Demonstration of the invariance principle for active sonar.

Northwest Electromagnetics and Acoustics Research Laboratory, Department of Electrical Engineering, Portland State University, Portland, Oregon 97201-0751, USA.
The Journal of the Acoustical Society of America (Impact Factor: 1.65). 04/2008; 123(3):1329-37. DOI: 10.1121/1.2836763
Source: PubMed

ABSTRACT Active sonar systems can provide good target detection potential but are limited in shallow water environments by the high level of reverberation produced by the interaction between the acoustic signal and the ocean bottom. The nature of the reverberation is highly variable and depends critically on the ocean and seabed properties, which are typically poorly known. This has motivated interest in techniques that are invariant to the environment. In passive sonar, a scalar parameter termed the waveguide invariant, has been introduced to describe the slope of striations observed in lofargrams. In this work, an invariant for active sonar is introduced. This active invariant is shown to be present in the time-frequency structure observed in sonar data from the Malta Plateau, and the structure agrees with results produced from normal mode simulations. The application of this feature in active tracking algorithms is discussed.

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    ABSTRACT: Random variability in shallow water will induce variability in a propagating acoustic field. The long-term goal of this research is to quantify how random variability in the ocean environment translates into random variability in the acoustic field and the associated signal processing algorithms. In the present funding cycle, the emphasis is on the effects of linear and non-linear internal waves on acoustic propagation in the mid-frequency (1-10 kHz) band. OBJECTIVES The specific objective for the current funding cycle is to understand the generation of new acoustic paths that occur due to the passage of non-linear internal waves. APPROACH During the Shallow Water 2006 Experiment (SW06), mid-frequency acoustic transmission data were collected over a continuous 7-hour period at range 550 m. The relatively short range was deemed desirable for isolating the effects of shallow water internal waves on acoustic propagation. At the SW06 site, both linear and non-linear internal waves were potentially important. Linear internal waves often are modeled as a background random process introducing small changes in the sound speed that cause fluctuations in the acoustic field. At range 550 m, mid-frequency transmissions between 1 and 10 kHz were thought to span the transition between the regimes where classical weak-and strong-scattering theories for random media would apply [1]. Non-linear internal waves are often modeled as a more event-like process causing strong, localized changes in the sound speed. Packets of non-linear internal waves are not unusual and it was anticipated that a 550 m acoustic path might permit individual waves in the packet to be isolated. Our approach is to use a statistical model for the acoustic fluctuations introduced by random background internal waves, and a more deterministic model for the acoustic effects introduced by passage of more event-like non-linear internal waves.
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    ABSTRACT: A physics-based method for beamforming signals measured on a horizontal array is developed with an application to underwater active sonar systems. The proposed striation-based beamformer coherently combines the pressure from each element in the array at different frequencies, and these frequencies are selected based on a striation hypothesis. The linear frequency shift and corresponding phase term introduced in the array weight vector accounts for multipath-induced fading, producing beam output with increased signal gain. The method is demonstrated using data collected on an array towed in the North Atlantic. The combination of the striation-based beamformer with the waveguide invariant concept to improve tracker performance is discussed.
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    ABSTRACT: The waveguide invariant has been widely studied and applied in the context of shallow water passive sonar. The invariant provides a simple linear relationship between the frequency content of a moving broadband source and the propagation range, and this relationship is not generally sensitive to small variations in environment parameters. Recent research has shown that a similar structure can be obtained for active sonar geometries, and it has been demonstrated that this structure may be exploited in advanced physics-based processing applications. However, the appearance of the active invariance striations depends on the sonar geometry (source-target-receiver ranges) as well as the scattering properties of the target. Controlled tank measurements with canonical scatterers (sphere, cylinder, plate) were performed to quantify the effect of the scattering function. Additionally, a theoretical formulation is presented to explain the invariance structure as a function of the geometry and target scattering properties. This paper presents results of these measurements, normal mode simulations, and theoretical expressions. The results show that targets which have wide (vertical) scattering patterns can couple ingoing and outgoing propagating modes, and this coupling results in time-frequency striations that are related to source-target-receiver ranges. Conversely, targets with no mode coupling produce striation patterns that correspond only to the time varying bistatic target range.