Arthur B. Baggeroer

The state of the art in underwater acoustic telemetry

Progress in underwater acoustic telemetry since 1982 is reviewed within a framework of six current research areas: (1) underwater channel physics, channel simulations, and measurements; (2) receiver structures; (3) diversity exploitation; (4) error control coding; (5) networked systems; and (6) alternative modulation strategies. Advances in each of these areas as well as perspectives on the future challenges facing them are presented. A primary thesis of this paper is that increased integration of high-fidelity channel models into ongoing underwater telemetry research is needed if the performance envelope (defined in terms of range, rate, and channel complexity) of underwater modems is to expand.

An overview of matched field methods in ocean acoustics

A short historical overview of matched-field processing (MFP) is followed by background material in both ocean acoustics and array processing needed for MFP. Specific algorithms involving both quadratic and adaptive methods are then introduced. The results of mismatch studies and several algorithms designed to be relatively robust against mismatch are discussed. The use of simulated MFP for range, depth and bearing location is examined, using data from a towed array that has been tilted to produce an effective vertical aperture. Several experiments using MFP are reviewed. One successfully demonstrated MFP at megameter ranges; this has important consequences for experiments in global tomography. Some unique applications of MFP, including how it can exploit ocean inhomogeneities and make tomographic measurements of environmental parameters, are considered. >

Matched field processing: Source localization in correlated noise as an optimum parameter estimation problem

Matched field processing is a parameter estimation technique for localizing the range, depth, and bearing of a point source from the signal field propagating in an acoustic waveguide. The signal is observed at an array in the presence of additive, spatially correlated noise that also propagates in the same ocean environment as the signal. In a weak signal‐to‐noise situation this parameter estimation requires the maximum exploitation of the physics of both the signal and noise structure which then must be coupled to optimum methods for the signal processing. We study the physics of this processing by modeling the ocean environment as a waveguide that is horizontally stratified with an arbitrary sound‐speed profile in the vertical. Thus, the wave equation describes the underlying structure of the signal and noise, and the signal processing via the generation of the replica fields. Two methods of array processing are examined: (i) the linear cross correlator (Bartlett) and (ii) the maximum likelihood method ...

Source Localization and Sensing: A Nonparametric Iterative Adaptive Approach Based on Weighted Least Squares

Array processing is widely used in sensing applications for estimating the locations and waveforms of the sources in a given field. In the absence of a large number of snapshots, which is the case in numerous practical applications, such as underwater array processing, it becomes challenging to estimate the source parameters accurately. This paper presents a nonparametric and hyperparameter, free-weighted, least squares-based iterative adaptive approach for amplitude and phase estimation (IAA-APES) in array processing. IAA-APES can work well with few snapshots (even one), uncorrelated, partially correlated, and coherent sources, and arbitrary array geometries. IAA-APES is extended to give sparse results via a model-order selection tool, the Bayesian information criterion (BIC). Moreover, it is shown that further improvements in resolution and accuracy can be achieved by applying the parametric relaxation-based cyclic approach (RELAX) to refine the IAA-APES&BIC estimates if desired. IAA-APES can also be applied to active sensing applications, including single-input single-output (SISO) radar/sonar range-Doppler imaging and multi-input single-output (MISO) channel estimation for communications. Simulation results are presented to evaluate the performance of IAA-APES for all of these applications, and IAA-APES is shown to outperform a number of existing approaches.

Acoustic telemetry - An overview

Acoustic telemetry from underwater submersibles and sensors has been pursued ever since it was recognized that the ocean could support signal transmission. While it has been evident that some form of communication is possible, the ocean has proved to be a distressingly difficult medium in which to achieve high data rates. High data rate transmission requires a wide bandwidth which is severely constrained in the ocean because of the absorption of high-frequency energy. Moreover, the ocean is a very reverberant environment with both time and frequency spreading of signals; this further limits data transmission rates. The net effect of the bandwidth and reverberation constraints has led to either acoustic telemetry systems with low data rates or to the use of tethered systems. Over the years, various forms of acoustic communication systems have been developed. These have included direct AM and SSB for underwater telephones, FM for sensor data, FSK and DPSK for digital data, and parametric sonars for narrow-beam systems. As offshore operations have increased, several other systems have been proposed and/or built to respond to particular needs. In this paper, we review the underwater channel and the limitations that it imposes upon acoustic telemetry systems. We then survey some of the systems that have been built (excluding military systems) and indicate how they use various communication system principles to overcome these limitations.

Communication over Doppler spread channels. Part I: Channel and receiver presentation

Scattering functions from several experiments demonstrate that acoustic underwater channels are doubly spread. Receivers used on these channels to date have difficulty with large Doppler spreads. A receiver to perform coherent communication over Doppler spread channels is presented in this first paper of two. The receiver contains a channel tracker and a linear decoder. The tracker operates by means of a modified recursive least squares algorithm which makes use of frequency-domain filters called Doppler lines. The decoder makes use of the channel tracker coefficients in order to perform minimum mean square error decoding. This first paper treats theoretical aspects whereas the second part presents implementation issues and results.

A test of basin-scale acoustic thermometry using a large-aperture vertical array at 3250-km range in the eastern North Pacific Ocean

Broadband acoustic signals were transmitted during November 1994 from a 75-Hz source suspended near the depth of the sound-channel axis to a 700-m long vertical receiving array approximately 3250 km distant in the eastern North Pacific Ocean. The early part of the arrival pattern consists of raylike wave fronts that are resolvable, identifiable, and stable. The later part of the arrival pattern does not contain identifiable raylike arrivals, due to scattering from internal-wave-induced sound-speed fluctuations. The observed ray travel times differ from ray predictions based on the sound-speed field constructed using nearly concurrent temperature and salinity measurements by more than a priori variability estimates, suggesting that the equation used to compute sound speed requires refinement. The range-averaged ocean sound speed can be determined with an uncertainty of about 0.05 m/s from the observed ray travel times together with the time at which the near-axial acoustic reception ends, used as a surroga...

ENVIRONMENTALLY TOLERANT BEAMFORMING FOR HIGH-RESOLUTION MATCHED FIELD PROCESSING : DETERMINISTIC MISMATCH

Standard adaptive beamforming or matched field processing requires accurate replica fields finely gridded over the search parameter space for localization with sidelobe control. Multiple constraints to the maximum likelihood method (MLM) technique are introduced in order to construct a beamformer (MCM) more tolerant to environmental mismatch of a deterministic nature. The result is a plane‐wave or matched field beamformer that accommodates some mismatch in the environment while still suppressing sidelobes. This beamformer maintains its localization and sidelobe control over a coarser grid of the search parameter space than the standard MLM beamformer which requires an extremely fine grid for localization and sidelobe control. Examples simulating the performance of the MCM beamformer for plane‐wave and matched field processing for an Arctic environment are given.

Spatial modulation experiments in the underwater acoustic channel

A modulation technique for increasing the reliable data rate achievable by an underwater acoustic communication system is presented and demonstrated. The technique, termed spatial modulation, seeks to control the spatial distribution of signal energy such that the single physical ocean channel supports multiple parallel communication channels. Given a signal energy constraint, a communication architecture with access to parallel channels will have increased capacity and reliability as compared to one with access to a single channel. Results from two experiments demonstrate higher obtainable data rates and power throughput for a system employing spatial modulation than for one that does not. The demonstrated benefits were characterized by an equivalent SNR gain of over 5 dB in the first experiment. In the second experiment, using two element source and receiver arrays with apertures of 0.9 m, a coherently modulated signal was shown to offer nearly 50% greater capacity by using spatial modulation than by using temporal modulation alone.

The Heard Island Feasibility Test

In January 1991, the Heard Island Feasibility Test (HIFT) was carried out to establish the limits of usable, long‐range acoustic transmissions. Coded acoustic signals transmitted from a source near Heard Island in the southern Indian Ocean were monitored at 16 sites in the North and South Atlantic, the North and South Pacific, the Indian Ocean, and the Southern Ocean. The question posed by HIFT, whether at such global ranges the signals would permit phase‐coherent processing and thus yield favorable signal‐to‐noise levels, was answered in the affirmative. There was no evidence of distress by the local marine mammal population in response to the acoustic transmissions. HIFT was prerequisite to a program for Acoustic Thermometry of Ocean Climate (ATOC). The principal challenges to such a program are discussed.