Showing papers presented at "OCEANS Conference in 1999"

Channel-estimation-based adaptive equalization of underwater acoustic signals

Abstract: To reduce computational complexity of signal processing and improve performance of data detection, receiver structures that are matched to the physical channel characteristics are investigated. A decision-feedback equalizer is designed which relies on an adaptive channel estimator to compute its parameters. The channel estimate is reduced in size by selecting only the significant components, whose delay span is often much shorter than the multipath spread of the channel. This estimate is used to cancel the post-cursor ISI prior to linear equalization. Optimal coefficient selection (sparsing) is performed by truncation in magnitude. The advantages of this approach are reduction in the number of receiver parameters, optimal implementation of sparse feedback, and efficient parallel implementation of adaptive algorithms for the multichannel pre-combiner, the fractionally-spaced channel estimators and the short feedforward equalizer filters. The receiver algorithm is demonstrated using real data transmitted at 10 kbps over 3 km in shallow water.

Direct sequence spread spectrum based modem for under water acoustic communication and channel measurements

Abstract: An underwater acoustic communication system based on direct-sequence spread-spectrum signaling is presented. The method is intended for use in multiple-access underwater communication networks. In this paper, the transmitter and receiver designs are proposed, discussed, and demonstrated using real data. At the receiver, a RAKE filter is employed. The same signaling scheme is also used to obtain an adaptive estimate of the time-varying channel coefficients. The channel estimator performance is demonstrated using simulations and experimental data.

Development and implementation of a low-cost LBL navigation system for an AUV

Abstract: A reliable navigation system is a key factor for the success of an operational mission with an AUV in a real scenario. We address the main issues involved in the implementation of a long baseline (LBL) navigation system for a REMUS AUV. This system replaces both the original hardware and software of the vehicle with a simpler, faster, less expensive and more precise system, based on a Kalman filter. We also discuss the influence of transponder location in the overall performance of the LBL navigation, and present results obtained with this new system in operational missions.

Beamforming and imaging with acoustic lenses in small, high-frequency sonars

Abstract: A high-resolution acoustic imaging system is an important aid in turbid water where optical systems fail. The three sonars described in this paper use acoustic lenses to form near-video-quality images. The first sonar, Limpet Mine Imaging Sonar (LIMIS), is diver-held, forms 64 beams, each with a beamwidth of 0.35/spl deg/ in the horizontal axis by 7/spl deg/ in the vertical axis. This sonar has a 20/spl deg/ field-of-view, operates at 2 MHz, has a practical range of 10 m, and forms between 5 and 12 images/second. The second sonar, Glendora Lake Acoustic Imaging System, (GLACIS), is used to monitor underwater tests and pans and tilts on a platform that can change depth. This sonar forms 64 beams, each with a beamwidth of 0.55/spl deg/ horizontal and 10/spl deg/ vertical. It has a 32/spl deg/ field of view, operates at 750 kHz, and forms 5 or 9 images/second at an operating range of 60 or 30 m, respectively. The third sonar, Acoustic Barnacle Imaging Sonar (ABIS), mounts on an ROV and forms 128 beams, each with a beamwidth of 0.25/spl deg/ horizontal by 10/spl deg/ vertical. This sonar has a 32/spl deg/ field-of-view, operates at 3 MHz, has a range between 1.8 m and 2.4 m, and forms 64 images/second. All three sonars use a set of thin, acoustic lenses made of polymethylpentene to focus sound on a 1-3 composite linear array. The acoustic lenses form beams at the speed of sound with no circuitry and thus eliminate the complexity and power consumption of conventional beamforming electronics. Two disadvantages are (1) the lenses and the spaces between the lenses add volume in front of the transducer array, and (2) multiple reflections between lens surfaces cause internal reverberation. The reverberation inside these sonars is about 40 dB down from the target echoes and scatters to form a slightly brighter background. No range-shifted "ghosts" of target images are seen.

The strength of correlations between geotechnical variables and acoustic classifications

Abstract: Seabed grain size, shear strength, bearing strength, and porosity were measured at 15 sites, all in bays and harbours around southern Vancouver Island. Sounder echoes at 38 and 200 kHz from the same sites were classified using the QTC VIEW/sup TM/ technology. This approach uses a feature set generated from the direct echoes by a set of algorithms and then reduced by multivariate analysis, with similar acoustic responses put into the same class. Canonical correlation analysis was used to uncover correlations between two data sets: the frequency of occurrence of each acoustic class, and the four geotechnical variables. Bearing strength was found to be the major contributor to the first geotechnical canonical variable, which correlated with classes from the 38-kHz echoes with a coefficient of 0.94.

Depth accuracy in seabed mapping with underwater vehicles

Abstract: The HUGIN 11 untethered underwater vehicle (UUV) provides detailed seabed survey services to the offshore industry on a commercial basis. This paper discusses the resulting depth accuracy of the digital terrain model (DTM) that can be achieved with an UUV using standard commercially available multibeam echosounders, navigation sensors, pressure transmitters, CTD sensors, sound velocity sensors and international standards for computation of salinity and density. The results are also applicable for remotely operated vehicles (ROV) and towed fish.

Beyond interferometry, resolving multiple angles-of-arrival in swath bathymetric imaging

Abstract: This paper presents a new swath bathymetric sidescan sonar concept that extends the single angle-of-arrival measurement technique used in interferometric sidescan to multiple angles-of-arrival. The new sonar concept utilizes a small array of vertically stacked sidescan transducer elements and affords high resolution 3D imaging and mapping of underwater geometries difficult to address using simple interferometry. The focus of the paper is the presentation of 3D sector-scan and sidescan results obtained using a prototype six element 300 kHz sonar. Results are presented for both target imaging and bottom mapping experiments.

AUVs-the maturity of the technology

Abstract: The development of autonomous underwater vehicles (AUVs), and their introduction into the military and offshore markets, has been a slow and costly process. The paper provides an overview of those using and developing AUVs, discusses the state of the art, and provides a projection on where the technology is heading and the hurdles it must overcome to reach maturity.

GNOME: NOAA's next-generation spill trajectory model

Abstract: The General NOAA Oil Modeling Environment (GNOME) is a new tool developed by the National Oceanic and Atmospheric Administration (NOAA) for oil-spill response. GNOME generates spill scenarios that support the NOAA standard for trajectory output by providing both "best guess" and "minimum regret" solutions. GNOME also serves as a tool for novice users, from high school on up, to learn more about oil-spill trajectories and actually run them. The GNOME model, regional location files, documentation, and training materials are available on the Internet at http://response.restoration.noaa.gov/software/gnome/gnome.html. Training courses are also offered by NOAA HAZMAT. Research is underway on how to effectively use nowcast/forecast models as sources of ocean current fields for the trajectory model. Questions of data size, time-step interpolation, and relevant physics are being investigated to use these models as data fields in GNOME.

Dynamics simulation of low tension tethers

Abstract: This paper presents the development of a mathematical model and computer simulation of an ROV tether operating in low-tension situations. This model makes use of a lumped mass approach in which the ROV tether is considered to be a system of point masses connected by visco-elastic springs. Using this approach, the formulation of the equations of motion for each of the point masses is explicit. The positions and velocities of the point masses at time t are used to calculate the internal and hydrodynamic forces which are, in turn, used to solve for the accelerations. Direct numerical integration is then used to calculate the positions and velocities at a time t+/spl Delta/t. Although this formulation is stable when the tension in the tether disappears, it is necessary to include bending effects in order to generate realistic results for low-tension maneuvers. The present work accomplishes this by first assuming that all sections of the tether have negligible rotational inertia. As a result, the curvature in the tether at any point can be related to the internal bending forces. This relation is then discretized using the Galerkin method of weighted residuals, to allow calculation of the bending forces at the node points. The mathematical model was implemented in C/C++ and was used to model several tether maneuvers. In a static validation test case in which a constant bending moment was applied at the ends of the discretized tether, the model agreed within 4% with the exact analytical solution. To facilitate a qualitative review of the bending model, the modeled tether was harmonically oscillated at one end. This generated slack, coiled sections of tether which showed a strong tendency to uncoil and straighten the tether.

Multi-sensor fusion for autonomous underwater cable tracking

Abstract: A multi-sensor fusion technique is proposed for autonomous underwater cable tracking. The work presented here is an extension of the vision based tracking system proposed earlier by Balasuriya and Ura (1997, 1998) and Balasuriya (1998). Two practical problems, encountered in vision based tracking are considered and a solution based on a sensor fusion technique is proposed. The two practical problems considered are; 1) situation when the cable is totally invisible in the image; and 2) situation when there are many similar cables appearing in the image. An experiment is conducted by setting an underwater cable with real world situations, especially with the above mentioned cases, and an autonomous underwater vehicle (AUV), the "Twin-Burger 2" is used to track the cable. The experimental results demonstrate that the proposed method is quite stable in handling the practical problems related to vision based tracking systems.

Automatic optical station keeping and navigation of an ROV; sea trial experiments

Abstract: In earlier work, the authors have described the development of a vision system in support of the autonomous operation or operated-assisted missions of AUVS and ROVs near the ocean bottom, S. Negahdaripour et al. (1998). The seafloor images, acquired by a down-look camera installed on the submersible platform, are processed by the vision system in order to detect and estimate its motion in real time. This information is utilized to realize a number of capabilities, including automatic station keeping, navigation and trajectory following, and the construction of a composite (mosaic) image of the seafloor. This paper describes preliminary sea trial experiments to evaluate the performance of the vision system on a Phantom XTL ROV.

HUGIN-use of UUV technology in marine applications

Abstract: The HUGIN untethered underwater vehicle (UUV) system is the result of a highly focused development program to provide a cost-effective tool for the offshore industry for high performance topographical seabed mapping and imaging. HUGIN is presently operated on a regular basis, providing commercial survey services to the off shore and scientific communities. The technology also holds great promise in future resource management and naval operations, being a hydrodynamically stable untethered platform that can carry a multitude of sensor systems and be operated ahead of or completely independent of surface ships. This paper describes the basic features of the HUGIN system and gives examples of practical results obtained from survey operations, maritime science data gathering, and deep water mine hunting research.

A 3-D laser line scanner for outcrop scale studies of seafloor features

Abstract: Manned submersibles and remotely operated vehicles make it possible to use many of the techniques of land geology on the seafloor. A fundamental aspect of geological maps is the documentation of the orientation of various planes and lines in space. Strike and dip typically characterize planes, and trend and plunge characterize lines. Sedimentary bedding, lava flow tops, dike margins, igneous layering, metamorphic foliations, joints and faults, etc. Land geologists determine the orientation of outcrop-scale features with various types of hand-held compasses and inclinometers. However, this type of instrument is not appropriate for use on the seafloor and different approaches are required to obtain orientation data. A 3D laser scanning system currently being developed by the Harbor Branch Oceanographic Institution Engineering Division, under National Science Foundation and Duke University sponsorship, affords a method of collecting orientation data. Similar to previously designed HBOI systems, surfaces of interest are rapidly scanned to produce high-resolution digital maps. The 3D map coordinates combined with the measured roll and pitch angles of the instrument are used to accurately determine orientation of the scanned geologically relevant planes and lines on seafloor outcrops. Unlike other techniques currently in use, this instrument does not need to be carefully positioned or placed on the rock surface and is not affected by magnetic fields. Furthermore, due to the high scan rate, the instrument need not be held stationary while scanning. During a single seafloor traverse of several hours, thousands of measurements can potentially be made.

Lamp Ray: ship hull assessment for value, safety and readiness

Abstract: Lamp Ray, an ROV based underhull inspection system, is a method for in-situ, underwater surveys of the hull condition. Comprising a hull-crawling inspection vehicle, which is integrated with a very precise hull tracking capability and data management system, the Lamp Ray processes quantifiable data on hull plate thickness, coating condition, galvanic potential field and hull form. Because the location of the Lamp Ray inspection sensors on the hull is always known with great accuracy, all survey data can be correlated to an exact plate position, yielding permanent and repeatable information which can be used for planning hull repairs and for long term trend analysis of hull condition. The paper describes the Lamp Ray inspection system and process, taking the reader through system calibration, deployment, hull form mapping and hull condition survey. The data products resulting from the Lamp Ray survey process are described and examples of the report are given. These reports include color visualizations of plate thickness gauging, plate diminution, coating thickness and cathodic potential field. A case study is also included of one Lamp Ray survey, which illustrates the significance of the technique. The Lamp Ray process should be of great interest to ship owners, ship managers, regulatory agencies, underwriters and inspection personnel who are interested in tools for condition-based maintenance and planning which provide cost effective alternatives to drydock and diver deployed survey methods.

Estuarine environment studies with Isurus, a REMUS class AUV

Abstract: The authors address the current developments of the "PISCIS" project. They focus on the experimental results from recent operational missions with an AUV in a estuarine environment. Besides describing mission planning and logistic details, they also present tools for oceanographic data processing and visualization. This is demonstrated with some examples of collected data.

Chirp FSK modem for high reliability communication in shallow water

Abstract: A high reliability shallow water acoustic modem has been developed for communication between AUVs and for general oceanographic use. The modem uses 56 narrowband chirp FM pulses, each centered at a unique frequency located in the range of 20 kHz to 30 kHz. Communication rates vary from 300 bps to 2400 bps, depending on the transmission mode. Packets of information are synchronized using two dedicated chirp pulses, followed by transmission format information. In the auto-baud mode, the modem uses information gathered from previous transmissions for adaptation of baud rate to the acoustic environment. At the lowest rate (300 bps), frequency hopping in combination with a one-of-eight mode of transmission is used to provide maximum SNR and immunity to multipath interference. Performance analysis is presented.

Multibeam sonar image matching for terrain-based underwater navigation

Abstract: This paper presents a terrain-referenced method for positioning an underwater vehicle equipped with a multibeam sonar. The local bathymetric profiles provided by this sensor are matched with a digital elevation map (DEM). The absolute position and heading of the vehicle with respect to this reference map are estimated by correlating a part of this map with the on-line altitude measurements. We propose an extended approach to the classical matching methods that takes into account the non-regular sampling of sonar data in the correlation space. This technique has been successfully applied to real terrain data obtained from the Var underwater canyon (France).

Experimental investigations into high resolution sonar systems

Abstract: Minehunting is performed today more often in shallower waters where smaller mines are more effective and burial more probable. Sonars with increased resolution, including the lower frequencies suited for sediment penetration, will be required to remain effective against this threat. A multinational Joint Research Program, entitled "Mine Detection and Classification", is investigating advanced sonar concepts for Autonomous Underwater Vehicles (AUVs). These will incorporate the most recent advances in sonar imaging technology: wideband sonar, bathymetric imaging, parametric and synthetic aperture sonar (SAS) and combinations of these. These techniques are also of great interest for a variety of commercial applications. The paper presents the experimental facilities, some of which are still under development, as well as the results of joint trials and scope of future work.

Effects of wind variability on scatterometry at low wind speeds

Abstract: Donelan and Pierson [1987] predicted that the microwave cross section of the sea at moderate incidence angles should show a dramatic increase in level at a certain, low value of wind speed, the threshold wind speed. This occurs because the wind speed must be high enough to overcome viscous damping of the short waves that scatter microwaves from the sea surface. Recent measurements in a wind-wave tank have demonstrated the validity of this prediction. Nevertheless, the present author shows that probability distributions of cross sections measured by NSCAT during its 10 month lifetime are not consistent with those expected if such a low-wind-speed threshold were present in the cross sections measured by the spaceborne scatterometer. The author shows that this is to be expected if sufficient variability of the wind exists within the very large footprint of the scatterometer on the sea surface. If this is the case, then the behavior of cross sections at low wind speeds, when taken over the whole data set, are more consistent with the model function that was used to process the NSCAT data. This model function was extrapolated linearly to very low wind speeds on a scale where the log of the cross section is plotted against the log of the wind speed. The author shows, however, that areas of the globe exist where this model function, which has a single form, cannot explain the observations and that a model function with a dependence on the wind variability may be required.

Coastal mooring design: taut elastomeric and chain catenary surface buoy moorings

Abstract: A comparison is made of the behavior of a buoy anchored with an elastomeric tether and a chain catenary mooring at the same water depth and under the same environmental conditions. The results include numerical predictions of the statics and dynamics of the two moorings under identical ocean current and sea state forcing. The at-sea recorded tensions in an elastomeric tension member buoy system are also studied. The results show that elastomeric tethers respond with one third of the dynamic tensions of a chain catenary mooring in severe sea states. The chain moorings also experience considerable frictional wear due to chain links interacting with the sea bottom. A drawback of elastomeric tether moorings is that they can be damaged or cut when entangled with fishing gear.

A reconfigurable mission control system for underwater vehicles

Abstract: This paper describes the mission control software used in the LSTS/FEUP underwater vehicles. This software follows the guidelines of the generalized vehicle architecture, adapts the original idea to encompass the current application requirements and constitutes a first implementation. The work is focused on the design and implementation of an application that can be easily adapted to different vehicle configurations or even to different vehicles. One of the desired goals was to enhance software reusability and to establish a development environment that allows developers with a minimal knowledge of coding details to upgrade the application. To assist this purpose, a CASE tool, which provides modern software development techniques, was used. A simulation environment was also developed whose purpose is to test the applications and to detect possible malfunctions before they occur during mission execution.

High-resolution beamforming for multibeam echo sounders using raw EM3000 data

Abstract: This paper deals with high-resolution beamforming for multibeam echo sounders with a linear receive-array. A fundamental limitation of the conventional beamformer is that the angular resolution depends upon the size of the receive-array. The use of subarray interferometry may increase the mapping resolution for oblique beams, but this method is not valid in the case of multiple scatterers. Therefore, it is interesting to examine how modern high-resolution beamformers work for multibeam echo sounders. Early research for this purpose, based on data simulation, showed promising results. The authors apply high-resolution beamformers to raw sonar data recorded at the receiver elements of Simrad EM3000 multibeam echo sounders. The estimation of spatial covariances plays an important role in high-resolution beamforming. A method to improve the estimation by subtracting noise covariances is applied. Among several beamformers they tested, MUSIC and ESPRIT gave the best results. Compared to the Fourier transform beamformer, high-resolution beamformers clearly improved the resolution. However, the noise level might also increase. It seems that a combined use of conventional and high-resolution beamforming may improve the sonar performance.

Multi input-multi output system identification of AUV systems by neural network

Abstract: Accurate identification of non-linear time variant multi input-multi output systems, especially for AUV systems are essential for implementation of control algorithms and navigation. Nonlinearities, which come from coupling effects between different motion modes of an AUV and also environmental effects, are very complicated and cannot be modeled easily by using conventional dynamics methods. In the paper, by implementation of a neural network identifier (NNI) a general idea for dynamic modeling of any six-degree of freedom rigid body is discussed, and then as an example, the proposed idea is applied to a four degree of freedom AUV system, named Twin Burger 2. This vehicle has four motion modes, called surge, sway, heave, and yaw. It is equipped with an inertial navigation system that can detect state variables of its motion. The NNI is made of two main parts. The first part is made of six independent single degree of freedom neural network identifiers (SDFNNI), which in the case of Twin Burger 2, is four. After training each independent SDFNNI, the second part or main part, named coupled model neural network identifier (CMNNI) is trained. In order to train all of the proposed neural network identifiers, a series of experiments were done and training data packs were collected. From the simulation results it can be seen that the proposed idea is satisfactorily able to identify dynamic behavior of the AUV system under consideration, and also it can be concluded that the proposed method will be useful for system identification of similar systems.

Integration of multibeam bathymetry and sidescan sonar data for geological surveys

Abstract: A geophysical survey conducted off Liverpool, Nova Scotia, Canada in November 1998 collected data using multibeam bathymetric, sidescan sonar, high-resolution subbottom profiler, and acoustic seafloor classification systems. During the survey, multibeam bathymetric and sidescan sonar data were processed on-site and imported into a Geographical Information System for further analysis and display. Shaded relief images derived from the multibeam bathymetric data and sidescan sonar mosaics were combined with data from maps and aerial photographs of the area. These maps and images formed the basis for a preliminary interpretation of geological processes and features on the seabed. Post-processing of the multibeam bathymetric data using newly-developed algorithms improved the resolution of seafloor features and provided acoustic backscatter intensity measurements. These data were used to define the distribution of coarse and fine-grained sediments and seabed features and to compare and contrast various system resolutions. Seafloor samples and photographs were also taken to provide information for the interpretation of the various acoustic data sets. A variety of seafloor features are presented, using both sidescan sonar and multibeam bathymetric data for illustration.

A fully interactive dynamic simulation of a semi-submersible towing a large towfish

Abstract: ISER and DREA are collaborating on the development of SIMRNIS, a fully interactive nonlinear submersible/cable/towfish six degree-of-freedom (DOF) time domain simulation. This capability is not found in standard tow system codes. SIMRMS is a meshing of the DREA Submersible Simulation Program (DSSP) and the DYNTOCABS tow cable/towfish simulator. DSSP is a nonlinear 6 DOF vehicle simulator that models control, propulsion, and ballasting. DYNTOCABS provides a three-dimensional, nonlinear, 3 DOF, finite segment simulation of the cable and includes a nonlinear 6 DOF model of an active towfish. The two programs have been merged so that the equations of motion for all system components are simultaneously integrated in time. New features and capabilities have also been developed. SIMRMS is used as a test bed to minimize technical risk for further development of a remote minehunting system. This paper discusses, and presents full scale sea trials data validating, the program's capabilities.

Quantitative stereo imaging from the Autonomous Benthic Explorer (ABE)

Abstract: The Autonomous Benthic Explorer (ABE) is a vehicle designed to perform long-term autonomous repeatable surveys of the deep ocean. In this paper we examine the basic methodology of obtaining quantitative stereo measurements with ABE. We utilize stereo imagery collected during an autonomous survey along a tectonically active segment of the Juan de Fuca Ridge. The ability to make repeatable measurements is crucial for being able to carry out tasks associated with the detection of spatio-temporal changes. As this is one of the stated goals of ABE we have been developing a stereo camera system to make quantitative measurements from underwater imagery. Pursuant to our goals of long term deployments and the low power levels that such a goal imposes on each vehicle subsystem, our efforts have been focussed on the design of a low-energy stereo camera system. In this paper we describe our system and the methodology for its alignment and calibration in the typical harsh operating conditions associated with working on research platforms at sea. We demonstrate photogrammetric measurements at very fine scales of actual pillow lavas near a diffuse hydrothermal vent. We use a calibrated object to independently ground truth our results. Finally, we discuss improvements to these techniques which we intend to pursue.

Performing ship hull inspections using a remotely operated vehicle

Abstract: The Naval Surface Warfare Center, Carderock Division, NSWCCD, has been using remotely operated vehicles (ROVs) to perform underwater ship hull surveys. These deployments have been in support of research efforts in the following areas: acoustic diagnostics; magnetic signatures; cathodic protection; hull coating inspection; and hull damage assessment. 26 surveys have been performed since 1990 on aircraft carriers, cruisers, destroyers, submarines, swath hulls and piers. This paper focuses on one aspect of underwater hull inspection, the measurement of hull paint thickness. An important part of ship hull maintenance is inspecting the thickness of the underwater hull coatings. This paper describes the issues that were dealt with when such a measurement probe was deployed on US Navy ships in an industrial harbor environment. The paper also addresses the physics behind the paint thickness sensor, and its preparation for underwater use on the ROV. Results from actual deployments are reported to show that this method of data collection is valid and the probe can be used to obtain a paint thickness profile of the hull. This information was subsequently used to assist in planning the drydock maintenance package for a ship.

Successful techniques for supporting multidisciplinary science programs with 'ROPOS'

Abstract: The ROPOS scientific ROV has successfully achieved the multidisciplinary objectives of up to thirty scientists on a single research cruise. These have included fine scale terrain mapping with a scanning downward looking sonar and video; collection of rock samples up to 2000 kg, including drilled cores; video observations of the behavior of animals down to centimetre size; collection of animals from bacteria to crabs, fluid samples; positioning of probes within millimetres, real-time chemical analysis with an on-board scanner; and deployment and recovery of a range of equipment. This success is partly due to versatility and adaptability of the system. ROPOS has two configurations, a 'caged' system for operations up to 5000 m depths, and a 'live-boating' system for shallow waters. The system is supplied with a set of 'core tools' including long-base-line navigation, two manipulators, 3-CCD Video, forward-looking scanning sonar, suction sampler, 'PacMan' grab, sample tray and bio-box. ROPOS readily accepts user-provided tools. Its hydraulic power packs can provide up to eight separate functions for the users' scientific tools. Its scientific telemetry system, independent of the vehicle telemetry, can multiplex up to seven bi-directional RS-232 channels for realime communication with, and control of many instruments. Analog and digital input and output are available through an external junction box. User provided tools have included a hot-fluid sampler, chemical scanner, ODP borehole data link, tubeworm stainer, rock-coring drill, rock-cutting chain saw, and downward looking scanning sonar. Operational procedures developed through experience have also had a big impact on effectiveness. Dive flexibility is maintained by carrying as many tools as possible at all times, which also minimizes changeovers on deck. The long dive times possible with an unmanned vehicle allow both survey and sampling on the same dive. This in turn allows the shipboard scientists to work efficiently.

Performance measurements of a diverse collection of undersea acoustic communications signals

Abstract: A diverse collection of underwater signaling waveforms provided by four acoustic modem developers was tested in the April ModemEx'99 experiment 6 km southwest of San Diego in 200-m water. These waveforms and a variety of probe signals were bidirectionally transmitted between a surface ship and an autonomous, bottom-deployed instrument called the telesonar testbed. The intent of this test was to relate communications performance in a variety of channels to signal design, decoding method, and the channel response. To reduce the number of free parameters, all waveforms were transmitted, received and digitized using a common suite of electronics. Although a complete waveform set could not be transmitted within the channel coherence time, they were all subjected to approximately the same channel geometries and noise. This paper describes the experimental design for ModemEx'99.