Kevin B. Smith

Underwater acoustic communication channel simulation using parabolic equation

High frequency acoustic communication (8--50 kHz) has attracted much attention recently. Significant advancements have been achieved in terms of data rates, communication range, and performance. At these high frequencies, various physical processes, including surface waves, subsurface bubbles, and ocean volume fluctuations, can significantly affect the communication channel. The time-varying underwater channel has both deterministic and stochastic features. While there is on-going work, the research community is still lacking adequate models that can provide realistic representations of the dynamic channel in the ocean. Advancements in underwater acoustic communication technology mainly rely on at-sea experiments, which are very costly. A realistic channel model not only can facilitate receiver design, help investigate channel limits, and aid in communication algorithm validation and comparison, it also can provide a basis for network level studies. A communication channel simulator is developed here through the use of parabolic equation modeling of acoustic propagation and scattering. Specifically, the simulator uses the Monterey-Miami Parabolic Equation model (MMPE) augmented with a linear surface model. The linear surface model generates an evolving surface based on theoretical or experimental directional surface spectrum and feed the surface displacement and its derivatives to the acoustic model. The time-varying acoustic field is calculated using successive MMPE runs when the surface evolves. At each single run, the model accounts for surface scattering effects based on the surface input. It also accounts for propagation through the water column and through the sediment based on other environmental measurements such as sound speed profile, bathymetry, and bottom properties. The channel simulator is also calibrated by experimental data obtained in the Pacific ocean in 2008. The surface model simulates a time-evolving surface from the directional surface spectrum obtained by a Waverider buoy in the experiment. Based on the surface input and other environmental measurements, the channel simulator generates realistic time-varying impulse responses. The output agreed well with the acoustic measurements in terms of arrival time structure and intensity profile. Acoustic communication performance comparison between the experimental and simulated data will be also reported in the conference.

Impact of wind turbine noise in The Netherlands

The Dutch government aims at an increase of wind energy up to 6 000 MW in 2020 by placing new wind turbines on land or offshore. At the same time, the existing noise legislation for wind turbines is being reconsidered. For the purpose of establishing a new noise reception limit value expressed in L den , the impact of wind turbine noise under the given policy targets needs to be explored. For this purpose, the consequences of different reception limit values for the new Dutch noise legislation have been studied, both in terms of effects on the population and regarding sustainable energy policy targets. On the basis of a nation-wide noise map containing all wind turbines in The Netherlands, it is calculated that 3% of the inhabitants of The Netherlands are currently exposed to noise from wind turbines above 28 dB(A) at the faηade. Newly established dose-response relationships indicate that about 1500 of these inhabitants are likely to be severely annoyed inside their dwellings. The available space for new wind turbines strongly depends on the noise limit value that will be chosen. This study suggests an outdoor A-weighted reception limit of L den = 45 dB as a trade-off between the need for protection against noise annoyance and the feasibility of national targets for renewable energy.

Near real-time improved UUV positioning through channel estimation - the Unscented Kalman Filter approach

The primary objective of this work is to enhance the navigational and positioning accuracy of Unmanned Underwater Vehicles (UUVs) by networking a number of Unmanned Surface Vehicles (USVs) utilizing underwater acoustic modems and acoustic travel time calculations. In a previous work [1], a tracking algorithm based on the Extended Kalman Filter (EKF) was developed presenting satisfactory results, but the assumption of the errors being Gaussian and zero mean at the base of the EKF is violated by the presence of nonlinearities in the measurement equation. In this work, a more suitable approach based on the Unscented Kalman Filter (UKF) is presented and its results compared to the existing approach. A combination of the EKF/UKF with a Smoothing Algorithm was developed and extensively tested with synthetic data. To validate the concepts, the tracking algorithms (EKF and UKF based) were applied to data collected during sea tests that took place in Monterey Bay in August, 2015.

An Efficient Normal Mode Solution to Wave Propagation Prediction

In this paper, an overview of a normal mode method of solving the Helmholtz wave equation to describe the underwater sound field for a fixed-point source in a plane multilayered medium is presented. The mode functions are well-defined at all depths of the medium as they are continuous across turning points of the separated depth-dependent differential equation. Comparisons of model results to a limited number of measured data sets and benchmark propagation codes are presented.

Scale model analysis of full-duplex communications in an underwater acoustic channel

Due to issues of in-band artifacts produced by a transmitter that would otherwise influence or jam an adjacent receiver in the near field, current underwater acoustic communication systems employ half-duplex algorithms. This limits the information transfer between nodes to exclusively send or receive at any given time. The use of full-duplex. communications, allowing simultaneous send and receive protocols to be employed, could help improve data transfer rates and error checking. A direct evaluation of the effectiveness of full-duplex transmissions is investigated in a scale-model, shallow water waveguide. The results of simultaneous two-way communications achieved through the use of adjacent, nonoverlapping frequency bands are presented. In this case, use of appropriate filtering of the receptions is being used to minimize cross-talk between bands. Different elements of the arrays are also assigned specific operating bands in order to help reduce cross-talk. The use of orthogonal signals (such as PRN sequences) over common bands is also discussed.

UUV localization using acoustic communications, networking, and a priori knowledge of the ocean current

Underwater navigation is particularly challenging due to the fact that a number of navigation aids, such as GPS and similar, are not available. In order to accurately estimate a UUVs position at any time during a mission, we are relying on acoustic communication between the UUV and a network of surface platforms at known locations (reference points). By using the acoustic modems and a model of the environment, the acoustic wave travel time from the UUV to the reference points can be measured and converted into a distance. These distance measurements are then used by a tracking algorithm to improve the UUV positioning accuracy. In previous work [1], a tracking algorithm based on the Unscented Kalman Filter (UKF) was developed presenting satisfactory results. As part of the UUV tracking model, the drift caused by the ocean current was modeled as a random walk and is part of the state of the system. Based on predictions for the ocean current made by different UUVs at different times, a consensus algorithm was developed [2]. The knowledge of the ocean current provided by the consensus algorithm is then used to improve the UUV positioning. The developed algorithms were initially tested using synthetic data. To validate the simulation results, the algorithms were applied to data collected during sea tests that took place in Monterey Bay in August, 2015.

Sensitivity Analysis of Hybrid Split-Step Fourier/Finite Difference Parabolic Equation Models

Traditionally, ocean acoustic propagation models assume the sea surface can be treated as an idealized pressure release boundary. For flat surfaces, this can easily be accomplished through a variety of modeling techniques. Rough surfaces, however, introduce additional complexities in numerical models which assume a pressure release condition. An alternative approach is to model the physical water/air interface in a manner analogous to the water/sediment interface of the bottom. However, the ocean surface boundary introduces a much larger interface discontinuity than the bottom interface. In this work, a previously developed hybrid split-step Fourier/finite-difference approach is implemented at the water/air interface. Results are compared with standard SSF smoothing approaches. Normal mode and finite element models are utilized to provide benchmark solutions. Tradeoffs between accuracy and stability are discussed, as well as the model’s ability to accurately compute transmission across the water/air inter...

Modeling acoustic vector fields for inverse problems

Acoustic vector sensors that measure pressure and orthogonal particle velocity are gaining widespread interest. Predicting their performance requires calculating the pressure field and the velocity fields, which require spatial gradients of the pressure field. In typical hydrophone applications, significant computational savings are realized by using reciprocity to generate the pressure field as a function of source position rather than receive position. However, the presence of the spatial gradients in the velocity fields means that reciprocity cannot be used to model the vector field for inverse problems. Instead, the inverse vector velocity field must be computed point by point, even for the simplest environments. Examples of this effect are demonstrated by the derivation of analytic expressions for pressure and particle velocity in a Pekeris waveguide. These simple waveguide results are extended to arbitrary, range-dependent, environment parameters using a parabolic equation model.

Scattered acoustic intensity field measurements of a rigid motionless sphere and cylinder

In this study, the properties of the scattered acoustic vector fields generated by a simple rigid motionless sphere and cylinder are investigated. Analytical solutions are derived from general acoustic pressure scattering models, and analyzed for wave numbers in the resonance region. The separable active and reactive components of the acoustic intensity are used to investigate the structural features of the scattered field components. The ability to extract scattered field features is illustrated with measurements obtained from in-air experiments using an anechoic chamber and acoustic intensity probes to measure the scattered acoustic vector field resulting from continuous plane wave illumination.

A full spectrum solution to wave propagation prediction

An overview of a normal mode method of solving the Helmholtz wave equation to describe the underwater sound field for a fixed point source in a plane multilayered medium is presented. The mode functions are well-defined at all depths of the medium as they are continuous across turning points of the separated depth-dependent differential equation. Comparisons of model results to a limited number of benchmark propagation solutions are presented.