acoustics

Because of the relatively low frequencies at which they operate, navy active sonars are often plagued by false-alarm returns resulting from geological structures. In the lexicon of sonar operators these false returns are referred to as geoclutter or simply clutter. Despite mounting evidence that human operators can aurally discriminate target returns from clutter, attempts to develop robust automatic classification algorithms have, as yet, met with limited success. This paper investigates the possibility of improving the performance of automatic classifiers by exploiting the signal processing employed in the human auditory system. This amounts to replacing the statistical signal features used by conventional automatic classifiers with perceptual signal features that reflect the way a human listener would perceive a given return. Drawing an analogy between active sonar returns and percussive musical timbre, this paper considers perceptual signal features—like loudness centroid, sub-band attack/decay, and sub-band correlation—that have previously been identified as underlying the perception of timbre. Values for each of these timbre features are measured for a series of target and clutter returns recorded during an experimental sea trial. The effectiveness of these perceptual features as target-clutter discriminators is then evaluated using a Gaussian linear discriminant classifier.

Aural classification of impulsive-source active sonar echoes

The goal of this effort is to develop automatic target classification technology for active sonar systems by exploiting knowledge of signal processing methods and human auditory processing. Using impulsive-source active sonar data, formal listening experiments were conducted to determine if and how human subjects can discriminate between sonar target and clutter echoes using aural cues alone. Both trained sonar operators and naive listeners at APL-UW were examined to determine a baseline performance level. This level was found to be well above chance for multiple subjects in both groups, validating the accepted wisdom that there are inherent aural cues separating targets from clutter. In a subsequent experiment, feedback was provided to the naive listeners and classification performance dramatically improved, demonstrating that naive listeners can be trained to a level on par with experts. Using these trained listeners at APL-UW, a multidimensional scaling (MDS) listening experiment was designed and conducted. The results of these experiments and an analysis of the data, particularly its correlation with the physical attributes of target and clutter echoes (i.e., signal features), will be presented.

An experimental laboratory investigation of the low-frequency acoustics of seagrasses

The acoustics of seagrass beds can impact sonar operation in shallow water and may also be exploited for remotely assessing the health of estuarine and littoral ecosystems. The acoustics are dominated by air channels (aerenchyma) within the plants and air bubbles produced by the plants during photosynthesis. In situ studies of seagrass beds have confirmed their unique acoustic properties with respect to side-scan sonar backscatter and propagation effects. The results of these studies suggest a richness of exploitable phenomena, but corresponding forward theoretical models have yet to appear. To address this question, low-frequency acoustic laboratory experiments were conducted on three freshly collected seagrass species, Thalassia testudinum, Syringodium filiforme, and Halodule wrightii. A one-dimensional acoustic resonator technique was used to assess the effective acoustic properties and gas content of individual plants, leaves, and rhizomes. We successfully tracked bubble formation and transport from above- to below-ground tissues and present our results with respect to model predictions based on independent optical gas content estimates obtained via microscopic cross-section imagery. [Work supported by the UT College of Engineering and the Texas Sea Grant College Program Grant R/ES-87.]

A long‐range, high‐resolution Doppler sonar for fishery stock assessment

A new approach is developed for horizontal‐looking active sonar that uses continuous bi‐/multistatic transmission of very long period m‐sequences. Compared to conventional monostatic ping rather than listen sonar, this approach improves the Doppler resolution by a factor of 20 and increases FOM by perhaps 13 dB by increasing the temporal coherent processing gain. The improved time/Doppler resolution helps to discriminate fish school from bottom reverberation, and improves the resolution of taxonomic parameters of fish assemblages. The improved FOM allows coverage of large areas of shallow oceans with source levels low enough to avoid environmental concerns for marine mammals. The principal obstacle to continuous transmission sonar operation is the masking of the target return by the time and frequency leakage of the direct path arrivals. Unique time‐correlation property of m‐sequences elimination time leakage and a process calls coordinate zeroing (CZ) eliminates all the Doppler leakage. A linear temporal Doppler search (LTDS) algorithm is used to shift the detection problem to a high‐resolution pulse arrival‐time/Doppler plane with a noise‐limited background. Ultrafast Hadamard transforms lighten the computational burden. The combination of these ideas is presented including the theory, numerical experiments that demonstrate the concept, and verification with real ocean signals.

A Tactical Approach to Environmental Uncertainty and Sensitivity

The issues of acoustic sensitivity to environmental parameters and the uncertainty in performance prediction due to lack of accurate environmental inputs are addressed. Special emphasis is given to the current state of affairs and to the importance of communicating in a simple, robust means the uncertainty to the sonar operator. A multistaged approach to estimating sensitivity, computing acoustic propagation uncertainty due to environmental variability is presented. A heuristic approach to estimating the most sensitive environmental parameters is developed. A measurements-based statistical approach is applied to environmental data taken in the Mediterranean Sea to estimate performance uncertainty due to sound speed and geoacoustics uncertainty

Where We Work: Naval Submarine Medical Research Laboratory

Where We Work: Naval Submarine Medical Research Laboratory

Modeling the Effectiveness of Tools to Assist Sonar Operators

The task of building the underwater picture from sonar data is made complex by high volumes of noise and multiple data that arrive from a variety of acoustic sources detected at great distances by modern, sonar equipment. Typically, acoustic sources from ships have a complex spectrum consisting of several base frequency components and related harmonics. The task for operators is to analyse the data to determine if there is a pattern that represents the signature of a known source, thereby leading to identification of a vessel. Since the task can be highly labour intensive automated decision aids may be of value to the operator. This project addresses how to predict and optimise the impact of new technologies in system re-design by using a modeling/simulation approach to operator-system functionality. A generic sonar analysis process was simulated and the effectiveness of a decision aid evaluated. The improvement in performance predicted by the aid was then validated experimentally.

Survey of dual frequency identification sonar (DIDSON) applications in fisheries assessment and behavioral studies

The Dual Frequency Identification Sonar (DIDSON) is a forward-looking sonar that operates in shallow riverine environments with rocky, uneven substrates and near concrete structures such as dams. This allows a number of fisheries applications in environments previously too hostile for reliable sonar operation. Currently 35 DIDSONs have been obtained by 16 groups to accomplish a variety of fish assessment and behavioral studies. This paper surveys the work of these groups and highlights novel assessments allowed by this new acoustic tool. The groups include Alaska Department of Fish and Game, U.S. Fish and Wildlife Service, NOAA, USGS, Bureau of Reclamation, California Department of Water Resources, Pacific Northwest National Laboratories, Nez Perce Tribal Fisheries, Puyallup Tribal Fisheries, Department of Fisheries and Oceans Canada, and Fisheries Engineering Japan. Assessments include: (1) Counting fish migrating up rivers of various sizes, bottom substrates, turbidity, and velocity; (2) Analysis of fish behavior at (A) prototype fish protection devices on dams, (B) irrigation intakes along muddy rivers, (C) intakes of trawl nets; and (3) Detection and measurement of redds in alluvial riverbeds. [Work for the survey supported by Sound Metrics Corp.]

A comparison of novices and experts in the identification of sonar signals

This research contrasted trained sonar operators with novices in their abilities to categorize sonar signals as being of biological or man-made origin. Although the sonar operators performed significantly better than the naïve subjects, the performance of the naïve subjects ( d′ = 1.39) was closer to that of the sonar operators ( d′ = 1.81) than to chance. Additionally, there were qualitative similarities in the two groups’ performances. In order to understand this, a new group of people listened to a subset of 76 of the sounds, and chose verbal descriptors from a detailed list of 146 words such as “pht”, “chuck-chuck”, “rainfall”, etc. to describe each sound. Analyses of the descriptor choices demonstrated that biological sound sources were identifiable as such simply because they sounded like animal noises. Man-made signals were identified as more percussive, a result confirmed by an acoustical analysis of the signals. Two scales were developed for the signals, indicating how “animal like” and how percussive they were, respectively. Either of the two scales could predict with about 75% accuracy the category of the signals, a level of accuracy equal to that of the novices, and not bested by neither a CART model nor a neural net model. A simple acoustical analysis of the percussiveness of the waveforms could predict people’s judgments, but not predict the objective classifications of the signals.

Continuous wide area monitoring of fish shoaling behavior with acoustic waveguide sensing and bioclutter implications

Field measurements are used to show that the detailed behavior of fish shoals can be continuously monitored at roughly 1-min intervals over wide areas spanning hundreds of square kilometers by long range acoustic waveguide sensing. The technique was used on the New Jersey Continental Shelf to produce unprecedented video images of shoal formation, fragmentation, and migration. Simultaneous line-transect measurements show the imaged shoals to contain pelagic fish with densities of at least one individual per meter3. The technique relies upon acoustic waveguide propagation in the continental shelf. Here, trapped modes dominate propagation and suffer only cylindrical spreading loss rather than the spherical loss suffered in free-space transmission or short-range propagation in the ocean. In contrast, standard methods for fish surveyance involve line transect measurements from slow moving research vessels that significantly under-sample fish distributions in time and space, leaving an incomplete behavioral picture. The implications of this bioclutter phenomenon on the Navy’s long range active sonar operations in continental shelf environments are discussed.

Blind deconvolution for non-stationary acoustical systems

Blind deconvolution is a signal-processing technique that has been shown to be highly effective for removing multipath propagation channel-induced corruption of a signal. These algorithms are usually implemented as adaptive filtering processes that learn finite impulse response inverse filters based on measurements of the corrupted signal. A major limitation of these techniques is that a stationary propagation environment is generally assumed. This assumption is not valid for many real-world applications, such as active or passive sonar operating in a shallow-water environment or for communications systems with moving sources or receivers. For slowly varying systems, standard algorithms with adaptive learning rates that improve convergence speed, such as frequency-domain implementations of the Infomax and Natural Gradient algorithms, may be sufficient. However, in systems where either the input signal or impulse response change rapidly, it becomes necessary to provide a means for tracking the statistical changes that occur in the input signal or impulse response. Approaches to tracking these changes through the use of a tracking algorithm such as Kalman filtering are discussed. [Work supported by Dr. David Drumheller, ONR Code 333, Grant No. N00014-00-G-0058.]

Thick aluminium nitride films deposited by room-temperature sputtering for ultrasonic applications

Materials in film form for electromechanical transduction have a number of potential applications in ultrasound. They are presently under investigation in flexural transducers for air-coupled ultrasound and underwater sonar operating at frequencies up to a few megahertz. At higher frequencies, they have the potential to be integrated with electronics for applications of ultrasound requiring high spatial resolution. However, a number of fabrication difficulties have arisen in studies of such films. These include the high temperatures required in many thick and thin film deposition processes, making them incompatible with other stages in transducer fabrication, and difficulties maintaining film quality when thin film––typically sub-1 μm––processes are extended to higher thicknesses. In this paper, we first outline a process which has allowed us to deposit aluminium nitride (AlN) films capable of electromechanical transduction at thicknesses up to more than 5 μm without substrate heating. As an ultrasonic transduction material, AlN has functional disadvantages, particularly a high acoustic velocity and weak electromechanical transduction. However, it also has a number of advantages relating to practicality of fabrication and functionality. These include the ability to be deposited on a variety of amorphous substrates, a very high Curie temperature, low permittivity, and low electrical and mechanical losses. Here, we present experimental results highlighting the transduction capabilities of AlN deposited on aluminium electrodes on glass and lithium niobate. We compare the results with those from standard simulation processes, highlighting the reasons for discrepancies and discussing the implications for incorporation of AlN into standard ultrasonic transducer design processes.

Multistatic broadband sonar (MSBA): Principles and measurements

Shallow-water (SW) coastal areas with depths between 40 and 200 m with bottom sediment layers (<25 m) formed by deposition and sea level variations are known to have frequency-dependent sound transmission and reverberation characteristics especially under the adverse conditions of downward refraction. Coastal ocean-dynamics complicates these environments by coupling the surface, volume, and bottom characteristics and can dramatically affect the temporal and spatial coherency of the signal. The broad-band signal and processing smear these frequency-dependent effects as well as signal excess fluctuations. Furthermore, recent experiments have shown that despite these complexities, explosive sources coupled with directional receivers have the capability to detect, localize, and classify sonar targets in the presence of clutter most likely due to bottom features, fish schools, and sunken objects. Active search sonar operations in SW are typically reverberation limited and experiments have shown that MSBA sonar when employed in bistatic source-receiver geometries can be effective. This paper reviews the principles underlying the MSBA sonar in shallow water and presents results from several experiments that demonstrated the utility of MSBA. The results are discussed in terms of a classical sonar model with basic statistical considerations to define limits on the detection, classification, and localization with multiple receivers.

Swath mapping the base of the arctic ice canopy

Satellite data have demonstrated that the areal extent of the Arctic ice canopy has decreased over the past few decades, however, whether sea ice thickness in the Arctic Ocean has substantially decreased remains controversial because of limited information available for the base of the ice canopy. Studies of sea ice thickness to date have involved analysis of ice draft data acquired by single-beam ice profiling sonars on US and British Navy submarines or single-beam moored sonars. During 1998 and 1999, a seafloor-mapping sonar mounted on the hull of the US Navy nuclear submarine USS Hawkbill serendipitously acquired sonar returns from the base of the arctic ice canopy despite design features intended to minimize such returns. Standard topographic processing of the data revealed coherent signals collected between the outgoing sonar pulse and the first returned bottom echoes. Existing processing software was modified to produce swath maps of the information collected from the beginning of ping transmission until the seafloor echoes were detected. These images demonstrate that the recorded water-column signals are different on the port and starboard sides of the system and that individual features can be traced from one side of the submarine to the other. Further software modification allowed us to produce the first wide-swath (2–6 km) sidescan images of the base of the Arctic ice canopy. Although the resultant maps cannot resolve the debate regarding changes in Arctic sea ice thickness, the approach will have far-reaching impact on the design of future systems, whether based on submarines or unmanned vehicles. Our analyses demonstrate that system performance is regulated by the ratio between sonar operating depth and height of the sonar above the seafloor. Where the ratio approaches 1, echoes from the seabed interfere with echoes from the ice canopy, preventing the creation of ice maps. When the sonar is operated below 135 m, signal attenuation results in ice maps with little useful information. However, where the sonar operational depth is less than 135 m and the total water depth ranges between 500 and 1200 m, our approach produces swaths up to 6 km wide exhibiting detailed lineations similar to those observed in higher frequency upward-looking sidescan sonar data.

Effects of a biocybernetic system on vigilance performance.

The present study was designed to determine whether a biocybernetic, adaptive system could enhance vigilance performance. Participants were asked to monitor the repetitive presentation of white bars on a computer screen for occasional increases in length. An index of task engagement was derived from participants' electroencephalographic (EEG) activity and was used to change the presentation rate of events among 3 values (6, 20, and 60 events/min). Under a negative feedback contingency, event rates increased if the engagement index decreased and, conversely, decreased if the index increased. Under positive feedback, the opposite contingency existed. Each experimental participant had a yoked control partner who received the same pattern of changes in event rates irrespective of his or her EEG activity. The results showed that better vigilance performance was obtained under negative feedback and that the performance of the yoked participants was similar to that of their experimental partners. These findings suggest that it may be possible to improve monitoring performance on critical activities such as air traffic control and radar and sonar operation through a pattern of event rate changes that do not rely on an operator's overt behavior.

Marine Mammal Resource

In the sonar world there is still a heavy reliance on the human ear for detection. To detect threats, the sonar operator must be familiar, not only with their acoustic signatures but also with the many and varied noises emanating from other sources in the ocean. These include noises of biological origin, particularly marine mammals. The Marine Mammal Resource is a hypertext document aimed at providing a reference set of sounds for training purposes. It contains textual descriptions on 18 different aspects of ecology for each of the species, summarizing the information available from the open literature. Subjects include vocalizations, habitat, distribution, hearing, and physical characteristics. The resource also contains images, diagrams, and movies for identification purposes. Data has been collated for over 40 species primarily found in the Atlantic Ocean. Information has been collated through a number of research organizations in America and Europe. The resource has application to the reduction of cetacean by-catch in trials, both in providing information in the derivation of environmental impact assessments pretrial and in the acoustic monitoring of the area during the course of a trial.

Determination of spatial position of multiple targets by ultrasonic binaural method

The binaural technique used in ultrasonic sonars has a higher performance speed in comparison with a mechanically scanned ultrasonic beam, however, in presence of multiple targets meets a very serious ambiguity problem. In this case the number of targets detected exceeds the actual number of targets, e.g., there are additional non-existing targets found. Objective of this research was development of a simulation tool enabling to model multi-channel sonar in the environment with multiple targets of arbitrary geometry and development of robust signal processing procedures, suitable for detection of multiple targets from the data collected using the binaural method. The developed software enables to simulate operation of multi-channel sonar in an environment with multiple reflectors and predict a time history of the reflected signals. The novel algorithm for separation of real targets from the virtual ones in presence of multiple targets has been developed. Performance of the proposed algorithm was investigated both in the simulated and a real environment. The results obtained indicate a significant improvement of the sonar performance.

Bottom reverberation in shallow water: Coherent properties as a function of bandwidth, waveguide characteristics, and scatterer distributions

Shallow water presents a difficult, reverberation-limited environment for active SONAR operations. It is important to understand the predictable structure of shallow-water reverberation in order to aid the design of processors and detectors which work properly in these environments. In this paper, the temporal characteristics of monostatic reverberation are predicted as a function of source bandwidth, source–receiver depth, and the propagation characteristics of range-independent shallow water. Results show that at early time, reverberation can be highly coherent across a vertical line array, violating the homogeneous noise assumption, while at late time the reverberation becomes increasingly uncorrelated. This is shown to be due to the insonification of independent bottom patches at late time. It is also shown that this decorrelation of the reverberation is dependent both on the propagation characteristics of the particular shallow-water environment, the correlation length scale of the scatterers, and the bandwidth of the source, with high-bandwidth sources causing decorrelated reverberation sooner than low-bandwidth sources. The results also show that there are several identifying characteristics in reverberation time series which may be useful for identifying the types of scatterers which cause reverberation during particular experiments.