Active Sonar Systems Research Articles

Study on the human ability to aurally discriminate between target echoes and environmental clutter in recordings of incoherent broadband sonar

Unacceptably high false-alarm rates due to the inability to discriminate between target echoes and environmental clutter are an issue for existing low-frequency active sonar systems operating in coastal environments. A research project at Defence R&D Canada—Atlantic is investigating the potential use of aural cues to tackle this challenge. One aspect of the project is to evaluate the human ability to aurally discriminate between target echoes and environmental clutter. The design and preliminary results from the study are presented here. Human subjects are presented with a series of sounds containing target echoes and clutter obtained from recordings of an incoherent broadband sonar experiment. The quantitative data collected in the study are the subjects’ decisions as to whether the echo heard was a target echo or clutter and their level of confidence associated with the decisions. Receiver-operating characteristic (ROC) analysis is used to produce a statistical model of the subjects’ performance. The study also includes a questionnaire: answers may prove useful in supporting the quantitative results and in providing a better understanding of the cues and decision techniques used by the subjects.

Multi‐static target tracking using a bistatic invariance constraint

The ability to track moving targets in shallow water channels is critically effected by bottom reverberation which can generate false alarms and substantially increase the domain of possible tracks. Even with application of advanced tracking algorithms, false alarms degrade performance and increase computational burden. The invariance principle, which predicts structure in the frequency‐time domain, has been applied to passive sonar systems but has not been considered for active systems. In this work, a mathematical expression (called the bistatic invariance principle, see companion paper) is proposed as a constraint to discriminate target detections from false alarms and thus reduce the search domain for multi‐static tracking algorithms. The constraint does not require knowledge of the underwater environment and is thus robust to environmental uncertainty. The bistatic invariance principle provides a relationship between changes in the bistatic range of a moving target to changes in the source frequency, and this relationship can be used to track the target position over time in broadband active sonar systems. Simulations of the bistatic invariance principle for multi‐static active systems are presented and discussed and multi‐static tracking performance is characterized for several choices of target scattering matrices and different environmental conditions.

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.

Inferring marine animal characteristics using multi-angle scatter

An important goal in using active sonar systems to assess underwater animals in situ is the inference of animal dimension. Although 180-deg backscatter provides an estimate of target reflectivity, in many situations the reflected energy is not easily related to animal size. An additional degree of freedom, rarely exploited, is to measure animal scatter at a multiplicity of angles. Although this is more challenging from a technical point of view, the benefit of this additional information can make it worthwhile. The interpretation of multi-angle scatter from marine organisms can be most easily understood in the context of diffraction tomography. Depending upon the details of the scattering process, in fortuitous circumstances a complete set of angular data can be inverted to compute the three-dimensional scattering potential which can ultimately lead to a complete inversion for animal morphology. In cases where it is impractical to collect a complete set of angular data it is often possible to infer characteristic dimensions of the animal from an analysis of the obtainable multi-angle data. The interpretation of multi-angle scatter data in the context of diffraction tomgraphy therefore presents opportunities to infer ecologically significant information about animal populations.

Acoustic coupling in capacitive microfabricated ultrasonic transducers: modeling and experiments

In the design of low-frequency transducer arrays for active sonar systems, the acoustic interactions that occur between the transducer elements have received much attention. Because of these interactions, the acoustic loading on each transducer depends on its position in the array, and the radiated acoustic power may vary considerably from one element to another. Capacitive microfabricated ultrasonic transducers (CMUT) are made of a two-dimensional array of metallized micromembranes, all electrically connected in parallel, and driven into flexural motion by the electrostatic force produced by an applied voltage. The mechanical impedance of these membranes is typically much lower than the acoustic impedance of water. In our investigations of acoustic coupling in CMUTs, interaction effects between the membranes in immersion were observed, similar to those reported in sonar arrays. Because CMUTs have many promising applications in the field of medical ultrasound imaging, understanding of cross-coupling mechanisms and acoustic interaction effects is especially important for reducing cross-talk between array elements, which can produce artifacts and degrade image quality. In this paper, we report a finite-element study of acoustic interactions in CMUTs and experimental results obtained by laser interferometry measurements. The good agreement found between finite element modeling (FEM) results and optical displacement measurements demonstrates that acoustic interactions through the liquid represent a major source of cross coupling in CMUTs.

Tidally phased emergence events in a strongly tidal estuary

Acoustic backscatter from an active sonar system over a range of six frequencies between 265 kHz and 3 MHz in the tidally dominated Damariscotta River estuary, Maine, United States, revealed that the major emergence event of the night commenced on the first tidal deceleration after dark (3.5-4 h after local slack), irrespective of flow direction. Emergence traps identified the mysid shrimp, Neomysis americana, as the dominant migrator. Water-column-integrated, acoustically estimated biovolume at our 10-m deep study location increased by a factor of about 6 during these large events, entirely dominating the holoplanktonic contribution and likely being a major component in benthic-pelagic coupling. Application of the same algorithm used to locate this nighttime emergence revealed a parallel but considerably smaller daytime emergence event near the same phase of the tide. Daytime trap samples failed to recover the organisms responsible, but transmissometry rejected the alternative hypothesis that we observed resuspension events. We suspect, but have yet only weak evidence, that animals emerging in daylight are copepods rather than mysids.

Light-cued emergence and re-entry events in a strongly tidal estuary

Acoustic backscatter from an active sonar system (TAPS-6) over a range of six frequencies between 265 kHz and 3 MHz revealed characteristics of emergence and reentry events within 1 h of sunset and sunrise, respectively, at a site 10 m deep in the Damariscotta River estuary, Maine. Emergence traps indicated that the mysid shrimpNeomysis americana was the dominant migrator. Daily fluctuation in irradiance influenced the timing of dusk emergence. Local variability in irradiance apparently caused populations to emerge occasionally before sunset or to leave the surface after the beginning of nautical twilight near dawn. Emergence before sunset was marked by slower-than-average ascent, and departure from the surface after initiation of nautical twilight was marked by faster-than-average descent. This pattern would be expected for populations avoiding visual predators by concealment in dark water. Mean ascent (0.29 ± 0.03 cm s−1 [±1 SE]) and descent (−0.26 ± 0.02 cm s−1) velocities showed little difference in magnitude, suggesting that a similar mechanism controls both. Ascent initiation times for the first such event of the night were consistent with a cue based on relative rate of change in light intensity and inconsistent with either the progress of isolumes or with absolute rate of change in light intensity.

Monitoring bubble growth in supersaturated blood and tissue ex vivo and the relevance to marine mammal bioeffects

There have been several recent reports that active sonar systems can lead to serious bioeffects in marine mammals, particularly beaked whales, resulting in strandings, and in some cases, to their deaths. We have devised a series of experiments to determine the potential role of low-frequency acoustic sources as a means to induce bubble nucleation and growth in supersaturated ex vivo bovine liver and kidney tissues, and blood. Bubble detection was achieved with a diagnostic ultrasound scanner. Under the conditions of this experiment, supersaturated tissues and blood led to extensive bubble production when exposed to short pulses of low frequency sound.

Long range acoustic imaging of the continental shelf environment: The Acoustic Clutter Reconnaissance Experiment 2001

An active sonar system is used to image wide areas of the continental shelf environment by long-range echo sounding at low frequency. The bistatic system, deployed in the STRATAFORM area south of Long Island in April-May of 2001, imaged a large number of prominent clutter events over ranges spanning tens of kilometers in near real time. Roughly 3000 waveforms were transmitted into the water column. Wide-area acoustic images of the ocean environment were generated in near real time for each transmission. Between roughly 10 to more than 100 discrete and localized scatterers were registered for each image. This amounts to a total of at least 30000 scattering events that could be confused with those from submerged vehicles over the period of the experiment. Bathymetric relief in the STRATAFORM area is extremely benign, with slopes typically less than 0.5 degrees according to high resolution (30 m sampled) bathymetric data. Most of the clutter occurs in regions where the bathymetry is locally level and does not coregister with seafloor features. No statistically significant difference is found in the frequency of occurrence per unit area of repeatable clutter inside versus outside of areas occupied by subsurface river channels.

Detection enhancement using multiple time-reversed guide sources in shallow water: Analysis of TREX04 data

Detection in a monostatic, broadband, active sonar system in shallow water is degraded by propagation-induced spreading. In the TREX04 experiment, performed south of the Hudson Canyon off the coast of New Jersey, 0.25 sec LFMs were transmitted with 500 Hz bandwidths chosen over a 0.5–3.5 kHz range using the NRL 64 element source-receiver array. The transmissions were then echo-repeated by a distant ship at a range varying between 1–5 km. The echo-repeating ship also transmitted one-way, 1 s, LFMs which were used as guide-source signals. These guide-source signals, which contain environmental information, form the basis of a technique for improving detection without having explicit environmental knowledge. Using an empirical-orthogonal-function representation of the set of monostatic guide-source signals, echoes were convolved with the time-reversed orthogonal functions as part of a filter bank. The result is improved probability of detection of noisy echoes using multiple guide-source signals in comparison with a baseline probability of detection using matched-filtering. In this talk, ROC curve improvement is analyzed as a function of the many parameters, e.g., target depth, source aperture, bandwidth, number of guide sources and their spatial and temporal separation. Comparison with numerical simulations is also made. [Work supported by ONR.]

Biomass assessment in commercial catfish ponds

With increasing seafood demand aquaculture is poised to become a major growth industry in the United States in the 21st century. In particular channel catfish represent an approximately $260 million industry in Mississippi with strong growth potential. A major portion of the costs associated with raising channel catfish are related to the cost of feed and aeration which are directly related to the total number of fish being raised in each pond. Crop insurance and bank loans are also contingent upon accurate population estimates. A high frequency, horizontally scanning, active pulse echo sonar system, the Aquascanner, has been developed to estimate pond populations. Commercial catfish ponds are typically 2 to 10 acres and, unlike many offshore fisheries, have fairly shallow depths of 1 to 2 m. The system components and its use will be presented along with results from field tests. [Work supported by US Dept. of Agriculture.]

Detection enhancement using multiple time-reversed guide sources in shallow water

Detection in a monostatic, broadband, active sonar system in shallow water is degraded by propagation-induced spreading. Improved detection has previously been achieved in the 3- to 3.5-kHz range by using the information in time-reversed one-way transmissions from multiple spatially separated guide sources. Detection results in the wider 500- to 3500-Hz band from an experiment south of the Hudson Canyon off the coast of New Jersey are presented in this talk. The target as well as the guide source was an echo repeater positioned at various water depths and ranges. Various subapertures of the 64-element NRL source receiver array were used to transmit 0.25-s LFMs and detect their echoes. The guide source data is analyzed to show the amount of information content depending on aperture and bandwidth. The improvement in probability of detection using multiple guide-source signals is compared with a baseline probability using matched filtering. Improvement in detection as a function of the number of guide sources and their range is also discussed. [Work supported by ONR.]

Issues for improved characterization of the acoustic field due to radiated noise of ships in shallow water environments

Estimation of the sound pressure levels radiated from ships to underwater sensors in littoral environments is a problem of increasing importance. Current areas of interest range from underwater surveillance and homeland security of ports, to the threat posed to naval vessels by influence mines. Recent literature addressing shallow water propagation has focused primarily on object detection using active sonar systems or on environmental characterization through inverse methods. While conclusions from these studies regarding the impact of environmental variability on acoustic propagation may be generally applicable, there are several additional areas that need to be investigated in order to understand the acoustic field at short ranges in shallow water. This presentation seeks to identify areas where further investigation may be warranted. In particular, existing propagation codes are examined for their applicability and/or limitations for the modeling of near-field radiation from nonsimple distributed sources, evaluation of complex source signals containing both broadband noise and narrow-band tones over a broad frequency range, and the impact of small-scale local variability. This presentation seeks to motivate a discussion toward new approaches and methodologies for addressing gaps between modeling requirements and capabilities. [Work supported by PEO-LMW, AIMS Program.]

Acoustic reflectivity measurments of sirenia (Florida manatees) at high frequencies

The Florida manatee (Trichechus manatus latirostris) is an endangered sirenian inhabiting shallow coastal waters of Florida (USA). Its population (∼3300) is dwindling, with 25%–30% of mortalities occurring due to collisions with boats (95 losses in 2002). An active sonar system that detects the presence of the animals, and hence alerts the boater, could help reduce the collisions. Experiments were performed on six captive manatees with several calibrated transducers in a 33×10×3 m3 pool to learn how much sound is reflected by these animals. At a frequency of 171 kHz we transmitted a brief sinusoid. An underwater video camera was aimed along the axis of the range direction of the sound transmission, permitting the co-registration of animal and acoustics. The camera and sonar were calibrated together by translating a 38-mm tungsten carbide sphere (TS=−39 dB@ 171 kHz) in a separate test tank facility. Results indicate that the reflectivity of the animals (not strictly target strength) is somewhat low, in the −49 to −40 dB range.

Multisensor registration for distributed active sonar systems

A technique is described whereby discrepancies between and biases within active sonar receivers may be corrected using acoustic signals. The method utilizes information contained in the direct blast recorded on each receiver over multiple active pings to estimate the ranges and bearings of the one or more sources relative to each receiver. Each sensor is assumed to have limited beamforming capability. In addition, the use of known acoustic scatters to resolve sensor misalignment will be discussed. Finally, results of an application of these techniques to active sonar data from the Deployable Experimental Undersea Multistatic Undersea Surveillance system (DEMUS) will be discussed and used to assess the algorithm. [This work was funded under ONR Contract No. N00014-00-G-0450-21.]

Broadband performance of a time reversing array with a moving source

The automatic spatial and temporal focusing properties of a time-reversing array (TRA) make it an attractive technology for active and passive sonar systems that may be deployed in unknown multipath environments. However, in these and other potential underwater applications of TRAs, either the source, the array, or both are likely to be moving. In this paper we present broadband-signal TRA performance predictions that include the influence of the Doppler effect on the time-reversal process for broadband signals transmitted from an arbitrarily moving source to a stationary vertical TRA through a shallow ocean environment. Here, the impact of source motion on TRA performance is predicted from analysis and numerical simulations using a formulation of the Doppler shifted field based on Fourier superposition of stationary but spatially distributed time-harmonic sources. Quantitative results for the size and location of the TRA’s retrofocus are presented as well as the correlation of the TRA retrofocus signal with the time-reversed original signal for various source motions in range-independent and range-dependent shallow water sound channels. Overall, source motion is predicted to have little effect on TRA operations with source speeds less than 20 m/s for signals having a center frequency of 500 Hz at source–array ranges of a few kilometers.

Survey of active sonar simulations

In lieu of well-characterized measured data sets, time series simulations are often employed for testing active sonar systems. This paper reviews the simulation techniques and models that are being employed today and discusses some of the issues of fidelity that may arise with regard to range dependence, bistatic geometries, reverberation statistics and bottom interactions.

Waveform Fusion in Sonar Signal Processing

In active sonar systems, proper selection of the transmitted waveform is critical for target detection and parameter estimation, especially with the existence of clutter (reverberation). Two commonly used waveforms (constant frequency (CF) and linear frequency modulated (LFM)) are studied. Their characteristics are complementary both with respect to their accuracies and their sensitivity to the blind zero-Doppler ridge. Several fusion schemes of the two kinds of waveforms are explored and fusion results are studied both analytically and from simulation. It is concluded that fusion of the information of different waveforms can be not only more robust, but in some cases outright preferable, in term of detection probability and estimation accuracy.

International Regulation Of Undersea Noise

In September 2002 seventeen whales were stranded off the coast of the CanaryIslands at a time when NATO was testing its active sonar system designed to detect silent enemy submarines.1The suggestion has been made that the use of sonar caused these whales to strand. In fact, sonar is just one of a variety of anthropogenic undersea sounds which, scientific research increasingly suggests, impacts negatively on marine biodiversity. Pollution of an acoustic nature is currently omitted from traditional works on the protection of the marine environment and is as yet the subject of very little jurisprudential discussion.2However the topic, which has received scientific attention for over 30 years, has recently been identified as acause for concern and consequently, for action, within the parameters of a number of global and regional environmental instruments.

Echo-to-reverberation enhancement using a time reversal mirror

Reverberation from rough ocean boundaries often degrades the performance of active sonar systems in the ocean. The focusing capability of the time-reversal method provides a new approach to this problem. A time-reversal mirror (TRM) focuses acoustic energy on a target enhancing the target echo while shadowing the boundaries below and above the focus in a waveguide, thereby reducing reverberation. The resulting echo-to-reverberation enhancement has been demonstrated experimentally using a time-reversal mirror in the 3–4 kHz band in shallow water.