NATO Undersea Research Centre Research Articles

Quantifying the effect of random roughness on synthetic aperture sonar image statistics

A perturbation-theory-based model has been developed to predict the effect of random seafloor roughness on synthetic aperture sonar (SAS) image statistics. The continuous variation in scattering strength produced by a random slope field is treated as an intensity scaling on image speckle produced by the SAS imaging process. Changes in image statistics caused by roughness are quantified in terms of the scintillation index (SI). Factors influencing the SI include slope variance, geo-acoustic properties of the seafloor, the probability density function describing the speckle and the signal-to-noise ratio. Example model-data comparisons will be shown for SAS images taken off the coast of Tellaro, Italy, by the NATO Undersea Research Centre, La Spezia, Italy (now the NATO Centre for Maritime Research and Experimentation) using the 300 kHz MUSCLE SAS system, and for data collected with the Norwegian Defence Research Establishment’s 100 kHz HISAS system off the coast of northwest coast of Elba Island, Italy, in ...

Beaked whale acoustic versus visual detection

Because of their sensitivity to anthropogenic noise, research on beaked whale habitat is particularly important. During 2010 and 2011, NATO Undersea Research Centre (NURC) conducted sea trials dedicated to marine mammals, in areas of potential beaked whale habitat. The first one took place in North Eastern Atlantic Ocean, Southwest of Portugal, and the second one took place in the Gulf of Genoa, Mediterranean Sea. For both trials: weather conditions allowing, during daylight there were two teams of visual observers, working in two shifts, scanning the horizon and taking note of marine mammal encounters. Acoustic data were collected with the CPAM (Compact Passive Acoustic Monitoring), designed by NURC. The total usable bandwidth is up to 80 kHz. The CPAM was deployed at a depth between 100 and 200 m for about 20 h a day Beaked whale detection obtained by visual observers and by passive acoustic are analyzed. The number of detections and the information obtained by each method are compared. The advantages and drawbacks are highlighted.

Analysis of some special buzz clicks

Toothed whales are known to click regularly to find prey. Once a prey has been detected, the repetition rate of the clicks increases; these sequences are called buzzes. Previous work shows that the buzz clicks spectrum slowly varies from click to click for various species. This spectrum similarity allows buzz clicks association as a sequence using multi-hypothesis tracking (MHT) algorithms. Thus buzz classification follows automatic click tracking. The use of MHT reveals that in some rare cases a variant of this property has been found, whereby sub-sequences of clicks exhibit slowly varying characteristics. In 2010 and 2011, NATO Undersea Research Centre (NURC, now CMRE Centre for Maritime Research and Experimentation) conducted sea-trials with the CPAM (compact Passive Acoustic Monitoring), a volumetric towed array comprised of four or six hydrophones. This configuration allows for a rough estimate of clicking animal localization. Some buzzes with sub-sequences of slowly varying characteristics were recorded with the CPAM. Localization may help to understand this new finding from a physiological point of view. The results of this analysis will be presented.

Comparisons of beaked whale signals recorded in eastern Atlantic to known beaked whales signals

Beaked whales are a group of more than 20 genetically confirmed species; they are very elusive and were among the least known species until a few years ago. Because of their sensitivity to sonar, an increased research effort dedicated to these species and in particular to their signals started 10 years ago. Despite this effort, the signals of many beaked whale species remain unknown. Signals of Blainville’s (Mesoplodon densirostris), Cuvier’s (Ziphius cavirostris), Gervais’ (Mesoplodon europaeus), and Northern Bottlenose Whales (Hyperoodon ampullatus) have been studied and described in different articles. The signals of three unknown species (different from the above mentioned species signals) have been reported. All seven species produce upsweep frequency modulated signals which seem species specific. In 2010, NATO Undersea Research Centre (NURC) conducted a sea-trial in Eastern Atlantic Ocean, Southwest of Portugal. Three different types of beaked whale signals were recorded. The characteristics of these signals will be presented and compared to the known signals of beaked whales. The conclusions of this analysis will be presented.

Improving Sonar Performance in Shallow Water Using Adaptive Beamforming

Multipath propagation degrades the performance of active, bottom-imaging sonars in shallow-water environments. One way to avoid multipath interference is to use a vertical array with a narrow enough angular response to separate the direct bottom return from the multipath. However, this requires a large array and is often infeasible for practical reasons. In this study, we focus on the use of adaptive beamforming on the receiver side to reduce multipath interference and hence improve the signal-to-noise ratio (SNR). Using a small, dense receiver array, we apply classical and adaptive beamformers to real data collected by the NATO Undersea Research Centre in a shallow-water environment. Our results show that the adaptive minimum variance distortionless response (MVDR) beamformer offers an improvement in the estimated SNR compared to a conventional beamformer in most cases. However, the MVDR beamformer is suboptimal when the receiver consists of only a few elements. We propose using the low complexity adaptive (LCA) beamformer, which is based on the same optimization criteria as the MVDR beamformer, but is robust in a coherent environment without the need for spatial smoothing. For two to 4-element receivers, we observe an improvement of about 0.5-2.5 dB in the estimated SNR when using the LCA beamformer. In cases where the model indicates that the direct bottom return and the dominating multipath arrive from nearly the same angle, little or no improvement is observed. This is typically the case for first- or second-order multipaths reflected off the seafloor toward the receiver. The results from this study also show that with a small vertical array, a narrow mainlobe width is more important than low sidelobe levels, in terms of maximizing the SNR. Consequently, an unweighted conventional beamformer performs better than a conventional beamformer with a Hanning window applied for sidelobe suppression.

Distributed Mobile Sensor Networks for Hazardous Applications

1Research Department for Underwater Acoustics and Marine Geophysics, Bundeswehr Technical Centre for Ships and Naval Weapons, Naval Technology and Research (WTD 71), Klausdorfer Weg 2, 24148 Kiel, Germany 2Navy Center for Applied Research in Artificial Intelligence, Naval Research Laboratory, 4555 Overlook Avenue S.W., Washington, DC 20375, USA 3Acoustic Research Laboratory, National University of Singapore, 21 Lower Kent Ridge Road, Singapore 119077 4 Systems Technology Department, NATO Undersea Research Centre (NURC), Viale S. Bartolomeo 400, 19126 La Spezia, Italy

Adaptive Multiview Target Classification in Synthetic Aperture Sonar Images Using a Partially Observable Markov Decision Process

The problem of classifying targets in sonar images from multiple views is modeled as a partially observable Markov decision process (POMDP). This model allows one to adaptively determine which additional views of an object would be most beneficial in reducing the classification uncertainty. Acquiring these additional views is made possible by employing an autonomous underwater vehicle (AUV) equipped with a side-looking imaging sonar. The components of the multiview target classification POMDP are specified. The observation model for a target is specified by the degree of similarity between the image under consideration and a number of precomputed templates. The POMDP is validated using real synthetic aperture sonar (SAS) data gathered during experiments at sea carried out by the NATO Undersea Research Centre, and results show that the accuracy of the proposed method outperforms an approach using a number of predetermined view aspects. The approach provides an elegant way to fully exploit multiview information and AUV maneuverability in a methodical manner.

On Van Holliday's shoulders

The year was 1992. The Cold War was over, and funding for resolving deep water issues was fading. I was at the NATO Undersea Research Centre, simultaneously searching for a new research challenge that I could sink my teeth into, and formulating a research program for the Centre, which would be appropriate for the post-Cold War era. I stumbled onto Van's classic paper on resonance scattering by fish, discussed it with him by telephone, and within an hour became a convert to marine bioacoustics. I became familiar with David Weston's work, discussed it with him, and soon thereafter started my research in bioacoustics. Van and David made important contributions to the design of my bioacoustics experiments. After I returned to the USA, I often met with Van to discuss a broad spectrum of fundamental and applied topics in bioacoustics. He was always generous with his time, knowledge, and insights. His explanations of seemingly complex phenomena were always readily understandable, concise, and precise. Standing on Van Holliday's shoulders, greatly expanded my vision of what is possible through technical innovation, interdisciplinary collaboration, dedication, and perseverance. I, along with many others, miss and will continue to miss this amazing and unique individual.

Geoacoustic inversion in shallow water using broadband synthetic aperture and single hydrophone acoustic data

A majority of geoacoustic inversion experiments sample the acoustic field on long arrays. This paper uses a moving single hydrophone to create a large synthetic aperture array for geoacoustic inversion. Practically, this is operationally attractive compared to using long arrays. For example, one possible application is the use of autonomous underwater vehicles (AUVs) to perform acoustic field sampling and pre-processing for geoacoustic inversion. The approach comprises synthetic aperture formation and a directed Monte Carlo Bayesian broadband frequency coherent geoacoustic inversion. This is demonstrated with simulated and real data from the MAPEX2000 experiment conducted by the NATO Undersea Research Center, using only one hydrophone of a towed array and a moored source in the Mediterranean Sea. The method yielded similar results compared to an equivalent physical array.

Interfaces between acoustic prediction, embedded signal processing, and behaviors at NATO Undersea Research Centre

The use of acoustic sensing systems for ASW in heterogeneous sensor networks utilizing marine robots has been a subject of research at the NATO Undersea Research Centre for the past several years. In this talk, we discuss the unique challenges of implementing ASW on autonomous, collaborative networks of AUVs, including the challenges of embedding the active sonar signal processing, implementing effective underwater messaging, and designing adaptive behaviors to optimize system performance. Theoretical studies, simulations, and results from the recent GLINT series of sea trials are shown and the way forward for autonomous sensor system studies at NURC is discussed.

Real-time sonar signal processing on-board an autonomous underwater vehicle

An active, bistatic signal processing system has been implemented for use on-board autonomous underwater vehicles (AUVs) using towed arrays. The NATO Undersea Research Centre's (NURC) AUVs are programmed to maneuver in order to best track a target. To perform this action autonomously, the vehicle must be able to fully process and track targets via its towed array data in real time or faster. The processor implemented includes processing from the array hydrophone data, through beamforming, matched filtering, Doppler processing, and tracking. This system adapts many previously developed algorithms to function in real time. Research and algorithm adaptations in the areas of normalization, CFAR detection, and array navigation, have also been developed to deal with practical issues which have arisen in sea trials. The implementation, practical issues, and steps taken to address them will be presented. This research is supported by the NURC Consolidated Programme of Work, Cooperative ASW program.

Decision support using the Multistatic Tactical Planning Aid (MSTPA)

The Multistatic Tactical Planning Aid (MSTPA) is a tool currently in development at NATO Undersea Research Centre which may be used to model the performance of a given multistatic sensor network in terms of the probability of detection of a submarine, the ability to hold a track and whether such a track could be correctly classified as such. The tool therefore considers the entire chain of events from an initial calculation of signal excess, the generation of a contact considering localisation errors, followed by the subsequent tracking and classification process. In its current form, the tool may be used to plan a particular multistatic scenario through operational analysis of many Monte Carlo simulations. The future development of MSTPA will transition towards a real-time decision support tool to assist operators and planners at sea. This study introduces a number of generic decision support techniques which may be wrapped around the MSTPA tool. The acoustic performance metric that will drive decisions will of course be subject to uncertainty relating to environmental measurements and extrapolations. The effect of this uncertainty on acoustic performance is examined here. Future studies will consider the sensitivity of the eventual decision—in terms of optimum sensor positions—to the acoustic uncertainty.

Dynamics of the circulation in the Sea of Marmara: numerical modeling experiments and observations from the Turkish straits system experiment

During September 2008 and February 2009, the NR/V Alliance extensively sampled the waters of the Sea of Marmara within the framework of the Turkish Straits System (TSS) experiment coordinated by the NATO Undersea Research Centre. The observational effort provided an opportunity to set up realistic numerical experiments for modeling the observed variability of the Marmara Sea upper layer circulation at mesoscale resolution over the entire basin during the trial period, complementing relevant features and forcing factors revealed by numerical model results with information acquired from in situ and remote sensing datasets. Numerical model solutions from realistic runs using the Regional Ocean Modeling System (ROMS) produce a general circulation in the Sea of Marmara that is consistent with previous knowledge of the circulation drawn from past hydrographic measurements, with a westward meandering current associated with a recurrent large anticyclone. Additional idealized numerical experiments illuminate the role various dynamics play in determining the Sea of Marmara circulation and pycnocline structure. Both the wind curl and the strait flows are found to strongly influence the strength and location of the main mesoscale features. Large displacements of the pycnocline depth were observed during the sea trials. These displacements can be interpreted as storm-driven upwelling/downwelling dynamics associated with northeasterly winds; however, lateral advection associated with flow from the Straits also played a role in some displacements.

Lightweight Autonomous Underwater Vehicles (AUVs) performing coastal survey operations in REP 10A

Lightweight Autonomous Underwater Vehicles (AUVs) were developed for Naval Special Warfare (NSW) Group 4 search and survey missions from a commercial AUV baseline (Iver 2) through integration of commercial off-the-shelf (COTS) hardware components, and through software development for enhanced on-board Command and Control functions. The development period was 1 year under a project sponsored by the Office of Naval Research TechSolutions Program Office. Hardware integration was completed by the commercial AUV vendor, OceanServer Technology, Inc., and software development was conducted by the Naval Undersea Warfare Center, Naval Oceanographic Office, and U MASS Dartmouth, with support from hardware and software application providers (YSI, Inc., Imagenex Technology Corp., and CARIS). At the conclusion of the integration and development period, an at-sea performance evaluation was scheduled for the Lightweight NSW AUVs with NSWG-4 personnel. The venue for this evaluation was the NATO exercise Recognized Environmental Picture 10A (REP 10A), hosted by Marinha Portuguesa, and coordinated by the Faculdade de Engenharia–Universidade do Porto. REP 10A offered an opportunity to evaluate the performance of the new AUVs and to explore the Concept of Operations (CONOPS) for employing them in military survey operations in shallow coastal waters. Shore- and ship-launched scenarios with launch/recovery by a single operator in a one-to-many coordinated survey, on-scene data product generation and visualization, data push to Reach Back Cells for product integration and enhancement, and survey optimization to streamline survey effort and timelines were included in the CONOPS review. Opportunities to explore employment of hybrid AUV fleets in Combined Force scenarios were also utilized. The Naval Undersea Warfare Center, Marinha Portuguesa, the Faculdade de Engenharia–Universidade do Porto, and OceanServer Technology, Inc., were the primary participants bringing in-water resources to REP 10A. Technical support and products were provided by the Naval Research Laboratory–Stennis Space Center, Naval Oceanographic Office, NATO Undersea Research Centre, University of Massachusetts–Dartmouth, and YSI, Inc. REP 10A proved to be a very effective exercise in meeting each of the critical goals. Results of the performance evaluation guided final development and Independent Verification and Validation (IV&V) for the Lightweight NSW AUV, leading to on-time, successful Factory Acceptance Testing and delivery of the three contracted vehicles to NSWG-4 in September, 2010.

Environmental knowledge and operational effectiveness at the NATO Undersea Research Centre.

One of the primary areas of research at the NATO Undersea Research Centre (NURC) is environmental knowledge and operational effectiveness. The objective of this work is to provide environmental information in support of maritime operations from the planning stages through to real‐time decision making. Generically, this is accomplished by parametrizing the environment according to need and providing the best knowledge available for each parameter including uncertainty. In practice, the environment is measured using a variety of mobile and remote sensors, including AUVs, with the aim of improving high‐resolution models of the oceanographic environment. In turn, these models and measurements are used as input for operational planning and prediction tools. This talk will describe recent and upcoming experimental work at the NURC designed to test these concepts and their application to acoustic based planning and prediction tools. The goal is to demonstrate the use of these concepts to improve prediction capabilities for transmission loss and signal excess in an operational area.

Lessons learned from time reversal experiments: Applications to target detection.

Over a decade beginning in 1996 we carried out a series of time reversal experiments jointly with the NATO Undersea Research Centre in coastal water at various frequencies (450 Hz, 850 Hz, 3.5 kHz, and 15 kHz). These experiments and follow‐up analysis suggested potential applications to active sonar. Time reversal focuses acoustic energy on a target enhancing the target echo while minimizing the reverberation from the boundaries below and above the focus, thereby resulting in echo‐to‐reverberation enhancement. These ideas have been confirmed under limited circumstances in our shallow‐water acoustic experiments. This talk will discuss acoustic and signal processing lessons learned from these experiments including an acoustic barrier concept and both active and passive reverberation nulling.

Measurements and interpretation of mesoscale seabed spatial variability.

Acoustic propagation, reverberation, and clutter may be controlled by sediment spatial variability in some environments. Many of the geoacoustic measurement approaches currently employed in the ocean acoustics community provide measurements of sediment properties at either fine scales, less than O(101) m (e.g., probes and cores) or at large scales spatially averaged over lateral scales greater than O(10III) m. Relatively little is understood about the mesoscale, defined here as scales from O(10I–103) m. Recent wide‐angle reflection measurements resolve lateral spatial variability down to a few meters over track lengths of order 10 km and provide early observations of sediment lateral variability at the mesoscale. [Work sponsored by the Office of Naval Research Ocean Acoustics Program and the NATO Undersea Research Centre.]

Passive acoustic monitoring of cetacean at Josephine Seamount, Portugal.

In the past years, the development of a variety of passive acoustics recorders has provided a unique way of acquiring information about marine mammal species in remote regions of the ocean. During the NATO Undersea Research Centre Sirena 10 cruise an ecological acoustic recorder (EAR) was deployed to monitor the presence of marine mammal. The EAR was deployed on May 13, 2010 at a depth of 944 m in the vicinity of Josephine seamount (37 deg; 02.087’N, 013¢ª 51.733W, south west Portugal) and programed for a recording time of 40 s every 2 min. For the detection and classification of echolocation clicks we integrated the visual analysis of spectrograms with the analysis performed by an automatic detector/classification system called M3R, developed by the Naval Undersea Warfare Center Division. M3R has been successfully operated for the analysis of recording from different US Navy ranges. Cetaceans detected include beaked whales, sperm whales, pilot whales, risso’s dolphins, and dolphins. Hourly and daily distributions of signals were calculated. Beaked whales have been regularly detected everyday and the echolocation activity has been found to be higher at night. This suggests that Josephine seamount represents a good habitat for these deep diving cetaceans. [NATO Undersea Res. Ctr. funded and operated the sea trial Sirena 10.]

A Decision Support Architecture for Maritime Operations Exploiting Multiple METOC Centres and Uncertainty

Maritime operations are affected significantly by meteorological and oceanographic (METOC) conditions. The availability of multiple METOC centres able to deliver more accurate forecasts in the near future (2/3 days) is a promising resource toward better operational planning. However, how a specific maritime operation can benefit from such forecasts is not straightforward. A decision support architecture is required to combine METOC forecasts and human knowledge and select the best action from a set of pre-defined actions for the maritime operation. This paper describes a decision support architecture developed at the NATO Undersea Research Centre that is an improvement over a previous version in two ways: i) it exploits forecasts coming from more than one METOC centre, and ii) it exploits uncertainty associated with METOC forecasts. The former allows for the exploitation of different abilities of METOC centres at different conditions. The latter allows for the propagation of input uncertainty on output products (the risks related to each action), thus allowing operators to assess if risks related to different actions are statistically different. Both features increase the robustness of the previous decision support architecture. The effectiveness of the new architecture is demonstrated on an underwater glider surfacing experiment carried out on data collected during a cruise in the Adriatic Sea in 2006.

Volumetric estimation of thermal fields inferred from glider‐like and remote‐sensing measurements in undersampled coastal regions

A method to estimate volumetric oceanographic conditions in a given coastal area merging information from remote sensing and gliders is investigated. The method relies on the application of a linear model that makes use of a degenerated 3‐D covariance model inferred from in situ and remote‐sensing measurements and takes into account the functional nature of glider data. The optimum weighting function results from a Fredholm integral equation of the first kind. A centroid collocation method with piecewise constant bases and a specific class of stabilizing functional are used to numerically solve the problem. The investigated methodology was validated during the oceanographic cruise TSS08 conducted by the NATO Undersea Research Center in September 2008 in a region of the Black Sea, offshore the Bosphorus Strait. Data sampled with a towed vehicle emulating a glider were merged with remote‐sensing information to estimate the 3‐D temperature field at unobserved locations. A set of the observations was only considered for validation. Estimations from the functional model are confronted with those obtained from a discrete approach. Results indicate a better performance of the functional model and raise the question of whether functional techniques are more appropriate than algebraic ones to analyze high‐resolution data obtained from new robotic platforms.