Stefan M. Murphy

The Dependence of Signal Coherence on Sea-Surface Roughness for High and Low Duty Cycle Sonars in a Shallow-Water Channel

It is anticipated that high duty cycle (HDC) sonars will typically maintain the same bandwidth as the pulsed active sonars (PASs) that they might replace. This will significantly increase their time–bandwidth product, but may not produce the increased gain anticipated, if there are coherence limitations of the acoustic channel. To compare performance of HDC with conventional PAS in the littorals, a set of experiments was conducted as part of the Target and Reverberation Experiment in spring 2013 (TREX13). This paper presents the results of an examination of short-range single surface-reflection echoes, and longer range target echoes from an air hose. The Pearson product–moment correlation coefficient (Pearsons

Examining the robustness of automated aural classification of active sonar echoes

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Testing the feasibility of a concurrent comparison of continuous and pulsed active sonar

) was used to confirm significance of the results. Measurements showed that for an 18-s HDC pulse, the mean (coherent) component of the specular arrival decreased by as much as 5 dB as root mean square (rms) surface roughness increased, whereas the 0.5-s PAS pulse echoes showed no correlation with roughness. The standard deviations of the mean levels were used to examine the incoherent (scattered) component of the specular arrivals. The incoherent component of the specular arrival increased with the product of the surface correlation length and the square of the rms roughness, for both HDC and PAS, with the PAS data having a 1-dB higher standard deviation. A normal mode propagation model and a rough surface scattering model used in conjunction with a simple model that accounts for coherence loss from the matched filter were successfully used to interpret the results.

Classifying sonar signals with varying signal-to-noise ratio and bandwidth

Active sonar systems are used to detect underwater man-made objects of interest (targets) that are too quiet to be reliably detected with passive sonar. Performance of active sonar can be degraded by false alarms caused by echoes returned from geological seabed structures (clutter) in shallow regions. To reduce false alarms, a method of distinguishing target echoes from clutter echoes is required. Research has demonstrated that perceptual-based signal features similar to those employed in the human auditory system can be used to automatically discriminate between target and clutter echoes, thereby reducing the number of false alarms and improving sonar performance. An active sonar experiment on the Malta Plateau in the Mediterranean Sea was conducted during the Clutter07 sea trial and repeated during the Clutter09 sea trial. The dataset consists of more than 95,000 pulse-compressed echoes returned from two targets and many geological clutter objects. These echoes were processed using an automatic classifier that quantifies the timbre of each echo using a number of perceptual signal features. Using echoes from 2007, the aural classifier was trained to establish a boundary between targets and clutter in the feature space. Temporal robustness was then investigated by testing the classifier on echoes from the 2009 experiment.

Validity of the frozen-surface approximation for large time-bandwidth signals

The performance of continuous active sonar (CAS) was compared to conventional pulsed active sonar (PAS) during the TREX13 (Target and Reverberation Experiment 2013) sea trial. The approach was to execute a one-hour CAS run followed closely by a one-hour PAS run to limit the environmental variability between runs and allow a fair comparison. This approach was possible because the sonar source and receiver were fixed to the seabed. A different approach was required for a more recent sea trial, LCAS15 (Littoral Continuous Active Sonar 2015), where the source and receiver were towed from a ship. Ship motion increases variability in sonar performance, therefore a simultaneous comparison of CAS and PAS was desired so that any motion effects were the same for both waveforms. The approach of transmitting CAS and PAS concurrently in two separate frequency bands was taken. The risk with this approach is that potential differences in propagation in the two bands could bias the comparison. A run in which equivalent C...

Some effects of ocean surface roughness on high and low duty cycle, broadband waveforms

An automatic aural classifier developed at Defence Research and Development Canada has demonstrated the ability to distinguish target echoes from clutter using perceptual-based features inspired by sonar operators. Initially, the classifier was tested with echoes from explosive sources, but more recent research involved transmitting broadband waveforms from transducer sources. In sonar transducer operation, there is a trade off between source level and bandwidth, and the goal of this paper is to study how these factors affect echo classification. Source level relates to signal-to-noise ratio (SNR), which inherently affects classification since signals with low enough SNR cannot be distinguished from noise, let alone other signals. The dependence of classification performance on bandwidth is less obvious; however, the aural classification technique is based on a sub-band type of processing that mimics the basilar membrane in the human auditory system, and this model is not well adapted for narrow bands. Performance of the aural classifier is therefore expected to degrade as bandwidth is decreased. In this paper, the effect of SNR and signal bandwidth on echo classification is examined using echoes of varying SNR, and in various bands selected using band-pass filters.

Comparison of signal coherence for continuous active and pulsed active sonar measurements in littoral waters

The physical modeling of underwater acoustic propagation, scattering, and reflection, and the signal processing associated with these processes, usually rely on the frozen-surface approximation. This longstanding approximation, which dates back to Carl Eckart’s seminal paper (The Scattering of Sound from the Sea Surface, 1953) assumes that the ocean surface can be modeled as being frozen in time throughout the entire pulse duration. The approximation is valid for short-duration pulses typically used by sonars in the decades following that paper; however, its applicability to present-day high duty cycle sonars is questionable. Although the assumption is based on the physics of the problem, it can have a profound effect on the signal processing, especially for large time-bandwidth signals. In this paper some experimental results will be presented to provide examples of how the approximation fails for a long duration, wideband pulse. Using this as motivation, the authors will explore some areas where incorrectly employing the approximation can introduce errors in the expected signal processing gain. While no attempt will be made to correct shortcomings in the approximation, it is hoped that the discussion may motivate renewed interest in this issue. Funding from ONR and ONR Global is gratefully acknowledged.The physical modeling of underwater acoustic propagation, scattering, and reflection, and the signal processing associated with these processes, usually rely on the frozen-surface approximation. This longstanding approximation, which dates back to Carl Eckart’s seminal paper (The Scattering of Sound from the Sea Surface, 1953) assumes that the ocean surface can be modeled as being frozen in time throughout the entire pulse duration. The approximation is valid for short-duration pulses typically used by sonars in the decades following that paper; however, its applicability to present-day high duty cycle sonars is questionable. Although the assumption is based on the physics of the problem, it can have a profound effect on the signal processing, especially for large time-bandwidth signals. In this paper some experimental results will be presented to provide examples of how the approximation fails for a long duration, wideband pulse. Using this as motivation, the authors will explore some areas where incorre...

Analysis/synthesis of sonar echoes as impact sounds.

In this paper, data from TREX13 are used to compare the impact of the environment on High Duty Cycle (HDC) sonar and Pulsed Active Sonar (PAS). This paper presents the results of an examination of short-range single surface-reflection echoes, and longer-range target echoes from an air hose. Measurements showed that for an 18 s HDC pulse, the mean (coherent) component of the specular arrival decreased with increasing rms roughness whereas 0.5 s PAS pulse echoes showed no correlation with roughness. The standard deviations of the mean echo levels are used to examine the incoherent (scattered) component of the specular arrivals. The incoherent component of the specular arrival increased with the product of the surface correlation length and the square of the rms roughness, for both HDC and PAS, with the PAS data having a 1 dB higher standard deviation. A normal mode propagation model and a rough surface scattering model used in conjunction with a simple model that accounts for motion-induced coherence loss f...

Testing the temporal robustness of an automatic aural classifier.

Military sonars must detect, localize, classify, and track submarine threats from distances safely outside their circle of attack. However, conventional pulsed active sonars (PAS) have duty cycles on the order of 1% which means that 99% of the time, the track is out of date. In contrast, continuous active sonars (CAS) have a 100% duty cycle, which enables continuous updates to the track. This should significantly improve tracking performance. However, one would typically want to maintain the same bandwidth for a CAS system as for the PAS system it might replace. This will provide a significant increase in the time-bandwidth product, but may not produce the increase in gain anticipated if there are coherence limitations associated with the acoustic channel. To examine the impact of the acoustic channel on the gain for the two pulse types, an experiment was conducted as part of the Target and Reverberation Experiment (TREX) in May 2013 using a moored active sonar and three passive acoustic targets, moored a...

Classifying Continuous Active Sonar Echoes for Target Recognition

Active sonar performance is sometimes limited by clutter that generates an unacceptable false alarm rate (FAR). High FAR is overcome through the use of signal classification, which is treated here using a sequence of techniques that mimic human perception. The techniques are applied to a corpus of signals that were measured during the experiment Clutter 09, which took place on the Malta Plateau in the spring of 2009. First, techniques for foreground/background separation are presented using whitening and thresholding in a time‐frequency representation adapted from computation techniques from acoustic scene analysis. The effects of thresholding are demonstrated with a few signals from the corpus. Modifications, suitable for the noisy sonar‐echoes in the corpus, of the natural sound paradigm of Aramaki is presented [Aramaki, et al., Comp. Mus. Mod. Retr. CMMR 2009 (2009)]. Preliminary results of this representation are presented aurally. [Research funded by the Office of Naval Research.]