Showing papers in "IEEE Journal of Oceanic Engineering in 2001"

Seaglider: a long-range autonomous underwater vehicle for oceanographic research

Abstract: Seagliders are small, reusable autonomous underwater vehicles designed to glide from the ocean surface to a programmed depth and back while measuring temperature, salinity, depth-averaged current, and other quantities along a sawtooth trajectory through the water. Their low hydrodynamic drag and wide pitch control range allow glide slopes in the range 0.2 to 3. They are designed for missions in a range of several thousand kilometers and durations of many months. Seagliders are commanded remotely and report their measurements in near real time via wireless telemetry. The development and operation of Seagliders and the results of field trials in Puget Sound are reported.

SLOCUM: an underwater glider propelled by environmental energy

Abstract: SLOCUM is a small gliding AUV of 40 000-km operational range which harvests its propulsive energy from the heat flow between the vehicle engine and the thermal gradient of the temperate and tropical ocean. The design of both the glider and the thermal engine are discussed including the design genesis and approach, field trial results, concept strength, and limitations and potential use.

The autonomous underwater glider "Spray"

Abstract: A small (50-kg, 2-m long) underwater vehicle with operating speeds of 20-30 cm/s and ranges up to 6000 km has been developed and field tested. The vehicle is essentially an autonomous profiling float that uses a buoyancy engine to cycle vertically and wings to glide horizontally while moving up and down. Operational control and data relay is provided by GPS navigation and two-way communication through ORBCOMM low-Earth-orbit satellites. Missions are envisioned with profile measurements repeated at a station or spaced along a transect. The initial instrument complement of temperature, conductivity, and pressure sensors was used to observe internal waves and tides in the Monterey underwater canyon.

The Energy Harvesting Eel: a small subsurface ocean/river power generator

Abstract: The Energy Harvesting Eel (Eel) is a new device that uses piezoelectric polymers to convert the mechanical flow energy, available in oceans and rivers, to electrical power. Eel generators make use of the regular trail of traveling vortices behind a bluff body to strain the piezoelectric elements; the resulting undulating motion resembles that of a natural eel swimming. Internal batteries are used to store the surplus energy generated by the Eel for later use by a small, unattended sensor or robot. Because of the properties of commercially available piezoelectric polymers, Eels will be relatively inexpensive and are easily scaleable in size and have the capacity to generate from milli-watts to many watts depending on system size and flow velocity of the local environment. A practical Eel structure has been developed that uses the commercially available piezoelectric polymer, PVDF. Future Eels may use more efficient electrostrictive polymer. Every aspect of the system from the interactions between the hydrodynamics of the water flow and structural elements of the Eel, through the mechanical energy input to the piezoelectric material, and finally the electric power output delivered through an optimized resonant circuit has been modeled and tested. The complete Eel system, complete with a generation and storage system, has been demonstrated in a wave tank. Future work on the Eel will focus on developing and then deploying a small, lightweight, one-watt power generation unit, initially in an estuary and then subsequently in the ocean. Such Eels will have the ability to recharge batteries or capacitors of a distributed robotic group, or remote sensor array, thus extending the mission life indefinitely in regions containing flowing water.

Model-based feedback control of autonomous underwater gliders

Abstract: We describe the development of feedback control for autonomous underwater gliders. Feedback is introduced to make the glider motion robust to disturbances and uncertainty. Our focus is on buoyancy-propelled, fixed-wing gliders with attitude controlled by means of active internal mass redistribution. We derive a nonlinear dynamic model of a nominal glider complete with hydrodynamic forces and coupling between the vehicle and the movable internal mass. We use this model to study stability and controllability of glide paths and to derive feedback control laws. For our analysis, we restrict to motion in the vertical plane and consider linear control laws. For illustration, we apply our methodology to a model of our own laboratory-scale underwater glider.

Underwater acoustic communication by passive-phase conjugation: theory and experimental results

Abstract: A new method for coherent underwater acoustic communication called passive phase conjugation is evaluated. The method is so named because of conceptual similarities to active phase conjugation methods that have been demonstrated in the ocean. In contrast to active techniques, however, the array in passive phase conjugation needs only receive. The procedure begins with a source transmitting a single probe pulse. After waiting for the multipathed arrivals to clear, the source then transmits the data stream. At each element in the distant receiving array, the received probe is cross-correlated with the received data stream. This cross-correlation is done in parallel at each array element and the results are summed across the array to achieve the final communication signal suitable for demodulation. As the ocean changes, it becomes necessary to break up the data stream and insert new probe pulses. Results from an experiment conducted in Puget Sound near Seattle are reported. Measurements were made at multiple ranges and water depths in range-dependent environments.

Underwater vehicle obstacle avoidance and path planning using a multi-beam forward looking sonar

Abstract: This paper describes a new framework for segmentation of sonar images, tracking of underwater objects and motion estimation. This framework is applied to the design of an obstacle avoidance and path planning system for underwater vehicles based on a multi-beam forward looking sonar sensor. The real-time data flow (acoustic images) at the input of the system is first segmented and relevant features are extracted. We also take advantage of the real-time data stream to track the obstacles in following frames to obtain their dynamic characteristics. This allows us to optimize the preprocessing phases in segmenting only the relevant part of the images. Once the static (size and shape) as well as dynamic characteristics (velocity, acceleration,...) of the obstacles have been computed, we create a representation of the vehicle's workspace based on these features. This representation uses constructive solid geometry (CSG) to create a convex set of obstacles defining the workspace. The tracking takes also into account obstacles which are no longer in the field of view of the sonar in the path planning phase. A well-proven nonlinear search (sequential quadratic programming) is then employed, where obstacles are expressed as constraints in the search space. This approach is less affected by local minima than classical methods using potential fields. The proposed system is not only capable of obstacle avoidance but also of path planning in complex environments which include fast moving obstacles. Results obtained on real sonar data are shown and discussed. Possible applications to sonar servoing and real-time motion estimation are also discussed.

Analysis of channel effects on direct-sequence and frequency-hopped spread-spectrum acoustic communication

Abstract: Multiuser underwater acoustic communication is one of the enabling technologies for the autonomous ocean-sampling network (AOSN). Multiuser communication allows vehicles, moorings, and bottom instruments to interact without human intervention to perform adaptive sampling tasks. In addition, multiuser communication may be used to send data from many autonomous users to one buoy with RF communications capability, which will then forward the information to shore. The two major signaling techniques for multiuser acoustic communication are phase-shift keying (PSK) direct-sequence spread-spectrum (DSSS) and frequency-shift keying (FSK) frequency-hopped spread-spectrum (FHSS). Selecting between these two techniques requires not only a study of their performance under multiuser conditions, but also an analysis of the impact of the underwater acoustic channel. In the case of DSSS, limitations in temporal coherence of the channel affect the maximum spreading factor, leading to situations that may be better suited to FHSS signals. Conversely, the multipath resolving properties of DSSS minimize the effects of frequency-selective fading that degrade the performance of FSK modulation. Two direct-sequence receivers potentially suitable for the underwater channel are presented. The first utilizes standard despreading followed by decision-directed gain and phase tracking. The second uses chip-rate adaptive filtering and phase tracking prior to despreading. Results from shallow water testing in two different scenarios are presented to illustrate the techniques and their performance.

Docking for an autonomous ocean sampling network

Abstract: In this paper, we examine the issues associated with docking autonomous underwater vehicles (AUVs) operating within an Autonomous Ocean Sampling Network (AOSN). We present a system based upon an acoustic ultrashort baseline system that allows the AUV to approach the dock from any direction. A passive latch on the AUV and a pole on the dock accomplish the task of mechanically docking the vehicle. We show that our technique for homing is extremely robust in the face of the two dominant sources of error-namely the presence of currents and the presence of magnetic anomalies. Our strategy for homing is independent of the initial bearing of the dock to the AUV, includes a method for detecting when the vehicle has missed the dock, and automatically ensures that the AUV is in a position to retry homing with a greater chance of success. Our approach is seen to be extremely successful in homing the vehicle to the dock, mechanically attaching itself to the dock, aligning inductive cores for data and power transfer, and undocking at the start of a fresh mission. Once the AUV is on the dock, we present a methodology that allows us to achieve the complex tasks with ensuring that the AUV is securely docked, periodically checking vehicle status, reacting to a vehicle that requires charging, tracking it when it is out on a mission, archiving and transmitting via satellite the data that the AUV collects during its missions, as well as providing a mechanism for researchers removed from the site to learn about vehicle status and command high-level missions. The dock is capable of long-term deployments at a remote site while respecting the constraints - low power, small size, low computational energy, low bandwidth, and little or no user input - imposed by the amalgamation of acoustic, electronic and mechanical components that comprise the entire system.

Autonomous underwater vehicle homing/docking via electromagnetic guidance

Abstract: Central to the successful operation of an autonomous undersea vehicle (AUV) is the capability to return to a dock, such that consistent recovery of the AUV is practical. Vehicle orientation becomes increasingly important in the final stages of the docking, as large changes in orientation near the dock are impractical and often not possible. A number of homing technologies have been proposed and tested, with acoustic homing the most prevalent. If AUV orientation is required as well as bearing and distance to the dock, an acoustic homing system will require high update rates, and extensive signal conditioning. An Electromagnetic Homing (EM) system is one alternative that can provide accurate measurement of the AUV position and orientation to the dock during homing. This system offers inherent advantages in defining the AUV orientation, when compared to high frequency acoustic systems. The design and testing of an EM homing system are given, with particular attention to one can be adapted to a wide class of AUVs. A number of homing, docking, and latching trials were successfully performed with the design. Homing data include dead reckoning computation and acoustic tracking of the homing track, and video documentation of homing into the dock.

Power systems for autonomous underwater vehicles

Abstract: In this paper, we examine the issues involved in designing battery systems and power-transfer (charging) techniques for Autonomous Underwater Vehicles (AUVs) operating within an Autonomous Ocean Sampling Network (AOSN). We focus on three different aspects of the problem, battery chemistry, pack management and in situ charging. We look at a number of choices for battery chemistry and evaluate these based on the requirements of maximizing power density and low temperature operation particular to AUVs. We look at the issues involved in combining individual cells into large battery packs and at the problems associated with battery monitoring, and the charging and discharging of packs in a typical AUV application. Finally, we present a methodology for charging an AUV battery pack in situ in support of long term deployments at remote sites.

Acoustic vector-sensor correlations in ambient noise

Abstract: Most array-processing methods require knowledge of the correlation structure of the noise. While such information may sometimes be obtained from measurements made when no sources are present, this may not always be possible. Furthermore, measurements made in-situ can hardly be used to analyze system performance before deployment. The development of models of the correlation structure under various environmental assumptions is therefore very important. In this paper, we obtain integral and closed form expressions for the auto- and cross-correlations between the components of an acoustic vector sensor (AVS) for a wideband-noise field, under the following assumptions concerning its spatial distribution: 1) azimuthal independence; 2) azimuthal independence and elevational symmetry; and 3) spherical isotropy. We also derive expressions for the cross-covariances between all components of two spatially displaced AVSs in a narrowband-noise field under the same assumptions. These results can be used to determine the noise-covariance matrix of an array of acoustic vector sensors in ambient noise. We apply them to a uniform linear AVS array to asses its beamforming capabilities and localization accuracy, via the Cramer-Rao bound, in isotropic and anisotropic noise.

An adaptive-hybrid algorithm for geoacoustic inversion

Abstract: This paper presents an adaptive hybrid algorithm to invert ocean acoustic field measurements for seabed geoacoustic parameters. The inversion combines a global search (simulated annealing) and a local method (downhill simplex), employing an adaptive approach to control the trade off between random variation and gradient-based information in the inversion. The result is an efficient and effective algorithm that successfully navigates challenging parameter spaces including large numbers of local minima, strongly correlated parameters, and a wide range of parameter sensitivities. The algorithm is applied to a set of benchmark test cases, which includes inversion of simulated measurements with and without noise, and cases where the model parameterization is known and where the parameterization most be determined as part of the inversion. For accurate data, the adaptive inversion often produces a model with a Bartlett mismatch lower than the numerical error of the propagation model used to compute the replica fields. For noisy synthetic data, the inversion produces a model with a mismatch that is lower than that for the true parameters. Comparison with previous inversions indicates that the adaptive hybrid method provides the best results to date for the benchmark cases.

Coordinated motion planning and control of autonomous underwater vehicle-manipulator systems subject to drag optimization

Abstract: A new motion coordination algorithm for an autonomous underwater vehicle-manipulator system (UVMS) is proposed. This algorithm generates the desired trajectories for both the vehicle and the manipulator in such a way that the total hydrodynamic drag on the system is minimized. Resolution of kinematic redundancy of the system is performed at the acceleration level so that this algorithm can be incorporated into the system dynamics. The dynamics of the UVMS are modeled using a quasi-Lagrange approach. A state-space formulation of the system along with a model-based controller design for trajectory-following tasks that includes thruster dynamics is also presented. The computer simulation results demonstrate the effectiveness of this proposed method in reducing the drag on the system.

Overview of SAX99: environmental considerations

Abstract: A 1-km/sup 2/ area located 2 km off the Florida Panhandle (30/spl deg/22.6'N; 86/spl deg/38.7'W) was selected as the site to conduct high-frequency acoustic seafloor penetration, sediment propagation, and bottom scattering experiments. Side scan, multibeam, and normal incidence chirp acoustic surveys as well as subsequent video surveys, diver observations, and vibra coring, indicate a uniform distribution of surficial and subbottom seafloor characteristics within the area. The site, in 18-19 m of water, is characterized by 1-2-m-thick fine-to-medium clean sand and meets the logistic and scientific requirements specified for the acoustic experiments. This paper provides a preliminary summary of the meteorological, oceanographic, and seafloor conditions found during the experiments and describes the important physical and biological processes that control the spatial distribution and temporal changes in these characteristics.

Decoupled stochastic mapping [for mobile robot & AUV navigation]

Abstract: This paper describes decoupled stochastic mapping (DSM), a new computationally efficient approach to large-scale concurrent mapping and localization (CML). DSM reduces the computational burden of conventional stochastic mapping by dividing the environment into multiple overlapping submap regions, each with its own stochastic map. Two new approximation techniques are utilized for transferring vehicle state information from one submap to another, yielding a constant-time algorithm whose memory requirements scale linearly with the number of submaps. The approach is demonstrated via simulations and experiments. Simulation results are presented for the case of an autonomous underwater vehicle navigating in an unknown environment with 110 and 1200 features using simulated observations of point features by a forward look sonar. Empirical tests are used to examine the consistency of the error bounds calculated by the different methods. Experimental results are also presented for an environment with 93 features using sonar data obtained in a 3 by 9 by 1 m testing tank.

Command, control, and navigation experimental results with the NPS ARIES AUV

Abstract: Provides an overview of the Naval Postgraduate School ARIES autonomous underwater vehicle and its control and navigation. An attempt is made to highlight its current operational capabilities and provide a description of future enhancements for greater mission utility and flexibility. An overview of the vehicle design along with descriptions of all major hardware components and sensors is given. A major discussion of the implementation of a modular, multirate, multiprocess software architecture for the ARIES is provided. The architecture is designed to operate using either a single computer processor or two independent, cooperating processors linked through a network interface for improved load balancing. A dual computer implementation is presented since each processor assumes different tasks for mission operation. Also included is a section on the underwater navigation method using a real-time extended Kalman filter that fuses all sensor data and computes the real time position, orientation and velocity. Experimental results for navigational accuracy using a DGPS/IMU/Doppler-aided navigation system are presented with DGPS pop-up maneuvers. Navigational accuracy is a requirement for the use of ARIES as a mobile communications network node. This work provides some examples of missions possible with such a node and the current state of its command and control system.

A system for high-resolution zooplankton imaging

Abstract: We discuss an in situ marine imaging system based on high-speed digital line scan cameras for collection of a continuous picture of microscopic marine particles ranging in size from 200 /spl mu/m to several centimeters The system is built to operate on a 53-cm-diameter autonomous underwater vehicle or a tethered platform The digital imaging system provides a continuous record of all particles passing through a symmetric 96/spl times/96 mm sampling tube and provides views from two orthogonal directions Data are compressed using a lossless encoding technique and stored onto a disk drive Over 50 h of continuous imaging is possible using the system Data are suitable for studies requiring sizing, identification, quantification, and spatial recording of semi-transparent and opaque particles This paper summarizes the mechanical, optical, and data processing design of this instrument and discusses recent improvements We also present images and quantitative results from recent deployments in the Gulf of Mexico

Enhancement of the inertial navigation system for the Morpheus autonomous underwater vehicles

Abstract: This paper presents the design and development of an enhanced inertial navigation system that is to be integrated into the Morpheus autonomous underwater vehicle at Florida Atlantic University. The inertial measurement unit is based on the off-the-shelf Honeywell HG1700-AG25 3-axis ring-laser gyros and three-axis accelerometers and is aided with ground speed measurements obtained using an RDI Doppler-velocity-log sonar. An extended Kalman filter has been developed, which fuses together asynchronously the inertial and Doppler data, as well as the differential Global Positioning System positional fixes whenever they are available. A complementary filter was implemented to provide a much smoother and stable attitude estimate. Thus far, preliminary study has been made on characterizing the inertial navigation system-based navigation system performance, and the corresponding results and analyzes are provided.

An overview of SAX99: acoustic measurements

Abstract: A high-frequency acoustic experiment was performed at a site 2 km from shore on the Florida Panhandle near Fort Walton Beach in water of 18-19 m depth. The goal of the experiment was, for high-frequency acoustic fields (mostly In the 10-300-kHz range), to quantify backscattering from the seafloor sediment, penetration into the sediment, and propagation within the sediment. In addition, spheres and other objects were used to gather data on acoustic detection of buried objects. The high-frequency acoustic interaction with the medium sand sediment was investigated at grazing angles both above and below the critical angle of about 30/spl deg/. Detailed characterizations of the upper seafloor physical properties were made to aid in quantifying the acoustic interaction with the seafloor. Biological processes within the seabed and the water column were also investigated with the goal of understanding their impact on acoustic properties. This paper summarizes the topics that motivated the experiment, outlines the scope of the measurements done, and presents preliminary acoustics results.

Enabling technologies for REMUS docking: an integral component of an autonomous ocean-sampling network

Abstract: A dock for an autonomous underwater vehicle (AUV) allows the vehicle to be left on station ready for deployment. However, it represents a significant engineering challenge, as docking requires an accurate navigation system so that the vehicle can find the dock, and complex mechanics to make the required underwater power and data connections. This paper describes the docking system built for the REMUS AUV. It outlines the basis for the design decisions, the as-built configuration, and its performance once deployed. It also delineates the lessons learned from the deployments, and the refinements in the vehicle that have been made since that time, that will improve the system's utility and reliability.

A comparison of multifrequency HF radar and ADCP measurements of near-surface currents during COPE-3

Abstract: A high-frequency multifrequency coastal radar operating at four frequencies between 4.8 and 21.8 MHz was used as part of the third Chesapeake Bay Outflow Plume Experiment (COPE-3) during October and November, 1997. The radar system surveyed the open ocean east of the coast and just south of the mouth of Chesapeake Bay from two sites separated by about 20 km. Measurements were taken once an hour, and the eastward and northward components of ocean currents were estimated at four depths ranging from about 0.5 m to 2.5 m below the surface for each location on a 2 by 2 km grid. Direction of arrival of the signals was estimated using the MUSIC algorithm. The radar measurements were compared to currents measured by several moored acoustic Doppler current profilers (ADCPs) with range bins 2-14 m below the water surface. The vertical structure of the current was examined by utilizing four different radar wavelengths, which respond to ocean currents at different depths, and by using several ADCP range bins separated by 1-m intervals. The radar and ADCP current estimates were highly correlated and showed similar depth behavior, and there was significant correlation between radar current estimates at different wavelengths and wind speed.

Performance metrics for oceanographic surveys with autonomous underwater vehicles

Abstract: Discusses tools to facilitate the effective use of AUVs to survey small-scale oceanographic processes. A fundamental difficulty is the coupling of space and time through the AUV survey trajectory. Combined with the finite velocity and battery life of an AUV, this imposes serious constraints on the extent of the survey domain and on the spatial and temporal survey resolutions. We develop a quantitative survey error metric which accounts for errors due to both spatial undersampling and temporal evolution of the sample field. The accuracy of the survey error metric is established through surveys of a simulated oceanographic process. Using the physical constraints of the platform, we develop the "survey envelope" which delineates a region of survey parameter space within which an AUV can successfully complete a mission. By combining the survey error metric with the survey envelope, we create a graphical survey analysis tool which can be used to gain insight into the AUV survey design problem. We demonstrate the application of the survey analysis tool with an examination of the impact of certain survey design and parameters on surveys of a simple oceanographic process.

Real-time path planning and obstacle avoidance for RAIS: an autonomous underwater vehicle

Abstract: This paper describes a navigation and guidance system (NGS) with real-time path planning and obstacle avoidance capabilities that has been developed for the autonomous underwater vehicle RAIS. The vehicle is designed to accomplish two missions: pre-deployment survey of sea bottom, and visual inspection of pipelines. In the first mission, the NGS must be able to track a predefined path while avoiding the unplanned occurrence of obstacles. In the second mission, the NGS must track a pipeline by locally reconstructing its location from visual information; also in this case, the unplanned occurrence of obstacles must be handled. Furthermore, the NGS must properly take into account the presence of ocean current and some drastic constraints due to sensor and actuator characteristics. Numerical and hardware-in-the-loop simulations have been developed to verify the effectiveness of the proposed NGS.

Buried object scanning sonar

Abstract: A sonar, designed to scan for objects buried in the seafloor, generates images of pipe and cable sections and ordnance buried in sand. The sonar operates by illuminating a broad swath of the seabed using a line array of acoustic projectors while acoustic backscattering from the illuminated sediment volume is measured with a planar hydrophone array. The line transmitter performs along track beamsteering to improve the SNR of buried target images by illuminating major target surfaces at normal incidence and to reduce volume scattering by limiting the volume of sediments illuminated. The output of the planar-hydrophone array undergoes nearfield focusing which allows the sonar to operate near the seabed where target images have the highest SNR and resolution. The nearfield focusing reduces scattering noise by approximately 12 dB, an improvement measured by comparing the SNR of target echoes in single channel data with the SNR of buried targets in the focused imagery. Plan and side views of the seabed generated from a three-dimensional matrix of focused data provide the position and burial depth of targets covered by sand off Hawaii. An energy detector automatically locates targets in the focused image data.

An interface system for autonomous undersea vehicles

Abstract: Central to the successful operation of an autonomous undersea vehicle (AUV) is the capability to return from a mission, in that there is consistent recovery or docking of the AUV In addition, some missions may require communication with and power transfer to the AUV after docking This paper describes an inductive system that provides a nonintrusive power and communications interface between the dock and the AUV The system makes up to 200 W of AC or DC power available to the AUV The communications interface is 10BaseT Ethernet and is platform- and protocol-independent The overall design of the system is given as well as results from wet laboratory and field tests

Bistatic synthetic aperture target detection and imaging with an AUV

Abstract: The acoustic detection and classification of completely and partially buried objects in the multipath environment of the coastal ocean presents a major challenge to the mine countermeasures (MCM) community. However, the rapidly emerging autonomous underwater vehicle (AUV) technology provides the opportunity of exploring entirely new sonar concepts based on mono-, bi- or multi-static configurations. For example, the medium frequency regime (1-10 kHz) with its bottom penetration advantage may be explored using large synthetic apertures, where acoustic information is accumulated over a series of sonar pings. The performance of such approaches is highly dependent on accurate platform navigation and timing, which poses a significant challenge to AUV developers, particularly because the navigation procedures are themselves dependent on the complicated multipath acoustic environment. Data from the GOATS'98 experiment have been analyzed to investigate the feasibility of combining seabed scattering data from consecutive pings of a fixed parametric source to form a bistatic synthetic aperture for target localization and imaging with an AUV based receiving platform. The paper describes different levels of bistatic processing including both incoherent and coherent beamforming and very large aperture interferometric approaches, and the associated performance tradeoffs are discussed.

Communication over Doppler spread channels. II. Receiver characterization and practical results

Abstract: For pt. I see ibid., vol. 25, no. 1, p. 62-72 (2000). A receiver for coherent communication through underwater communication channels is analyzed. The receiver performance and stability versus delay spread, Doppler spread, and signal-to-noise ratio is quantified. The stability is governed by the ill-conditioning of a correlation matrix estimate and it sets the limit on how many taps should be used for a channel with a given number of degrees of freedom. The receiver is used extensively on both simulated and real data that are Doppler spread, and good performance in these channels is verified.

Underwater acoustic receiver employing direct-sequence spread spectrum and spatial diversity combining for shallow-water multiaccess networking

Abstract: This paper proposes an underwater adaptive-array receiver structure that utilizes direct-sequence code division multiple access and spatial diversity combining in order to achieve reliable low-data rate multiuser communication in an asynchronous shallow-water network. The performance of the proposed receiver architecture has been verified by means of offline processing of data acquired during sea trials in the summer of 1999 in the North Sea. Results show that this computationally efficient structure is near-far resistant and provides successful multiuser operation in the shallow-water channel.

A passive sonar system based on an autonomous underwater vehicle

Abstract: Autonomous underwater vehicles (AUVs) are typically used to "actively" probe the ocean, using sensors that are mounted on a vehicle that moves through the water column. A more challenging task is to use an AUV to position a bottom-mounted stationary platform, such as a passive sonar that requires a large aperture. This paper describes the development of a passive sonar, which was mounted as a payload to the Ocean Explorer II, AUV developed at the Florida Atlantic University. The payload was designed so that the AUV could land on the seafloor and deploy the sonar. The passive sonar developed for this system can be used to measure and localize broadband sources that generate ambient noise in the coastal region. The sonar uses a sparse volume array of only six elements and a 2 m maximum dimension. With this array, it is possible to generate three-dimensional images consisting of thousands of independent pixels for any look direction or chosen image plane. This paper describes the ambient noise sonar and the variable buoyancy system used in deploying the array, and then presents some typical results obtained using the sonar.