Bruce K. Newhall

Nonambiguous beamforming for a high resolution twin‐line array

Technology of towed line arrays and associated signal processing has progressed in recent years to the point that wideband multiline array coherent signal processing can now be performed in real time. Earlier implementation (1993) of a multiline array beamformer centered around the ability to convolve a conventional line array beam response with that of an orthogonal dipole. Performance of this implementation provided a unique nonambiguous right/left beam with roughly −20‐dB energy rejection in the null steered direction over a significant range of vertical arrival angles. However, performance suffered significantly with other than straight and parallel towed lines. Recent (1994) implementation of a beamformer for a twin‐line towed array has shown the ability to form deep backlobe nulls (−37 dB). Implementation of this beamformer is similar to the MVDR method except an isotropic noise field is assumed in the horizontal plane. This technique allows for uniform suppression of sidelobes at all off‐MRA azimut...

Improved estimation of the shape of towed sonar arrays

Low frequency passive sonar systems often use towed line arrays. In order to properly beamform the elements in the array, an accurate estimate of the array shape is required. We consider a submarine towed instrumented with 12 heading sensors used to estimate its shape. Submarines may make drastic maneuvers, producing significant bends in the array. These maneuvers also produce vibrations which introduce severe noise into the heading sensors. A Kalman filter has been developed to produce an accurate estimate of the array shape in the presence of severe heading noise. The Kalman filter uses an adaptively weighted average of the heading sensor data with an improved dynamic model to produce the estimate of shape. During periods of nominally straight tow, the filter relies strongly on the heading sensor data. The filter adaptively estimates the variance of the heading sensors, so that it automatically recognizes vibration-inducing maneuvers, and relies more heavily on the dynamic model during those times. Performance of the filter is controlled by adjusting the ratio of the sensor to process noise. Since the filter works on all sensors simultaneously to produce a single shape estimate, it is capable of recognizing a few bad sensors that are inconsistent with the majority. The Kalman filter algorithm has been successfully applied to eliminate contaminations from noisy sensors.

Continuous Reverberation Response and Comb Spectra Waveform Design

A model for the matched filter response to continuous reverberation from the transmission of broadband waveforms is developed. The application is for reverberation from a rough interface, based on perturbation theory. The model is developed for both the stationary rough bottom and the moving ocean surface interfaces. The mean reverberation is predicted as a function of the Doppler speed of the matched filter replica. Application is made to the design of waveforms with comb-like spectra. A uniform train of impulses produces a comb spectrum that is shown to significantly reject reverberation for a certain range of Doppler speeds. A similar low-reverberation response is produced from a continuous source emitting a wavetrain composed of adjacent hyperbolic-frequency-modulated (HFM) pulses. A waveform design technique is demonstrated to ensure continuity of the entire HFM wavetrain. Finally, waveforms with geometrically spaced comb spectra are considered. A new geometric comb waveform with constant amplitude is specified. However, this waveform requires a large bandwidth which may be difficult to obtain with practical high-power sources. Hard and soft-clipped versions of the comb spectra waveform are considered which provide useful compromises between the amount of reverberation suppression, the transmitted energy efficiency, and the utilization of available bandwidth.

Unambiguous noise and reverberation measurements from a dual line horizontal array

Single horizontal line arrays are often utilized to resolve the azimuthal character of the low‐frequency noise dominated by long‐range shipping sources. However, such arrays are subject to an azimuthal ambiguity on conjugate bearings. Coherent beamforming of an array composed of two parallel horizontal lines was recently employed to eliminate this left/right ambiguity using a minimum variance adaptive algorithm with single‐point main response constraints under an assumed horizontally isotropic noise field. The ability of this array and beamformer to resolve the directional characteristics of the acoustic noise field was demonstrated in the Atlantic and the Mediterranean. Median left/right rejection of 20 dB or more was observed over a wide frequency and azimuthal band. The measured beam noise exhibits temporal and spatial variability that could not have been quantified with a single‐line system. Median noise gain improvements of up to 7 dB above that of a single line were measured. Distant low‐frequency r...

Classification of signals in spherically invariant random clutter

The generalized likelihood ratio test (GLRT) is derived for the case of a signal subspace in acoustic clutter characterized by a spherically invariant random variable (SIRV). This result is a generalization of two previous results. First, the GLRT for the detection of a one dimensional signal in SIRV clutter has been previously given. However, featureless classification work has previously considered signals that are members of a multidimensional subspace but only in Gaussian clutter. The SIRV model extends that to the non-Gaussian clutter case. A SIRV is the product of two random variables: a non-Gaussian scalar times a complex multivariate Gaussian vector. The general GLRT result is then applied to a generalized gamma distribution for the SIRV scalar. When the generalized gamma exponent parameter is −2 this produces a product SIRV whose acoustic intensity has a generalized Pareto distribution. When the exponent parameter is + 2, the SIRV intensity is k distributed. This demonstrates that two widely used...

Underwater acoustics at the Johns Hopkins University Applied Physics Lab

The Johns Hopkins University Applied Physics Lab (JHU/APL) was founded in 1942. JHU/APL began research in underwater acoustics in 1970 and became known for full scale ocean testing. JHU/APL developed the first technique to accurately measure the shape of a towed array. Towed array testing culminated in 1991–1992 with the deployment of a 5 km aperture measuring signal coherence and beam noise statistics in the Pacific, Atlantic, and Mediterranean. In 1982, JHU/APL began investigations into low frequency (LF) active sonar. Initially, airguns and explosives were employed to measure bottom and surface scattering strengths. Then, tests with a stationary controlled source were conducted from 1986–1989, activating both stationary and towed receiver arrays. In 1989, JHU/APL outfitted the Cory Chouest, adding a two story back deck superstructure. The lower level housed a three aperture LF source array and long towed receiver array, while the upper story berthed 50 scientists and engineers. This ship conducted a se...

Validation of a physical model for surface clutter.

There is a strong need to more accurately represent active sonar system false contacts in various environments and conditions for the purpose of active acoustic simulation and synthetic training. A computationally viable approach for the generation of physics‐based false contacts in a raw beamformed time series that can be injected into a sonar processor was previously developed for bottom clutter. That model is now extended for clutter due to rough ocean surface scattering, and is compared with ASIAEX01 data. Considerations in extending the existing model include: characterizing appropriate statistics of the rough surface, adding surface scattering clutter to the sonar model, evaluating the realism of the simulation, and evaluating the computational burden (for real‐time trainers). The output of the simulation was analyzed for those considerations and then compared to the data. The use of a vertical line array allowed physical mechanisms in the data to be isolated, and their clutter data statistics separ...

Modeling persistent clutter.

Clutter is the collection of targetlike returns from nontarget sources that are observed in active sonar use. Persistent clutter is the set of clutter returns that persist across multiple transmissions and thereby form false target tracks in sonar processing. The U.S. Navy is developing active sonar simulators for training purposes. Effective training requires physically realistic clutter simulation including modeling of persistent clutter. Potential physical sources of persistent clutter are discussed, and a variety of modeling approaches are reviewed. Measures of effectiveness for model comparison with persistent clutter data are given.

Validating physics‐based clutter from seafloor roughness and biologic scattering

Two physics‐based clutter models were developed for comparison and to study the merits and limitations of both approaches. The stochastic approach [Newhall and Arvelo, J. Acoust. Soc. Am. 118, 2041 (2005)] makes use of perturbation theory, to relate the mean seafloor scattering strength to the mean surface wavenumber spectrum, and the extrapolation of this spectrum to the Bragg wavenumber to generate the clutter distribution from the seafloor roughness. The semi‐deterministic approach [Monjo and Arvelo, J. Acoust. Soc. Am. 118, 2041 (2005)] is based on 2‐D power‐law fractal realizations of the ocean floor, coherent modal addition, and the microscale bathymetric slopes. Biologic scattering was later included where spatial distribution and nonstationarity of schools of fish and individual marine mammals are realistically represented [Frankel et al. J. Acoust. Soc. Am. 119, 3437 (2006)] through individual‐based modeling of their movements, school size, and target strengths. Clutter distributions from both ap...

Realistic modeling of adaptive beamformer performance in nonstationary noise

Most adaptive beamformers (ABFs) operate under the assumption that the noise field is quasistationary. They estimate the present noise field by averaging, assuming stationarity over the estimation time. The adaptive beamformer responds to slow changes in the noise field across multiple estimation intervals. Unfortunately, in many low‐frequency underwater sound applications, the shipping noise may change rapidly, due to nearby ship motion. This motion can be significant during the estimation interval and degrade ABF performance. A realistic model has been developed, including two effects of source motion on horizontal towed arrays. Bearing rate produces a differential Doppler shift across the array. Range rate produces an amplitude modulation as the multipath interference pattern shifts along the array. The ABF model begins with a realization of ship locations and motion based on historical shipping density. Each ship generates realistic random noise composed of tonals in a broadband background. The noise ...