Adam Zielinski

Performance analysis of digital acoustic communication in a shallow water channel

For an accurate automatic measurement of ship’s attitude the paper proposes an intelligent on-line sensing system which uses four servo-type accelerometers and one servo-type inclinometer appropriately located on the ship. Through an adequate location of the accelerometers, the heaving, rolling, and pitching signals of the ship are separated from each other with adequate linear combinations of the four sensors’ outputs. Furthermore, the inclinometer is utilized to extract a bias signal of the pitching. By introducing linear dynamic models and linear observation equations on the three signals, their on-line measurement is reduced to the state estimation of the linear dynamic systems. A bank of Kalman filters are used to execute the on-line state estimation and to overcome changes in parameters in the dynamic models with time.Abstruct- High data rate acoustic transmission is required for diverse underwater operations such as the retrieval of large amounts of data from bottom packages and real time transmission of signals from underwater sensors. The major obstacle to underwater acoustic communication is the interference of multipath signals due to surface and bottom reflections. High speed acoustic transmission over a shallow water channel characterized by small grazing angles presents formidable difficulties. The reflection losses associated with such small angles are low, causing large amplitudes in multi-path signals. In this paper we propose a simple but effective model for multipath interference, which is then used to assess the performance of a digital communication system operating in a shallow water channel. The results indicate that transmission rates in excess of 8 k-bits/s are possible over a distance of 13 km and channel depth of only 20 meters. Such a system offers improved performance in applications such as data collection from underwater sensors.

An eigenpath underwater acoustic communication channel model

Ocean exploration can require high data rate transmission over an underwater acoustic channel. The properties of underwater acoustic channels are thus very important in the design of high data rate acoustic telemetry systems. The authors review the essential properties of such channels. Based on some experimental results and analytical considerations they point out the limitations of the Rayleigh fading model commonly used in modeling underwater acoustic channels. This model has been adapted from microwave and radio communications in which received signals arrive along scattered paths. In an underwater acoustic channel there can be several distinct paths (eigenpaths) over which a signal can propagate from transmitter to receiver (eigenpath signals). Each eigenpath signal contains a dominant, stable component and many smaller, randomly scattered components (sub-eigenpath components). The envelope of the eigenpath signal can therefore be described using a Rice fading model. Based on the above the authors develop a novel channel model in which each eigenpath signal is described using a certain signal-to-multipath ratio, propagation time delay and Doppler shift. They propose a structure for a simulator of such a channel, to be used in conjunction with different transmission schemes.

Precise Multibeam Acoustic Bathymetry

The maximum error in ocean depth measurement as specified by the International Hydrographic Organization is 1% for depth greater than 30m. Current acoustic multibeam bathymetric systems used for depth measurement are subject to errors from various sources which may significantly exceed this limit. The lack of sound speed profiles may be one significant source of error. Because of the limited ability of sound speed profile measurement, depth values are usually estimated using an assumed profile. If actual sound speed profiles are known, depth estimate errors can be corrected using ray-tracing methods. For depth measurements, the calculation of the location at which a sound pulse impinges on the sea bottom varies with the variation of the sound speed profile. We demonstrate that this location is almost unchanged for a family of sound speed profiles with the same surface value and the same area under them. Based on this observation, we can construct a simple constant-gradient equivalent sound speed profile t...

An iterative method for array pattern synthesis

The authors propose a novel iterative method for synthesizing radiation patterns of linear arrays with arbitrary sidelobe envelopes, with nonisotropic elements, and with nonuniform spacing between elements. Various design examples are presented and compared with other methods. It is shown that the proposed iterative method converges rapidly and yields superior results over other methods. >

High rate shallow water acoustic communication

High rate communication over a very shallow channel is discussed. A multipath for such a channel is proposed and is used to predict performance of a phase modulated acoustic communication system. The results obtained indicate that transmission rates in excess of 6 kbits/s are possible over a distance of 3.5 km and in a water depth of only 15 meters. Such a system can find application for collecting data from underwater sensors.<<ETX>>

Lossless compression of hydroacoustic image data

Despite rapid progress in improving mass-storage density and digital communication system performance, compression of hydroacoustic image data is still significant in many engineering and research areas since it can overcome data storage and transmission bandwidth limitations. In this paper, we present a novel and effective approach for lossless compression of hydroacoustic image data which consists of two stages. The first stage reduces the information redundancy. We propose several new techniques to remove redundancy between data samples, data blocks, and data frames. The second stage uses a newly developed cascade coding scheme. This simple scheme can achieve an efficiency of 97%. A decomposition algorithm is presented for finding the optimal cascade coding parameters. The algorithm decomposes a multivariable optimization problem into a series of one-variable optimizations. Our two-stage algorithm offers a compression ratio of 2-3 and provides an exact recovery of the original data. Because of its simplicity, the algorithm can be incorporated into a variety of echo sounder systems. The compression algorithms can also be implemented using low-level assembly language to meet the requirements of real-time applications.

Matrix formulation for Dolph-Chebyshev beamforming

Several methods have been developed for tapering or shading of line arrays to minimize minor lobes. A method developed by C. L. Dolph makes it possible to optimize the patterns so that 1), for any specified minor lobe, the narrowest possible major lobe is achieved; or 2), for any specified major-lobe width, the lowest possible minor-lobe levels are achieved. The procedure to calculate the required set of shading coefficients involves algebraic manipulation of Chebyshev polynomials. This manipulation becomes increasingly tedious as the array size becomes larger. The letter presents a compact matrix formulation of Dolph-Chebyshev beamforming which can provide additional insight to the general problem of array shading. A special case of this formulation leads to a close-form solution.

Equivalent linear array approach to array pattern synthesis

An effective method for array radiation pattern synthesis that allows formulation of the synthesis of a desired pattern as an optimization problem is presented. The solution proposed involves matrix operations based on the equivalent computation. The advantage of this approach is that it does not involve any numerical optimization routines-only ordinary matrix operations are required. Illustrations are presented to highlight the various aspects of the method. The proposed algorithm can be used to design arrays of uniformly spaced elements with nonisotropic and unequal radiation patterns. >

Tsunami detectability using open-ocean bottom pressure fluctuations

Rationale for the measurement of open-ocean tsunami signatures are presented, and available pertinent data are reviewed. Models for tsunami signature and background noise are proposed in order to synthesize an optimum tsunami receiver. Using these models, the minimum tsunami amplitude (in cm) to yield the probability of correct tsunami detection P_{D} = 0.999 and probability of false alarm P_{F} = 10^{-3} is found to be 0.718/\sqrt{f_{0}} , where f_{0} is the tsunami dominant frequency (in cycles/h). A realizable receiver is proposed and its performance is evaluated using actual tsunami signatures. It is demonstrated that the detection of a tsunami with an average amplitude as small as 0.7 cm is possible for the P_{D} and P_{F} as above. Simulation results using synthesized background noise are shown. Tidal effects on the receiver performance also are considered and are found negligible for a certain range of the receiver parameters, resulting in a considerable reduction of the signal processing required.

A novel array of ring radiators

An array of ring radiators which generates a symmetric search-light-type narrow beam with greatly reduced sidelobes is proposed. Such a narrow beam can find several applications related to acoustic remote sensing, telemetry, and specialized sonars. A design procedure which benefits from design techniques developed for linear arrays is detailed. The developed methodology requires only simple matrix operation and does not involve nonlinear optimization. The results indicate that radiation patterns with arbitrary sidelobe suppression can be achieved. >