Jeffrey S. Rogers

Passive broadband source localization in shallow-water multipath acoustic channels

This paper addresses the problem of direct-path passive broadband localization of a shallow-water acoustic source with a sparsely distributed set of sensors in the presence of uncertain multipath propagation. The classical approach to broadband source localization with sensors spaced many wavelengths apart involves time delay estimation (TDE) via generalized cross-correlation (GCC) methods. In shallow-water scenarios, however, multipath arrivals yield spurious peaks in cross-correlation outputs resulting in anomalous estimates of source location. In this paper, cross-correlations are normalized by an expected direct-path delay and geometrically averaged over multiple array orientations in order to disambiguate multipath returns from direct-path arrivals. In shallow-water channels, this allows for more robust estimates of the intersensor time delay due to direct-path propagation. Simulation results indicate a significant improvement in source localization performance over GCC methods operating in multipath environments.

Maximum-likelihood spatial spectrum estimation in dynamic environments with a short maneuverable array

This work concerns the development of field directionality mapping algorithms for short acoustic arrays on mobile maneuverable platforms that avoid the left/right ambiguities and endfire resolution degradation common to longer non-maneuverable line arrays. In this paper, it is shown that short maneuverable arrays can achieve a high fraction of usable bearing space for target detection in interference-dominated scenarios, despite their lower array gain against diffuse background noise. Two narrowband techniques are presented which use the expectation-maximization maximum likelihood algorithm under different models of the time-varying field directionality. The first, derivative based maximum likelihood, uses a deterministic model while the second, recursive Bayes maximum likelihood, uses a stochastic model for the time-varying spatial spectrum. In addition, a broadband extension is introduced that incorporates temporal spectral knowledge to suppress ambiguities when the average sensor array spacing is greater than a half-wavelength. Dynamic multi-source simulations demonstrate the ability of a short, maneuvering array to reduce array ambiguities and spatial grating lobes in an interference dominated environment. Monte Carlo evaluation of receiver operating characteristics is used to evaluate the improvement in source detection achieved by the proposed methods versus conventional broadband beamforming.

Evaluation of the resolution of a metamaterial acoustic leaky wave antenna

Acoustic antennas have long been utilized to directionally steer acoustic waves in both air and water. Typically, these antennas are comprised of arrays of active acoustic elements, which are electronically phased to steer the acoustic profile in the desired direction. A new technology, known as an acoustic leaky wave antenna (LWA), has recently been shown to achieve directional steering of acoustic waves using a single active transducer coupled to a transmission line passive aperture. The LWA steers acoustic energy by preferential coupling to an input frequency and can be designed to steer from backfire to endfire, including broadside. This paper provides an analysis of resolution as a function of both input frequency and antenna length. Additionally, the resolution is compared to that achieved using an array of active acoustic elements.

Compressive beamforming with co-prime arrays

The results of compressive beamforming using arrays formed by Nyquist, co-prime samplers, Wichmann rulers, and Golomb rulers are shown along with forms of array gain, resolution and latency as measures of performance. Results will be shown for the idea case of few sources with Gaussian amplitudes in spatially white Gaussian white noise. Results will also be shown for data taken on the Five Octave Research Array (FORA). [This work was supported by ONR.]

An online method for time-varying spatial spectrum estimation using a towed acoustic array

This paper addresses the problem of time-varying field directionality mapping (FDM) or spatial spectrum estimation in dynamic environments with a maneuverable towed acoustic array. Array processing algorithms for towed arrays are typically designed assuming the array is straight, and are thus degraded during tow ship maneuvers. In this paper, maneuvering the array is treated as a feature allowing for left and right disambiguation as well as improved resolution towards endfire. A new method for online spatial spectrum estimation is presented. The maximum likelihood of the time-varying field is solved for using a single expectation maximization step after each received data snapshot. A multi-source simulation is used to illustrate the proposed algorithms ability to suppress ambiguous towed-array backlobes and resolve closely spaced interferers near endfire which pose challenges for conventional beamforming approaches, especially during array maneuvers.

Beamspace compressive spatial spectrum estimation on large aperture acoustic arrays

For large aperture sonar arrays, the number of acoustic elements can be quite sizable and thus increase the dimensionality of the l1 minimization required for compressive beamforming. This leads to high computational complexity that scales by the cube of the number of array elements. Furthermore, in many applications, raw sensor outputs are often not available since computation of the beamformer power is a common initial processing step performed to reduce subsequent computational and storage requirements. In this paper, a beamspace algorithm is presented that computes the compressive spatial spectrum from conventional beamformer output power. Results from CALOPS-07 experiment will be presented and shown to significantly reduce the computational load as well as increase robustness when detecting low SNR targets. [This work was supported by ONR.]

A Study of Active Sonar Reverberation using Ultrasonic Experiments in a Shallow-water Tank

This paper describes a study of active sonar reverberation level (RL) estimates made using scaled ultrasonic water-tank data. Such laboratory experiments provide a means of upper bounding the performance of RL estimates made in real shallow-water ocean environments. An ocean-to-tank scaling factor of 1000:1 was used to model a 180-220 Hz active source in a 100 m. deep, 2 km. extent of ocean using a 180-220 kHz. ultrasonic source in a 2 m. diameter tank filled to 0.1 m depth. Time-frequency analysis of backscattered ultrasonic data yielded intensity striations which compare favorably with theoretical models. Reverberation level (RL) estimates were computed as a function of frequency and range for both conventional frequency-invariant processing and more recently proposed waveguide-invariant processing. Experimental results indicate that waveguide invariant RL estimates provides an average 8 dB mean-square-error improvement over conventional methods.

Far-field superresolution imaging using shaped acoustic vortices

The limitations on resolution due to the effects of diffraction have presented a significant barrier to generating and observing small features with acoustic or electromagnetic waves. Previously proposed methods to overcome this limit, and therefore achieve superresolution, have largely been restricted to operating within the near-field region of the aperture. In this work, we will describe how acoustic helicoidal waves generated using a phased acoustic aperture (such as a traditional phased array or acoustic metasurface) can create acoustic vortices that are well below the resolution limit, and how this can enable far-field superresolution acoustic imaging. The acoustic vortices generated in this manner propagate from the near-field into the far-field through an arrangement of stable integer mode vortices, thereby enabling the generation of far-field superresolved features in the acoustic pressure field. Through the use of non-axisymmetric vortex beam distributions, splitting of the on-axis vortex occurs. This leads to arbitrary off-axis arrangements of vortices, enabling more complicated superresolved structures to be created such as squares, triangles and multi-point stars. In this paper, theoretical and numerical results will be presented for an acoustic aperture which is capable of generating superresolved far-field features in the radiated acoustic pressure, and results will be shown illustrating the superresolution capability of this novel technique.

Beamforming using chip-scale atomic clocks in a controlled environment

Recently developed low-power Chip-Scale Atomic Clocks (CSACs) hold promise for underwater acoustics applications because they enable time-coherent processing, critical for estimating the directionality of the sound field, when acoustic array elements cannot share a timing reference. Controlled, tank-based experiments with a small acoustic array (N = 4) featuring CSAC-equipped elements show that optimal disciplining is important for continued array coherence. Clock drift equivalent to a 10% wavelength error at 0.3, 1, and 10 kHz was reached at approximately 25, 10, and 3 days, respectively. Within application-specific limits, this technology brings enhanced capabilities to acoustic thermometry, geoacoustic, biological, and under-ice acoustic oceanography.

A performance metric for screen selection with the acoustic single pixel imager

In the recent literature, an Acoustic Single-Pixel Imager has been successfully developed for source localization in a two-dimensional waveguide. Source bearing angle estimation was carried out by applying sparse recovery techniques on sensor measurements taken over different imaging screens. This paper shows that the Mutual Coherence of the sensing matrix can be used as a metric to predict the source localization capability of the single-pixel imaging system. In particular, this papers analysis focuses on the sparsity of open cells within the imaging screen and the number of imaging screens used to maximize the probability of correct detection over varying levels of source sparsity. In this work, a simulation environment to demonstrate how the mutual coherence of the sensing matrix correlates with source localization performance over source sparsity, sparsity of open screen cells, and number of measurements used for sparse recovery is developed. The analysis shows that the leading factor in source localization performance gains is primarily from the number of imaging screens used to measure the acoustic wave-field.