Benjamin R. Dzikowicz

Underwater Acoustic Navigation Using a Beacon With a Spiral Wave Front

In this paper, a method for performing underwater acoustic navigation using a spiral wave-front beacon is examined. A transducer designed to emit a signal whose phase changes by 360° in one revolution can be used in conjunction with a reference signal to determine the aspect of a remote receiver relative to the beacon. Experiments are conducted comparing spiral wave-front beacon navigation to Global Positioning System (GPS) onboard an unmanned surface vehicle. The advantages and disadvantages of several outgoing signals and processing techniques are compared. The most successful technique involves the use of a phased array projector utilizing a broadband signal. Aspect is determined by using a weighted mean over frequencies. Sources of error for each of the techniques are also examined.

Navigation on an unmanned surface vehicle using an acoustical spiral wave front beacon.

An acoustical spiral wave front beacon described previously [Dzikowicz and Hefner, J. Acoust. Soc. Am. 127, 1748 (2010)] is tested using an unmanned surface vehicle (USV). The beacon transmits from two transducers, one with a signal whose phase varies with aspect and a reference signal whose phase is constant with aspect. A remote hydrophone’s aspect relative to the beacon’s can be determined by comparing the phase of the two signals. The beacon is positioned at a fixed depth, location, and orientation in the acoustic test pond at the Naval Surface Warfare Center, Panama City Division. A remotely operated USV is equipped with a hydrophone at the same depth as the beacon. The vehicle is also equipped with a differential global positioning system (DGPS) receiver to determine its exact position. The USV is driven in several routes including areas of varying water depth and reverberation levels. The DGPS positions are then compared to the aspect as calculated by the spiral wave font beacon. Single, dual, and ...

Aspect determination using a transducer with a spiral wavefront: Prototype and experimental results.

An underwater acoustical navigational beacon is built whose operation is similar to the VHF omnidirectional ranging systems used in aircraft navigation. The beacon consists of two disk transducers whose direction of motion is radial: a circular transducer with constant radius and a spiral transducer whose radius varies linearly by one wavelength over 360 deg. To determine aspect, short tone bursts are then sent from each of the transducers. A remote hydrophone can then determine its aspect to the beacon by comparing the phase between the two transducers, with the circular transducer acting as reference. Results from tank experiments demonstrate the viability of this technique. The vertical beam patterns are also determined and compared to numerical results. In addition, other techniques, such as producing a spiral wavefront using a phased array, are also discussed. [Work sponsored by the Office of Naval Research.]

Doubly focused backscattering from finite targets in an Airy caustic formed by a curved reflecting surface

Caustics can be formed in the water column when sound scatters off a curved-reflecting surface such as the ocean floor or surface. The simplest caustic is an Airy caustic formed by the merging of two rays. Small targets lying in or near Airy caustics have backscattered echoes that can be focused both to the target and upon return. For a point target, the doubly focused backscattering amplitude is proportional to the square of an Airy function whose argument depends on the target location through the changes in relative return times of contributing rays. For a finite sized target, the symmetry is broken and the amplitude unfolds into a hyperbolic umbilic catastrophe. The arguments for the hyperbolic umbilic function are calculated using the relative return times of transient echoes. These doubly focused echoes can lead to amplitudes larger than that of direct or singly focused echoes (echoes which focus once, either to the target or upon return). Experiments using a cylindrical half-pipe as a reflecting su...

Cylindrical heat conduction and structural acoustic models for enclosed fiber array thermophones

Calculation of the heat loss for thermophone heating elements is a function of their geometry and the thermodynamics of their surroundings. Steady-state behavior is difficult to establish or evaluate as heat is only flowing in one direction in the device. However, for a heating element made from an array of carbon fibers in a planar enclosure, several assumptions can be made, leading to simple solutions of the heat equation. These solutions can be used to more carefully determine the efficiency of thermophones of this geometry. Acoustic response is predicted with the application of a Helmholtz resonator and thin plate structural acoustics models. A laboratory thermophone utilizing a sparse horizontal array of fine (6.7 μm diameter) carbon fibers is designed and tested. Experimental results are compared with the model. The model is also used to examine the optimal array density for maximal efficiency.

Isolating scattering resonances of an air-filled spherical shell using iterative, single-channel time reversal

Iterative, single-channel time reversal is employed to isolate backscattering resonances of an air-filled spherical shell in a frequency range of 0.5-20 kHz. Numerical simulations of free-field target scattering suggest improved isolation of the dominant target response frequency in the presence of varying levels of stochastic noise, compared to processing returns from a single transmission and also coherent averaging. To test the efficacy of the technique in a realistic littoral environment, monostatic scattering experiments are conducted in the Gulf of Mexico near Panama City, Florida. The time reversal technique is applied to returns from a hollow spherical shell target sitting proud on a sandy bottom in 14 m deep water. Distinct resonances in the scattering response of the target are isolated, depending upon the bandwidth of the sonar system utilized.

Small scale test bed for studying multiaspect and multistatic sonar systems.

A nominally 1:50 scale acoustic test bed is operational at the Naval Surface Warfare Center‐Panama City Division (NSWC‐PCD) to image free‐field, bottom, and buried targets using multiaspect, bistatic, backscatter, forward scatter, and synthetic aperture sonar (SAS) techniques. The test bed is designed to test and study novel geometries and techniques faster and cheaper than can be done in the field. Using precise translational positioning systems mitigates any issues associated with positioning of multiple sonar platforms in the field. The use of two transmit and receive transducer platforms is superior to a single platform, which is restricted to purely backscattered imaging, for producing SAS images. With two platforms, one can generate four images, two using backscattered return data and two using data sent from one platform to the other. Thus, the system is inherently both dual aspect and bistatic. The setup, including the scaled sediment, acoustic sources and receivers, and computer controlled transl...

Aspect determination using a transducer with a spiral wavefront: Basic concept and numerical modeling.

A circular cylindrical acoustic transducer whose direction of motion is radial produces an outgoing signal whose wavefronts are concentric circles. A similar transducer can be designed to generate spiral wavefronts where the phase of the outgoing wave changes by 360 deg in one revolution around the transducer. If these two transducers are used in tandem as a beacon, short tone bursts fired consecutively from each transducer can be used to determine the aspect angle to a receiver relative to the beacon. This is an acoustic version of the VHF omnidirectional ranging systems currently in use in airplane navigation. There are several ways of designing a transducer to produce the spiral wavefront required for the beacon. Several of these designs have been modeled numerically to understand the details of the outgoing field and their impact on aspect determination. [Work supported by ONR.]

Acoustic propagation from a spiral wave front source in an ocean environment

A spiral wave front source generates a pressure field that has a phase that depends linearly on the azimuthal angle at which it is measured. This differs from a point source that has a phase that is constant with direction. The spiral wave front source has been developed for use in navigation; however, very little work has been done to model this source in an ocean environment. To this end, the spiral wave front analogue of the acoustic point source is developed and is shown to be related to the point source through a simple transformation. This makes it possible to transform the point source solution in a particular ocean environment into the solution for a spiral source in the same environment. Applications of this transformation are presented for a spiral source near the ocean surface and seafloor as well as for the more general case of propagation in a horizontally stratified waveguide.

Modeling the response of an acoustical spiral wave front transducer.

An acoustical spiral wave front transducer radiates a field whose phase varies linearly with the azimuthal angle around the transducer. The phase of the field, therefore, carries information about the location of a receiver relative to the spiral transducer. There are two transducer designs that can produce this type of wave front. The first is a “physical spiral” transducer that creates the phase change by physically deforming the active element of the transducer. While this is the simplest design, the physical deformation produces a discontinuity in the active element that affects both the amplitude and phase of the outgoing field. The effect of this discontinuity is examined through both a finite element model and a baffled source approximation. The second technique uses an array of elements, each driven with an appropriate phase to produce a “phased spiral” transducer. The output of this transducer depends on the wavenumber of the outgoing field as well as on the radius and number of the elements of t...