James C. Luby

Vector sensors and vector sensor line arrays: Comments on optimal array gain and detection

This paper examines array gain and detection performance of single vector sensors and vector sensor line arrays, with focus on the impact of nonacoustic self noise and finite spatial coherence of the noise between the vector sensor components. Analytical results based on maximizing the directivity index show that the particle motion channels should always be included in the processing for optimal detection, regardless of self noise level, as long as the self noise levels are taken into account. The vector properties of acoustic intensity can be used to estimate the levels of nonacoustic noise in ocean measurements. Application of conventional, minimum variance distortionless response, and white-noise-constrained adaptive beamforming methods with ocean acoustic data collected by a single vector sensor illustrate an increase in spatial resolution but a corresponding decrease in beamformer output with increasing beamformer adaptivity. Expressions for the spatial coherence of all pairs of vector sensor compon...

Autoregressive Modeling of Nonstationary Multibeam Sonar Reverberation

This work describes a technique for simulating nonstationary multibeam Gaussian sonar reverberation sequences. The primary objective is to simulate quadrature-demodulated complex digital vector sequences that have a prescribed time-variant covariance function. We use a first-order integral scattering model to relate geometrical, environmental, and sonar parameters to the assumed locally stationary multivariate reverberation power spectrum for each of a set of ranges spanning the total observation range of interest. At each range we compute the multivariate autoregressive (AR) canonical factorization of the power spectrum. The resultant autoregressive models are used to generate multivariate correlated stationary overlapping realizations for each range. Next we combine stationary realizations adjacent in range with an overlapped windowing scheme. This scheme allows the spectral characteristics of the reverberation to change smoothly with time. Finally, a time-dependent gain is applied to the entire realization to adjust the average intensity.

Underwater acoustic measurements with the Liberdade/X‐Ray flying wing glider

An underwater glider based on a flying wing design (Jenkins et al., 2003) presently is under development by the Marine Physical Laboratory, Scripps Institution of Oceanography and the Applied Physics Laboratory, University of Washington. This design maximizes the horizontal distance between changes in buoyancy to minimize mechanical power consumed in horizontal transport. The prototype wing has a 6.1 m wing span and is 20 times larger by volume than existing gliders. Initial at‐sea tests indicate that the lift‐to‐drag ratio is 17/1 at a horizontal speed of about 1.8 m/s for a 38‐liter buoyancy engine. Beamforming results using recordings of the radiated noise from the deployment ship by two hydrophones mounted on the wing verify aspects of the prototype wing flight characteristics. The payload on the new glider will include a low‐power, 32‐element hydrophone array placed along the leading edge of the wing for large physical aperture at midfrequencies (above 1 kHz) and a 4‐component vector sensor. Data pre...

Gliders, floats, and robot sailboats: autonomous platforms for marine mammal research

Passive acoustic monitoring (PAM), now widely used for marine mammal research, is typically conducted using hydrophone arrays towed behind ships, providing real-time data from large areas over short time spans (days to weeks), or using fixed autonomous hydrophones, providing non-real-time data from small areas over long time spans (months to years). In contrast, mobile platforms can supply near-real-time data over spatiotemporal scales large in both space and time. These systems are deployed from a vessel, communicate via satellite with shore stations for navigation and control updates, and report in near-real time upon detecting marine mammal or other sounds of interest. Acoustically-equipped gliders are buoyancy-driven devices that are capable of traversing long distances (hundreds to thousands of kilometers) over weeks to months of autonomous operation. Autonomous floats such as QUEphones drift with currents or park on the seafloor, rising to the surface upon detecting sounds of interest. Robot sailboa...

Passive‐acoustic monitoring of odontocetes using a Seaglider: First results of a field test in Hawaiian waters.

In fall 2009 the University of Washington, Applied Physics Laboratory conducted in collaboration with the Oregon State University, a comprehensive field test of a passive‐acoustic Seaglider along the western shelf‐break of the island of Hawaii. During the 3 week mission, a total of approximately 170 h of broadband acoustic data [194 kHz sampling rate] were collected. The recordings were manually analyzed by an experienced analyst for beaked whale (Ziphiidae), dolphin (Delphinidae), and sperm whale (Physeter macrocephalus) echolocation clicks as well as echo sounder pings emitted by boats in the area. Here we present and discuss first results of these data analysis, which revealed that more than 50% of the recorded files (each of 1‐minute duration) contain bioacoustic signals. Furthermore the recorded data and the results of the manual analysis are used to validate and optimize an automated classifier for odontocete echolocation clicks, which was developed in a collaborative effort with San Diego State Uni...

Underwater acoustic measurements with a flying wing glider

Liberdade, a new class of underwater glider based on a flying wing design, has been under development for the past 3 years in a joint project between the Marine Physical Laboratory, Scripps Institution of Oceanography and the Applied Physics Laboratory, University of Washington. This hydrodynamically efficient design maximizes the horizontal distance traveled between changes in buoyancy, thereby minimizing average power consumed in horizontal transport to achieve ‘‘persistence.’’ The first fully autonomous glider of this class, ‘‘XRay,’’ was deployed and operated successfully in the Monterey Bay 2006 experiment. Communications, including real‐time glider status reports, were accomplished using an underwater acoustic modem as well as with an Iridium satellite system while on the surface. The payload included hydrophone array, with 10 kHz per channel bandwidth, located in a sonar dome along the leading edge of the 6.1‐m‐span wing. Narrowband tones from 3.0 to 8.5 kHz were transmitted from a ship‐deployed controlled underwater source. During the glider’s flight, lift‐to‐drag ratios (equal to the inverse of the glide slope) exceeded 10/1. However, specific flight behaviors that deviated from this efficient horizontal transport mode allowed for improved detection and localization by the hydrophone array. [Work sponsored by the Office of Naval Research.]

Broadband mine detection and classification—Preliminary results from a set of low‐frequency shallow‐water experiments

Mine counter measures (MCM) sonars have seen accelerated development over the last few years as Navy interests have shifted into shallow‐water operations. To find mines, most MCM sonar technologies use relatively high‐frequency and narrow‐band signals to produce high‐resolution images of the seafloor. Even so, the problem of discriminating between proud mines and similarly sized false targets and of detecting buried mines at ranges of several hundred meters remains very difficult. The Applied Physics Laboratory‐University of Washington in conjunction with Arete Engineering and Technologies Corporation‐San Diego have been conducting shallow‐water experiments with a relatively low‐frequency and very broadband (2–20 kHz) sonar. The use of low frequencies permits greater penetration into bottom sediments and very broadband signals mitigate the lack of spatial resolution expected from a narrow‐band analysis. A description of the sonar and of the experiments conducted in Puget Sound using mines and minelike fal...

Broadband mine detection and classification—Target responses from a set of 2‐ to 20‐kHz shallow‐water experiments

The problem of discriminating between underwater mines and similarly sized false targets at ranges of several hundred meters remains a challenge. The Applied Physics Laboratory, University of Washington, in conjunction with Arete Engineering and Technologies Corporation, San Diego, have been developing detection and classification signal processing schemes using data collected in several shallow‐water experiments with a broadband (2–20 kHz) sonar. The use of relatively low frequencies for mine hunting permits greater penetration into bottom sediments for improved detection of buried or partially buried mines. Wide bandwidth signals mitigate the lack of spatial resolution expected from a narrow‐band analysis at such frequencies, and provide a wealth of information for classification. A brief description of the sonar and of the experiments conducted in Puget Sound using mines and minelike false targets will be given. Target responses as a function of time, frequency, and aspect angle will be presented, as w...

Optimal detection with vector sensors and vector sensor line arrays

The detection performance of single vector sensors and vector sensor line arrays is degraded by nonacoustic self‐noise and spatial coherence of the noise between vector sensor components. Results based on optimizing the directivity index for a single vector sensor show that the particle motion channels should always be included in the processing for optimal detection, regardless of self‐noise level, assuming these levels are properly taken into account. The vector properties of acoustic intensity can be used to estimate the levels of nonacoustic noise in ocean measurements. Application of conventional, minimum variance distortionless response, and white‐noise‐constrained adaptive beamforming methods with ocean acoustic data collected by a vector sensor illustrates increase in spatial resolution but corresponding decrease in beamformer output and introduction of bias with increasing beamformer adaptivity. Expressions for the spatial coherence of all pairs of vector sensor components in homogeneous, isotrop...

Autoregressive modeling of nonstationary multibeam sonar reverberation

This work describes a technique for simulating nonstationary, multibeam Gaussian sonar reverberation sequences. The primary objective is to simulate quadrature‐demodulated complex digital vector sequences that have a prescribed time variant covariance function. We use a first‐order integral scattering model to relate the geometrical, environmental, and sonar parameters to the assumed locally stationary multivariate reverberation power spectrum for each of a set of ranges spanning the total observation range of interest. At each range we compute the multivariate autoregressive canonical factorization of the power spectrum. The resultant autoregressive models are used to generate multivariate, correlated, stationary realizations for each range. Next we combine stationary realizations adjacent in range with an overlapped windowing scheme. This scheme allows the spectral characteristics of the reverberation to change with time. Finally, a time variable gain is applied to the entire realization to adjust the a...