R.F.W. Coates

Analysis of the performance of an underwater acoustic communications system and comparison with a stochastic model

This paper describes the testing of an experimental 50 kHz DPSK acoustic data telemetry system developed at The University of Birmingham. The experiment was conducted across the Solent, at Southampton in the United Kingdom. The Solent is a extremely shallow channel with a severe multipath and high shipping noise. Excellent performance was obtained under favourable transmission conditions. The performance of the system is characterised by the bit error rate at various symbol and bit rates and ranges. The calculated performance is then compared to a theoretical model. The channel is characterised using ray theory in a constant temperature medium to initialise the impulse response, then randomly varying the delay and amplitude of the separate arrivals. This represents the effect of the fluctuating sea surface upon the reflected sound waves. The validity of this comparison is then discussed in the context of other published experimental work in this field.<<ETX>>

The design of transducers and arrays for underwater data transmission

Consideration is given to the design of underwater acoustic projector transducers for communication purposes. Particular interest is focused upon broadband designs which provide the potential for high data-rate communication. The author introduces the underlying physics of sound propagation in solid-bar and disc structures typically used for manufacturing underwater acoustic transducers. The variability of sound speed with bar dimensions, in bar resonators, is discussed. Consideration is given to the nature and identification of different modal states in such structures. The possibilities and limitations in modeling and design are reviewed. An examination is made of several specific designs, including thin-disc and broadband high-frequency transducers as well as piston and tonpilz structures modified electrically and mechanically to produce broad, controlled passband responses. A variety of more exotic structures is also included. Attention is given to arrays of transducers and, particularly, to the problems involved in overcoming element interaction and inadequacies in element behavior. >

Potential performance of parametric communications

From a designer viewpoint, a parametric array should not appear different from any other type of acoustic transducer and should be described by a limited set of design equations together with their range of validity. In this paper, these design equations are stated and discussed. They are used to optimize the acoustic parameters of an underwater communication system using parametric transduction and to evaluate its performance in terms of signal-to-noise ratio and data-rate limits as a function of transmission range. It turns out that, for a maximum data transmission rate at a given range, there is a set of optimum design parameters which is a function of the array size only. This means that, once given an operational range, the primary frequency, the electrical power, the maximum acoustic source level, and the directionality of the transducer can be deduced directly from the array diameter.

An experimental study of rough surface scattering and its effects on communication coherence

The paper addresses some reflection and scattering phenomena for a model telemetry channel with a periodically fluctuating surface. Limitations on the extent of surface fluctuations are derived from the reflection and scattering area, Rayleigh parameter and shadow function. These results are related to the observed pulsed response of the surface. Finally the effect of the scattering process on communication coherence is briefly discussed.<<ETX>>

Measured channel sounding characteristics and their relationship with the performance of a parametric communication system

This paper presents the results of experiments carried out in the Mediterranean Sea using parametric transduction to effect acoustic communication. Short duration, pulsed carrier signals were used to estimate the impulse response of the channel and these observations were related to the performance of a parametric communication system operating under the same conditions. The statistical characteristics and spectra of the amplitude and phase fluctuations were used to analyse the pulsed response over short observation periods. Typically, the amplitude fluctuations experienced by the main path were Ricean in nature with a spectral content in the sub-Hertz range whilst the phase fluctuations tended towards more Gaussian-like behaviour with a comparable frequency spectra. Under these conditions a high data rate differential phase shift keyed (DPSK) telemetry link was established over a 1.7 km path. The ensuing performance of the communication system, characterised by the bit error rate for various symbol rates, is presented and demonstrates the viability of a parametric telemetry system.

Underwater acoustic communication utilising parametric transduction with M-ary DPSK modulation

A real-time M-ary differential phase-shift keying (MDPSK) communication system utilising parametric transduction has been constructed. The system employs a 50 kHz primary frequency and a 5 kHz difference frequency. It has been tested in the Gulf of Lion and at Cap Ferrat in the Mediterranean Sea. Experimental results indicate that the system can be used in shallow water to realise real-time acoustic communications at ranges of tens of kilometres and can achieve data rates of 1, 2, and 3 kb s/sup -1/ for 2-, 4-, and 8-DPSK respectively.

The development of a low frequency barrel-stave transducer for tomographic applications using finite element and boundary element modelling

Ocean acoustic tomography requires wide bandwidth, compact, and efficient low frequency sources of sound. The paper describes the design process for such a transducer in which the principal objectives are a centre frequency of 400 Hz and a bandwidth of 100 Hz, in as compact a device as possible. A finite element method is used to model the structure, and this is coupled to a boundary element method to predict the performance in water. Quadratic elements are used throughout, and treatment of the piezoelectric properties of the driver is included. Both convex and concave barrel shapes are considered, and the effects of various parameters are determined; these include the thickness and curvature of the staves, the number of staves, and the overall size of the device. A design is presented which theoretically meets the specified objectives.<<ETX>>

Transmission Line Matrix Modeling Applied To Problems In Underwater Acoustics

Transmission Line Matrix (TLM) modeling is a relatively new numerical method for solving partial differential equations which has some advantages over competing techniques: it is easy to include nonlinear effects, it is inherently stable and it offers physical insight into the processes being modeled. TLM modeling produces an electrical network which contains 1,2 or 3-dimensional transmission lines as space-discretizi ng elements. Electrical terminations determine boundary conditions. This paper introduces the theoretical basis behind TLM and discusses current and future applications of TLM in underwater acoustics. The method has already found use in volume scattering and in acoustic propagation when modeling time-varying phenomena in strongly multipath channels and is now also being applied to transducer structure modeling as well as the thermal behaviour of transducers.

Combined finite element-boundary element analysis of a viscoelastic anechoic panel for underwater applications

In this paper the combined finite element-boundary element method is applied to the analysis of the sound-structure interaction around a multi-layered anechoic baffle. The baffle is made of three layers: an aluminium backing supports a second layer of highly butyl rubber. The third layer, loaded polyurethane, presents a regular grid of steep shaped cones with a fluid-matching function. The viscoelastic materials are fully modelled into the finite element method using linear integral constitutive relationships. The combined FE-BE algorithm is based on a partial application of the Burton and Miller Helmholtz gradient formulation to overcome non-uniqueness problems. Examples of analysis of acoustic scattering in a range of frequencies typical of underwater communications are given together with a practical application of the structure.

The design and testing of a free-fall device for recording seismic activity beneath the ocean floor

For the last thirty years ocean bottom seismometers (OBSs) have been used to survey the oceans. However, the quality of the data they produce is often degraded due to poor coupling or high levels of ambient noise. In contrast, sensors placed beneath the seafloor avoid many of these problems. For the last two decades burials have relied on Ocean Drilling Program (ODP) boreholes. This approach is expensive and demands numerous human resources. In addition, the locations of these boreholes, determined by the ODP, are rarely desirable for monitoring seismic activity. In this paper a sub-bottom seismic acquisition system is presented that uses free-fall devices, or deep ocean seismic penetrators (DOSP), to place sensors several tens of metres beneath the seafloor. The DOSPs weigh approximately 1800 kg, achieve terminal velocities between 30-50 m/s and penetrate to depths of 20-30 metres in soft sea sediments. Once buried, they record seismic activity and transmit data back to the surface using a frequency shift keyed (FSK) modulation technique. The results of an experiment conducted in the Mediterranean using this system are presented. These confirm the predicted dynamic and kinematic behaviour of the DOSP and allow an assessment of the ambient seismic noise level at a depth of /spl ap/30 metres beneath the seafloor. In conclusion this paper discusses the potential use of free-fall devices to increase our understanding of processes in the deep oceans, with particular emphasis on their applicability to future deep ocean seismology.