Ronald A. Wagstaff

The AWSUM filter: a 20-dB gain fluctuation-based processor

The advanced WISPR summation (AWSUM) filter, a natural extension of the WISPR filter for higher filter order numbers, is presented and its performance is compared to the performance of the WISPR filter and the conventional summation processor. It is shown that the AWSUM filter achieves substantial gains in various measures of processor performance above those of the other two processors in spatial and spectral resolution, minimum detectable level (MDL), clutter reduction, and signal-to-noise ratio (SNR) gain. The important processing parameters are shown to be the percentage of overlap of the voltage time series and the number of FFTs averaged. SNR gains in excess of 20 dB were shown to be achievable for low-fluctuation amplitude tonals using measured data.

A computerized system for assessing towed array sonar functionality and detecting faults

A computer-based system that processes hydrophone and beam noise data from a towed horizontal line array sonar has been developed. The system also produces various displays that can be used to help assess the functionality of the sonar and to identify faults that cause degraded performance. The system-and various statistics used for characterizing or quantifying a given aspect of the sonars performance are discussed. The results are presented in visual formats to aid in rapid assessment and quantification of the sonars performance. Examples obtained from the system during recent towed array ambient noise measurement exercises are given to illustrate its utility for real-time performance monitoring and its capability for providing clues to aid in fault localization. >

DIET AWSUM: a fluctuation-based deconvolution technique for enhancing signal-to-noise ratio and resolution

A technique for simultaneously improving resolution and signal-to-noise ratio gain, the directivity improved estimation technique (DIET) combined with the advanced WISPR summation (AWSUM), or DIET AWSUM, is introduced. The DIET AWSUM method combines a straightforward deconvolution technique with the highly fluctuation-sensitive AWSUM filter. DIET AWSUM results generated for ocean acoustic data are compared with estimates produced using the maximum entropy method (MEM), a well-tested standard. The DIET AWSUM method achieved superior results in both increased resolution and improved gain for stable signal identification chiefly because of its greater ability to discriminate between stable signals and fluctuating signals and noise.

Exploiting phase fluctuations to improve temporal coherence

A special-purpose definition is proposed for phase fluctuations to overcome the obstacle of unpredictable dynamic changes in the phase angle. This definition implies a specific time history for each phase sample and any deviation is termed a phase fluctuation. Its application to acoustic data led to the development of a technique for temporally aligning the phase angles of the acoustic pressure phasors. This alignment process transforms the signal phasors to the real half-space of a rotated complex plane, while the corresponding noise is distributed with random phase angles. Signal processing conducted in the rotated plane improves the temporal coherence of the signals without significantly altering the incoherence of the noise. Coherent attenuation and cancellation of signals is common with temporal coherence and vector averaging. These were eliminated when the aligned-phase angles were substituted for the original unaligned phase angles. Thus, the transformation produces a net temporal coherence gain. Furthermore, it significantly improves the robustness of the signal processor to source and receiver motion. An automatic identifier of signals in the transformed plane also is introduced. Signal identification is based on aligned-phase angle temporal coherence, which significantly improves identification of signals. Results are included for both ocean and atmosphere acoustic data.

The Wagstaff’s integration silencing processor filter: A method for exploiting fluctuations to achieve improved sonar signal processor performance

There is a class of signal processing algorithms that achieves gain by exploiting the amplitude fluctuations in the spectrum analyzed acoustic power. One of those processors is presented. The causes of fluctuations that influence the performance of such processors are discussed, and processed results from a fluctuation-based processor are presented. Gains relative to the average power processor in signal-to-noise ratio (SNR) of as much as 10 dB, and gains in spatial and spectral resolution, minimum detectable levels, and other measures of processor performance have been achieved. For the most part, the additional gains from exploiting fluctuations are independent of the frequency resolution and the array aperture (or number of elements if the noise is “white”) and depend on the number of averages and percent of overlap of consecutive time-to frequency fast Fourier transforms (FFT).

Three-Dimensional Noise Field Directionality Estimation from Single-Line Towed Array Data

For many reasons, the three-dimensional (3-D) arrival structure of the undersea ambient noise field is of interest to the research and development community. One reason is that the arrival structure can be used to estimate the beam noise of an array, which may be required to estimate the performance of the array as an operational Navy asset or a scientific measurement tool. Another reason is that there are clues inherent in the vertical arrival structure that relate to the nature of the acoustic propagation along the azimuths of the noise sources. Similarly, there are also clues in the horizontal arrival structure of the undersea ambient noise field that relate to the azimuthal distribution of the noise sources. Both of these classes of clues are important in the verification and validation of undersea ambient noise models. The ideal measurement tool to measure the 3-D arrival structure of the noise field is a high-resolution volumetric array sonar system. Unfortunately, such a system is not generally ava...

Exploitation of fluctuations to enhance target detection and to reduce clutter and background noise in the marine environment

Amplitude and phase fluctuations are an inherent characteristic of many types of propagation media including sound in the marine environment. Generally, fluctuations are regarded as a nuisance to be ignored, avoided, or eliminated. However, this paper shows that fluctuations can be effectively exploited to enhance the detection of targets that fluctuate less in amplitude than the clutter and background noise. In fact, a case can be made that the exploitation of fluctuations constitutes a third dimension for achieving gain to supplement the other two well know dimensions of frequency resolution gain and array aperture gain. Results from measurements at sea are presented to support the claim that a new dimension of gain is being accessed and to demonstrate that the additional gains can be substantial.

Infrasound measurements during hurricane Katrina

Infrasound is used to detect both natural and man‐made events. Microbarometric measurement stations have been placed worldwide in order to detect infrasonic signals in the atmosphere. Such signals range in frequency from less than 0.01 to 10 Hz. Infrasound is produced by many natural phenomena including violent storms, gravity waves, the Aurora Borealis, and even some animals. Because infrasound signals have such low frequencies, they can travel great distances in the atmosphere, while maintaining good temporal coherence. This makes the atmosphere an excellent medium for infrasound signal propagation and corresponding detection of large storms, such as hurricanes and tornados, which generate strong infrasonic signals. This paper will discuss hurricane Katrina’s infrasound spectral results obtained from data measured by piezo‐ceramic infrasound sensors in Oxford, MS, about 400 miles to the north of New Orleans. Spectral and temporal coherence results will be presented and discussed. [Work supported by ARDEC.]

Phase variations in a fluctuation-based processor

Fluctuations are always present in underwater sound propagation, and are generally viewed as a complication in signal detection and identification. However, in some cases where the signals fluctuate less than the noise, it is possible to take advantage of the different magnitudes of fluctuations of signal and noise to improve detection. Wagstaffs integration silencing processor (WISPR) is an example of such a processor. The original version of the WISPR processor utilized power values derived from complex pressures in a given frequency bin, but ignored the phases of these complex pressures. An improved processor that takes advantage of the phase as well as the amplitude is described below. Its performance is verified using measured data, where detection has been accomplished by a margin of 4 decibels. Simulations using synthetic data show that the new processor can be effective for signal-to-noise ratios greater than minus 20 decibels.

Signal processor for detection of signals in cluttered environments.

The well‐publicized experience of the USS Cole in a foreign port demonstrates the potential danger our military ships are frequently exposed to. When docked, they are the most vulnerable to attack by small fast boats that can hit and run before their threat is recognized, and defenses can be activated. This is a challenge that has many facets. Attack by a small high‐powered fast‐boat is just one type of threat. However, it is important. One way of quickly identifying a fast‐boat is with an underwater acoustic sensing system. Such a system is not without challenges. Harbors are typically busy and contaminated by many forms of acoustic clutter. Detecting and separating the signals of fast‐boats from among the clutter are difficult tasks. Fortunately, a signal processor has been developed with highly coherent fast‐boat signals, and harbor clutter in mind. This fast‐boat processor invokes temporal coherence constraints, by degree, to strip away incoherent noise and less coherent shipping signals, and leaves t...