This article deals with the detection of low-frequency noise sources and the estimation of their direction of arrivals using an acoustic vector sensor hosted on a buoyancy Slocum glider. The sensor used is a 3-D directional hydrophone capable of acquiring both the acoustic pressure and the components of the particle velocity vector. This article presents experimental results with data collected at sea and describes the signal processing chain, including detection, direction of arrival, and clustering. The vehicle is equipped with a modem and is supported by an ultrashort baseline (USBL) located on a mother ship or gateway to achieve the navigational accuracy required for correct target localization. The surface vessel localizes the glider via the USBL and then transmits position fixes, which are used by the underwater vehicle to reset the drift error of its navigation filter. The results discussed in this article demonstrate two important achievements, i.e., an underwater glider can produce accurate bearing estimates on a target and the feasibility of the integration of an acoustic modem and a USBL positioning system on a glider. The results obtained open up the possibility of using multiple autonomous sensorized gliders integrated in intelligent surveillance robotic networks.

For the first time at CMRE, an acoustic vector sensor, namely the 3D GeoSpectrum M20-040, has been integrated into one of CMRE Slocum buoyancy gliders. This sensor is able to provide directionality in the band from few Hz to 3 kHz, despite its limited size. The sensor beampattern was first measured in a water tank. The glider has been equipped for the first time also with an acoustic modem, able to communicate with a USBL system deployed from a mother ship or a gateway buoy. Through the USBL, a Command and Control Station can get R/T measurements of the glider position and receive its detection alarms during the underwater missions, without need for the glider to surface. First at-sea tests of navigation, underwater communications and acoustic data collection were conducted during REP18-Med trials (Palmaria Island, Italy, Oct. 2018). Evaluating the smoothness of the glider navigation is fundamental to understand whether a glider is suitable to conduct passive survey with a sensor sensitive to acceleration. The USBL position measurements were compared to the estimate achieved in post-processing by filtering the proprioceptive data collected on-board the glider, corrected with the available GPS fixes. The paper provides a detailed description of the system, along with the discussion of preliminary water-tank and at-sea results.

This paper reports first in-field results of a passive acoustic system installed on an Autonomous Underwater Vehicle (AUV) for tracking low frequency underwater sources in very shallow waters. The problem of passive acoustic detection and Direction-of-Arrival estimation has been approached by using a DIFAR (Directional Frequency Analysis and Recording) vector sensor, which consists of an omnidirectional hydrophone and two dipole sensors oriented orthogonally on the horizontal plane. A mechanical and electrical design study has been conducted, identifying the most appropriate technical solution for the implementation of the DIFAR vector sensor on board of an AUV by minimizing the effects of interference between the vehicle and the payload. Bearing estimation algorithms have been implemented both in time and frequency domain. Tests in real scenarios have been conducted. Results of these measurements show that the bearing estimates are consistent with the reference ground truth. The computationally light methods implemented and the low-cost instrumentation used for data processing are suitable for being used in real-time processing on heterogeneous autonomous vehicles.

This paper presents the experimental activities performed by the NATO STO Centre for Maritime Research and Experimentation (CMRE) during the CommsNet17 trial where a persistent Underwater Acoustic Sensor Network (UASN) was deployed. The CommsNet17 trial was held from the 27 th of November to the 6 th of December in the Gulf of La Spezia (IT), close to the CMRE premises, using the CMRE Littoral Ocean Observatory Network (LOON) as one of its key components. A network consisting of up to eleven nodes was deployed, including static and mobile assets. Various aspects related to persistent UASNs were addressed, including autonomous and distributed network discovery and node configuration, node localisation and navigation, self-adjustment of the network topology in support to the assigned tasks, underwater docking, wireless battery recharging and data offloading. The collected results show that the employed solutions were able to successfully complete all these tasks, thus demonstrating the effective deployment of a persistent, distributed and ad-hoc UASN.

A key to developing autonomous maritime systems for NATO is communication among data gathering platforms below and above the water. However, unlike the mature technologies that are used for terrestrial networks, underwater communications is still in an early stage. This paper provides an overview of CMRE's activities in the field of underwater communications where emphasis has been put in addressing the specific issues that may help accelerating the development of the state-of-the art. For its importance for NATO, interoperability is a key focus area for CMRE. Additionally, experimentation at sea and real-world data collection also play a crucial role as they represent the only viable means for science and technology validation.

The CommsNet 2013 experiment took place in September 2013 in the La Spezia Gulf, North Tyrrhenian Sea. Organized and scientifically led by the NATO S&T Org. Ctr. for Maritime Research and Experimentation (CMRE, formerly NURC), with the participation of several research institutions, the experiment included among its objectives the evaluation of on-board acoustic Ultra-Short Base Line (USBL) systems for navigation and localization of Autonomous Underwater Vehicles (AUVs). The ISME groups of the Universities of Florence and Pisa jointly participated to the experiment with one Typhoon class vehicle. This is a 300 m depth rated AUV with acoustic communication capabilities originally developed by the two groups for archaeological search. The CommsNet 2013 Typhoon, equipped with an acoustic modem/USBL head, navigated within the fixed nodes acoustic network deployed by CMRE. This allows the comparison between inertial navigation, acoustic self-localization and ground truth represented by GPS signals (when the vehicle was at the surface). The preliminary results of the experiment show that the acoustic USBL self-localization is effective, and it has the potential to improve the overall vehicle navigation capabilities.

The interest in underwater communication has grown rapidly in the last few decades, as the ability to deploy assets at sea with increased levels of autonomy naturally led to the problem of getting data to and from them. Despite it's mature research topic status, underwater acoustic communications still faces high barriers when it comes to reaching consensus, not only on the communication processes but also on modelling and validation methodologies under which to perform objective comparison of methodologies. Having access to a persistent unattended infrastructure that enables long term testing of physical and logical underwater communications processes allows researchers to have access to precious real world data without the full cost of a sea trial and with the added bonus of potentially being able to capture variability on the seasonal scale. This paper presents the 2014 version of the Littoral Ocean Observatory Network (LOON): a test bed implemented by the NATO STO Centre for Maritime Research and Experimentation (CMRE), envisioned to foster cooperative development of underwater communications and networking. The data collection infrastructure provides a comprehensive data set of environmental, acoustic and packet measurements relevant to study the the communication processes for the physical and logical layers. This document focuses on the description of the test bed in its 2014 version, the expected benefits and the opportunities for the underwater communications research community.

A line of highly innovative products is presented, which particularly addresses acoustical oceanographic measurements, and includes digital hydrophones and digital acoustic arrays. Major features are very low self-noise, wide dynamic range, wide bandwidth, possibly very high working depth, simultaneous non-acoustic data integration (possible options: CTD data, sound velocity and depth, orientation, GPS time and position, etc.), portability and ease of use, and high cost effectiveness. The systems being digital, the digitizer is very close to the hydrophones, which prevents from any electromagnetic interference and signal weakening along cable, even at high frequency. The systems are particularly suitable for ambient noise measurements. The wide bandwidth from 4 Hz to 90 kHz is particularly appropriate for marine mammal survey. Orientation and positioning data are fundamental for array applications aimed to noise source localization. The integration of acoustic data with data from a CTD or from a sound velocity profiler is particularly innovative, and is of major interest to who needs to have both noise and oceanographic survey simultaneously and in real time, along the same deployment, with significant time saving and better knowledge of the environment.

The chapter describes the development of a payload-carrying capability on the Folaga vehicles (eFolaga). In the eFolaga design, lightweight, small dimensions, low-cost characteristics have all been maintained, as well as high manoeuvrability and hovering capacities. A general methodology to derive lumped parameter models of eFolaga like vehicles has been described, where the identification of the eFolaga buoyancy change and mass displacement actuators has also been reported. By judicious design, it is possible to lift the eFolaga modularity also at the software level, and indeed to have intelligent payloads implementing specific autonomous behaviours setting up a dialogue with the native eFolaga GNC.

The possibilities opened with the increased use of autonomous underwater vehicles and their potential interactions with existing or prospective submerged sensor networks create an end-user technological pull on the communication capabilities for the underwater domain. Simulation models, while fundamental in the scientific and technological development process cannot offer the feature richness of the physical environment and may potentially mask software and hardware behaviours exposed by the real world. This paper presents a testbed implemented by the NATO STO Centre for Maritime Research and Experimentation (CMRE), deployed to foster cooperative development of underwater communications and networking by providing an “hardware-in-the-world” capability to scientists and engineers. The data collection infrastructure provides a comprehensive data set of environmental measurements relevant to underwater acoustic propagation, arbitrary waveform generation within two frequency bands (useful for channel probing and testing of modulation schemes), full band raw acoustic data recording and access to two sets of fundamentally different commercially available acoustic modems. This structured data collection allows for a comprehensive analysis of the environment variables, their impact on the acoustic channel evolution and how this affects end-to-end connectivity of acoustic modems which can be used to steer the design choices for networking protocols.

Detection and tracking of vessels is important in confined areas such as marine parks or harbors. Nowadays, the presence of ships can be accurately monitored either by radar or via AIS system, while small vessels, which have weak radar signature, may be easily missed. The paper presents the detection and localization algorithms optimized for small- and mid-sized boats and based on data either from a single underwater sensor station of four hydrophones, or from data fusion between two hydrophone volumetric arrays. Each platform hosts a sparse tetrahedral array of broadband hydrophones and pan, tilt, compass and depth sensors. Both acoustic and non-acoustic data from the two stations are transferred to shore, where they are stored and processed on a PC. The basis of localization algorithm is the cross-correlation between pairs of hydrophones along time (crosscorrelogram). The wavevector estimation of a vessel from each tetrahedron is achieved through Least Mean Square method. Adequate data association algorithms allow the fusion of estimates obtained from each array in order to provide precise and robust tracking of each vessel. At-sea results demonstrate the system capability for detecting and localizing small vessels in a shallow-water harbor environment. [Work partially funded by EU within ARGOMARINE Project]

This work describes the progress in the developing of a hydrophonic sensors array, based on fiber laser technology, tailored for underwater acoustic surveillance of harbors, naval forces, and, in general, of maritime areas of strategic relevance; the same apparatus can also find application for marine mammals coastline surveying, simply addressing a suitable frequency detection band. The sensors are Distributed Bragg Reflectors Fiber Lasers. The laser active medium is an Er+ doped fiber included between two Bragg mirrors that are photo-imprinted through UV radiation on the fiber. The acoustic water pressure variations produce a longitudinal strain on the fiber laser structure with a consequent modulation of the emission wavelength. An in-fiber un-balanced Michelson interferometer transforms the wavelength modulation into phase modulation, enhancing the detection sensitivity. An acousto-optic modulator, mounted on one arm of the interferometer, generates a frequency carrier to allow conventional demodulation techniques. This apparatus has demonstrated a noise-equivalent level of less than 1 mPa/(Hz)(1/2) in the 0.5-5 kHz frequency band. Experimentations in marine environment of sensor arrays are in progress, and the first results obtained on a couple of sensors written on a same fiber are presented.

Underwater gliders are autonomous vehicles that use small changes in their buoyancy in conjunction with wings to convert vertical motion to horizontal, and thereby propel themselves forward, with very low power consumption, through the ocean for a long period of time. Gliders typically make measurements such as temperature, conductivity (to calculate salinity), currents, chlorophyll fluorescence, optical backscatter and bottom depth. However, such a platform could be a good candidate if properly equipped with an acoustic payload to persistently monitor the underwater acoustic environment. For that reason, NURC and the Technical University of Catalonia (UPC) decided in 2010 to jointly develop a glider acoustic payload that would provide the recording of two hydrophones but also, which is quite unique, provide in addition a real-time detection / classification (DC) capability. The DC capability will allow, while the glider being at-sea, to provide real-time feedback on the acoustic environment the glider is passing by, instead of only providing recording capability for postprocessing work as previously done in the past. The purpose of the paper is to describe the characteristics of the system that has been developed and additionally reports at-sea results from a deep-water WEBB glider operating in the Mediterranean Sea. Those results demonstrate the capability of the developed acoustic payload to detect and classify marine mammals in real-time within the glider. Examples of the noise generated by the glider are also presented.

Passive acoustic monitoring systems that can be deployed for a long period of time (few months) is a needed for the work NURC is doing in the field of marine mammals risk mitigation but also would be very suitable for the port protection project where the deployment of long-term acoustic systems could be considered/required for the detection of threats such as divers and small surface vessels. Fortunately, advances in low-power and high-data-capacity consumer computer technology during the past decade have facilitated the autonomous recording of sounds from marine environment, and in particular for marine mammals over long periods of time. Different systems, with different characteristics, currently exist but are unfortunately not always commercially available and/or not exactly meeting NURC requirements. For that reason, NURC decided, in 2010, to develop its own system, called HYDRA for High Yield Data Recording Array, for enhancing the performance of current available systems in combining and extending the advantages of each of them such as low cost, large bandwidth, high dynamic range, long storage and large autonomy. This paper will give an exhaustive description of the system and compared it to current existing systems. The developed system is capable of simultaneously recording up to 8 acoustic channels, with 24 bits resolution, sampled up to 144 kHz and will have 53 days autonomy and be able to record today up to 2 TBytes of data. The system will be capable of being deployed up to 2000 meters water depth. Complementary to the recording capability, the proposed system has been enhanced by an onboard processing capability that will allow performing embedded real-time detection and classification of acoustic events.

In recent years there is growing concern that the use of mid-frequency (1-10 kHz) tactical sonar may be causing atypical mass stranding of some species of whales. Beaked whales form a family of 21 species of mostly deep diving whales that range from 4 to 13 meters long with a characteristic `beak' similar to dolphins. The beaked whale species common in the Mediterranean Sea, the Cuvier's beaked whale (Ziphius cavirostris), seem uniquely vulnerable to the effects of mid-frequency sonar. These whales make deep dives (nearly 2000 m), typically lasting 20 to 30 minutes but as long as 85 minutes, and together with their minimal surface expression, make visual detection difficult. Although the detailed cause and effect of sonar-related whale strandings are still unknown and subject to intensive research, NURC has been developing a small towed system that not only detects deep-diving whales but also allows for real-time localization. The goal is to detect marine mammals in an area of acoustic experiments that involve high power sonar sources. The Compact Passive Acoustic Monitor (CPAM) lets researchers to monitor the environment with persistent data collection and results reporting. It uses advanced passive acoustic processing technology to capture and analyze very high frequency, short transient signals from marine mammals. A small volumetric array of four hydrophones together with depth and orientation sensors results in true three-dimensional direction finding. In this paper, we describe CPAM and some of the data obtained from engineering tests as well as a recent large-scale marine mammal survey cruise conducted in the Atlantic Ocean.

Detecting a target by measuring its forward scattered field is of interest for harbor surveillance because target strength levels are generally higher in the forward direction than in the backward direction for simple geometries. An acoustic barrier based on forward scattering was demonstrated in a nearly range-independent shallow water environment. The experimental location was characterized by high reverberation, low temporal signal coherence, and, as a result, few stable multipath arrivals due to the fluctuating sea surface. This high-frequency experiment utilized a vertical source array, broadcasting broadside pulses, and a vertical receiver array spanning the water column. The signal of interest was the aberration (in space and time) caused by the acoustic forward scattering field of crossing targets (2-m-long aluminum cylinder, 1-m-diameter steel sphere and pair of scuba tanks). Hence, the spatial and temporal coherence of the recorded acoustic signals was first investigated to assess the stability of the early acoustic arrivals in this rapidly fluctuating coastal environment. A principal component analysis of the stable portion of the recorded acoustic signals was then used to determine the crossing time of the target and to isolate some of its scattered wavefield components.

In the present paper, the authors report the results of the Universal Newborn Hearing Screening (UNHS) project at the University Hospital of Ferrara. A total of 6,759 full-term newborns and a total of 1,016 NICU babies were tested at the University Hospital of Ferrara, from January 2000 to December 2006. The paper presents information from clinically acceptable screening procedures developed and tested during the 6 years of the program and addresses two questions pertinent to hearing screening: (i) the cost-estimate of a UNHS program based on European economical and administration premises and (ii) the development of a database-structure for the evaluation of the UNHS/NHS performance and the individual patient tracking.

The typical 70 mm diameter towed array was developed for blue-water detection at long range and low frequencies in the 1960s. Since then, there has been a need for towed arrays that are lighter and less expensive, especially since the maturing field of autonomous vehicles has expanded the potential of such arrays. The marriage of AUVs and lightweight towed arrays is a natural progression in the development of littoral autonomous sensing networks for applications such Anti-Submarine Warfare, marine mammals, or ambient noise measurements. In August 2007, NURC began to design and build a new thin diameter (31 mm) high-frequency (up to 20 kHz) nested towed array for ASW purposes. An engineering at-sea trial of the array towed by OEX AUV was performed beginning of November 2007. The flow noise level of the array while towed and the potential influence of the AUV self-noise on the acoustic array were also measured. This paper will first describe the array design, its acquisition system and its integration on the OEX AUV. Then, the results obtained from the data analysis are presented. It is shown that the SLITA array has performance that will make it easily fit requirements of the applications previously mentioned.

The increasing problems related to homeland security and harbour/infrastructure protection have increased the level of interest on vector sensors. Market surveys carried out during the last three years gave the conclusion that there is a need for a new generation, small size, and low-cost underwater sensors capable of measuring particle velocity in three dimensional plain within a broad frequency band (2 Hz - 50 kHz) and with high angular resolution. The small size MEMS-based sensors developed by Microflown Technologies BV Inc are the world's only commercially available transducers that are capable of measuring the particle velocity, instead of pressure, in air. The development of a new generation, innovative and low-cost underwater sensors and technologies based on that in-air nano technology is therefore considered. This technology has a great potential to become a revolutionary underwater acoustic sensor using nano-technology, capable of finding many applications like sensors for Autonomous Underwater Vehicles, sensors for directional receivers for underwater acoustic systems, Floating autonomous systems, Sensors for seismic towed arrays for underwater oil and mineral prospecting and harbour and water-side infrastructure protection. This paper describes how Microflown technology can be adapted to underwater applications.