An Overview of Next-Generation Underwater Target Detection and Tracking: An Integrated Underwater Architecture <s> I. INTRODUCTION <s> Abstract : Ray equations based on an acoustic Hamiltonian, and formulated by Jones et al. (1986) for spherical coordinates in NOAA's three dimensional ray tracer HARPO, are adapted to ellipsoidal coordinates in the oceanic waveguide. The ensuing modified HARPO is used to model very long range (up to antipodal) acoustic paths in which the difference between geodesics and great circles is measurable. The eventual objective of this modeling is to extract the predictable part of the travel time trend and fluctuations along several long paths that will be used to monitor hypothetical global warming effects. The requirements for easy assimilation and representation of realistic environmental inputs are discussed. These requirements, when coupled with the possibility of classical chaos in the ray paths, dictate a new software architecture. We use the existing software, however, to breadboard and test features of new ray tracers in the global boundary layer, and to support the experimental design of a forthcoming pilot experiment that will use a transmitter located at Heard Island in the Indian Ocean near Antarctica. <s> BIB001 </s> An Overview of Next-Generation Underwater Target Detection and Tracking: An Integrated Underwater Architecture <s> I. INTRODUCTION <s> Underwater sensor nodes will find applications in oceanographic data collection, pollution monitoring, offshore exploration, disaster prevention, assisted navigation and tactical surveillance applications. Moreover, unmanned or autonomous underwater vehicles (UUVs, AUVs), equipped with sensors, will enable the exploration of natural undersea resources and gathering of scientific data in collaborative monitoring missions. Underwater acoustic networking is the enabling technology for these applications. Underwater networks consist of a variable number of sensors and vehicles that are deployed to perform collaborative monitoring tasks over a given area. In this paper, several fundamental key aspects of underwater acoustic communications are investigated. Different architectures for two-dimensional and three-dimensional underwater sensor networks are discussed, and the characteristics of the underwater channel are detailed. The main challenges for the development of efficient networking solutions posed by the underwater environment are detailed and a cross-layer approach to the integration of all communication functionalities is suggested. Furthermore, open research issues are discussed and possible solution approaches are outlined. � 2005 Published by Elsevier B.V. <s> BIB002 </s> An Overview of Next-Generation Underwater Target Detection and Tracking: An Integrated Underwater Architecture <s> I. INTRODUCTION <s> An interesting possibility for improved surveillance capabilities in littoral waters is the integration of multisensor systems by means of data fusion. Here, we describe how data fusion can be used for localisation and tracking of targets by means of passive underwater acoustic and electric field sensors. The data was fused using a Kalman filter. The filter was applied on bearing estimates from the acoustic data and estimates of the target position from the electrode array. The results show the positions of the targets are gained by fusion between the acoustic bearing and the electrode sensors, within the limits of their sensitivity. <s> BIB003 </s> An Overview of Next-Generation Underwater Target Detection and Tracking: An Integrated Underwater Architecture <s> I. INTRODUCTION <s> The performance comparison of target localization for active sonar and effective fusion algorithms for target localization of multistatic sonar is investigated. Active sonar can be categorized into monostatic, bistatic, and multistatic, depending on the number of receiver elements. Target localization performance depends on the system configuration. The target localization performance of the monostatic, bistatic, and multistatic sonar systems is compared assuming that each element can receive both range and azimuth information of the target. In addition, we propose the weighted least square (WLS) algorithm, which incorporates judicial weighting to the conventional least square (LS) method, and an efficient sensor arrangement rule for target localization in the multistatic sonar system. The representative experimental results demonstrate that the target localization performance of multistatic sonar configuration is superior in terms of root-mean-square error (RMSE), to monostatic sonar and bistatic sonar by 35.98% and 37.45%, respectively, while the proposed WLS algorithm showed an improvement of 2.27% compared with the LS method. <s> BIB004 </s> An Overview of Next-Generation Underwater Target Detection and Tracking: An Integrated Underwater Architecture <s> I. INTRODUCTION <s> Using traditional sonar arrays may be difficult and impractical in some mission-critical scenarios, because they should be mounted on or towed by a ship or a submersible. Alternatively, underwater acoustic sensor networks (UW-ASNs) offer a promising solution approach. In this paper, a target-tracking algorithm for UW-ASNs, i.e., 3-D underwater target tracking (3DUT), is presented. The objective of 3DUT is to collaboratively accomplish accurate tracking of underwater targets with minimum energy expenditure. Based on the time of arrival of the echoes from the target after transmitting acoustic pulses from the sensors, the ranges of the nodes to the target are determined, and trilateration is used to obtain the location of the target. The location and the calculated velocity of the target are then exploited to achieve tracking. To realize energy-effective target tracking, 3DUT incorporates a new target-movement-based duty-cycle mechanism. To avoid rapid depletion of the energy resources of boundary nodes due to continuous surveillance, 3DUT employs an adaptive procedure to find, designate, and activate new boundary nodes. Performance evaluation shows that 3DUT is a promising alternative to the traditional sonar-based target-tracking approaches, particularly for on-demand surveillance applications. <s> BIB005 </s> An Overview of Next-Generation Underwater Target Detection and Tracking: An Integrated Underwater Architecture <s> I. INTRODUCTION <s> To deploy successful underwater networks in the face of challenges such as low bandwidth, long propagation delay, half-duplex nature of links, high packet loss and time variability, we require highly optimized network protocols with low overhead and significant cross-layer information sharing. UnetStack is a network stack designed to provide a good balance between separation of concern, and information sharing. By replacing a traditional layered stack architecture by an agent-based architecture, we provide additional flexibility that allows novel protocols to be easily implemented, deployed and tested. In discrete-event simulation mode, UnetStack can be used on desktop/laptop computers or computing clusters to simulate underwater networks and test protocol performance. In realtime simulation mode, it can be used to interactively debug protocol implementations, and test deployment scenarios prior to an experiment. Once tested, the protocols can simply be copied to an underwater modem with UnetStack support, and deployed in the field. The stack implementation has been extensively tested, not only through carefully calibrated simulations, but also in several field experiments. We provide an overview of UnetStack and briefly discuss a few deployments to illustrate some of its key features. <s> BIB006 </s> An Overview of Next-Generation Underwater Target Detection and Tracking: An Integrated Underwater Architecture <s> I. INTRODUCTION <s> The Sapienza University Networking framework for underwater Simulation Emulation and real-life Testing (SUNSET) is a toolkit for the implementation and testing of protocols for underwater sensor networks. SUNSET enables a radical new way of performing experimental research on underwater communications. It allows protocol designers and implementors to easily realize their solutions and to evaluate their performance through simulation, in-lab emulation and trials at sea in a direct and transparent way, and independently of specific underwater hardware platforms. SUNSET provides a complete toolchain of pre-deployment and deployment time tools able to identify risks, malfunctioning and under-performing solutions before incurring the expense of going to sea. Novel underwater systems can therefore be rapidly and easily investigated. Heterogeneous underwater communication technologies from different vendors can be used, allowing the evaluation of the impact of different combinations of hardware and software on the overall system performance. Using SUNSET, underwater devices can be reconfigured and controlled remotely in real time, using acoustic links. This allows the performance investigation of underwater systems under different settings and configurations and significantly reduces the cost and complexity of at-sea trials. This paper describes the architectural concept of SUNSET and presents some exemplary results of its use in the field. The SUNSET framework has been extensively validated during more than fifteen at-sea experimental campaigns in the past four years. Several of these have been conducted jointly with the NATO STO Centre for Maritime Research and Experimentation (CMRE) under a collaboration between the University of Rome and CMRE. <s> BIB007 </s> An Overview of Next-Generation Underwater Target Detection and Tracking: An Integrated Underwater Architecture <s> I. INTRODUCTION <s> Underwater Wireless Sensor Networks (UWSNs) are considered as tangible, low cost solution for underwater surveillance and exploration. Existing acoustic wave-based UWSN systems fail to meet the growing demand for fast data rates required in military operations, oil/gas exploration, and oceanographic data collection. Electromagnetic (EM) wave-based communication systems, on the other hand, have great potential for providing high speed data rates in such scenarios. This paper will discuss the challenges faced in the utilization of EM waves for the design of tactical underwater surveillance systems and evaluate several EM wave-based three-dimensional (3D) UWSN architectures differing in topologies and/or operation principles on the performance of localization and target tracking. To the best of our knowledge, this is the first of its kind in the field of underwater communications where underwater surveillance techniques for EM wave-based high speed UWSNs have been investigated. Thus, this will be a major step towards achieving future high speed UWSNs. <s> BIB008 </s> An Overview of Next-Generation Underwater Target Detection and Tracking: An Integrated Underwater Architecture <s> I. INTRODUCTION <s> There are two different types of primary users (natural acoustic and artificial acoustic), and there is a long propagation delay for acoustic links in underwater cognitive acoustic networks (UCANs). Thus, the selection of a stable route is one of the key design factors for improving overall network stability, thereby reducing end-to-end delay. Software-defined networking (SDN) is a novel approach that improves network intelligence. To this end, we propose a novel SDN-based routing protocol for UCANs in order to find a stable route between source and destination. A main controller is placed in a surface buoy that is responsible for the global view of the network, whereas local controllers are placed in different autonomous underwater vehicles (AUVs) that are responsible for a localized view of the network. The AUVs have fixed trajectories, and sensor nodes within transmission range of the AUVs serve as gateways to relay the gathered information to the controllers. This is an SDN-based underwater communications scheme whereby two nodes can only communicate when they have a consensus about a common idle channel. To evaluate our proposed scheme, we perform extensive simulations and improve network performance in terms of end-to-end delay, delivery ratio, and overhead. <s> BIB009 </s> An Overview of Next-Generation Underwater Target Detection and Tracking: An Integrated Underwater Architecture <s> I. INTRODUCTION <s> A coherent-noncoherent joint processing framework is proposed for active sonar to combine diversity gain and beamforming gain for detection of a small target in shallow water environments. Sonar utilizes widely-spaced arrays to sense environments and illuminate a target of interest from multiple angles. Meanwhile, it exploits spatial diversity for time-reversal focusing to suppress reverberation, mainly strong bottom reverberation. For enhancement of robustness of time-reversal focusing, an adaptive iterative strategy is utilized in the processing framework. A probing signal is firstly transmitted and echoes of a likely target are utilized as steering vectors for the second transmission. With spatial diversity, target bearing and range are estimated using a broadband signal model. Numerical simulations show that the novel sonar outperforms the traditional phased-array sonar due to benefits of spatial diversity. The effectiveness of the proposed framework has been validated by localization of a small target in at-lake experiments. <s> BIB010 </s> An Overview of Next-Generation Underwater Target Detection and Tracking: An Integrated Underwater Architecture <s> I. INTRODUCTION <s> The ocean covers nearly two-thirds of the surface on the Earth, and there has been great interest in developing underwater wireless sensor networks (UWSNs) to help us explore the ocean realm. A great deal of efforts have been devoted to it, and significant progress has been made since the beginning of 2000s. However, most of the networks are isolatedly developed currently, inherently hardware-based and application-oriented with inflexible closed-form architectures, which are difficult to reconfigure, reprogram and evolve. They also lack the capability in sharing resources, and are far from service-oriented networks. These limitations impair their capacity for wide range of applications. To further propel the development of UWSNs, next-generation UWSNs have been proposed recently, which are robust, flexible, adaptive, programmable, support resource-sharing feature and are easy to manage and evolve. Moreover, a number of novel software-defined techniques and paradigms, such as software-defined radio, cognitive acoustic radio, network function virtualization, software-defined networking, Internet of Underwater Things, and sensor-cloud, have been emerging. These software-defined technologies have the capability of softwarizing network resources, and then redefining them to satisfy diverse application requirements, improve resource utilization efficiency and simplify network management. Consequently, these evolving technologies are envisioned as critical building blocks and major driving forces, which will transform conventional UWSNs toward software-based, programmable, user-customizable, and service-oriented next-generation UWSNs. In this paper, we provide a comprehensive review of existing works on implementing these techniques, and also present discussions for future research. We hope to inspire more active research on these areas and take a step further toward realizing next-generation UWSNs. <s> BIB011 </s> An Overview of Next-Generation Underwater Target Detection and Tracking: An Integrated Underwater Architecture <s> I. INTRODUCTION <s> In this paper, we describe an unmanned underwater vehicle (UUV) behavior designed to track nearby vessels using bearing-only measurements obtained from a rigidly mounted planar hydrophone array—one that was originally designed for active sonar use but is repurposed for passive sonar use. Upon detecting a target, a maneuver is executed to resolve the port/starboard bearing ambiguity. The maneuver is heuristically designed with the aim of reducing the effects of end-fire, self-noise during maneuvers, and other noise/spurious measurements on the bearing ambiguity decision. After resolving the bearing ambiguity, the vehicle continuously adjusts its heading to track the target by keeping it broadside to the array. The performance of this behavior, as well as that of the passive sonar itself, is evaluated through field trials in the approaches to Boston Harbor using a Bluefin-21 UUV. In total, 19 successful behavior tests are described where the UUV resolves the bearing ambiguity and tracks either a static source emitting a predefined waveform or the platform noise of a medium-sized catamaran underway. <s> BIB012
The networking in the ocean is an advancing technology that has been drawing abundant attention for the last two decades. A technology that allows communication among different acoustic users dealing with different applications that ranges from the depths of the ocean to the sea surface is called as underwater acoustic network (UAN). Due to the limitations of resources and complexity of ocean environment, widerange of efforts has been dedicated to tackle the problems of communications in underwater networks BIB002 -  . Similarly, the target detection and tracking in the ocean is becoming a critical issue due to increased demand of attaining manageable development of ocean resources. We consider target detection and tracking as a communication problem, since The associate editor coordinating the review of this manuscript and approving it for publication was Fang Yang. several entities need to be involved to make the detection process secure and reliable. Target tracking is one of the important applications of underwater wireless sensor networks (UWSNs). It is a sophisticated process that estimates the state (position, velocity, acceleration) of single or multiple moving targets by conducting the possible measurements that can be available from different types of sensors. Different unmanned underwater vehicles (UUVs) can be used to detect the underwater targets that cannot be detected by fixed sensor nodes. Literature has several sonar-based target tracking algorithms that utilize underwater vehicles (mostly autonomous underwater vehicles (AUVs)) to submerge the equipment into water BIB008 . Sonar systems have the capability of estimation of both spatial and temporal wave fronts however, the most essential mission of sonar arrays is the estimation of direction-of-arrival (DOA), i.e. bearing information BIB003 . Two types of sonar systems are VOLUME 7, 2019 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see http://creativecommons.org/licenses/by/4.0/ available for underwater target tracking: a sonar system that listen the echoes after transmitting the pulses is known as active sonar whereas the one who detects the noise made by others is called passive sonar. Active sonar systems provide both range and bearing information BIB012 . Three different types of active sonar systems are used to detect and track the target: monostatic (both transmitter and receiver are co-located), bistatic (receiver is separated from transmitter), and multi-static (one transmitter and multiple receivers). To estimate the target bearing and range with special diversity, a broadband signal model is used that has been proved in the simulation results of BIB010 . Passive sonar systems only provide bearing information. Solving bearing ambiguity is the most common issue of passive sonar systems. The systems have arrays of linear geometry that causes difficulties in distinguishing the signals; either they are originating from the right or left of the arrays BIB012 . Passive sonar systems are used to analyze the features of UUVs especially AUVs. Due to the advantages of target tracking based on three-dimensional (3D) measurements and sensor management, active sonar systems have received great attention over passive sonars BIB005 , BIB004 . However, due to the advancement in technology, the smart next-generation underwater devices are stealthy which cause difficulty for the defense agencies to predict the sounds or echoes of the target on time. This leads to another reason that passive sonar is not suitable for robust detection in the modern next-generation underwater technology. Sound propagation in the ocean is affected by several acoustic properties of the medium. To have timely measurements, the tracer should be fast and flexible BIB001 . Several ray tracing models have been developed to provide acoustic simulations timely. Though, the accuracy of any ray tracing model depends on the validity and implementation of the ray theory. A few simulators/emulators (UnetStack, DESERT, WOSS, and SUNSET BIB006 - BIB007 ) for underwater communication systems have been designed to conduct experiments and test the validity of different algorithms. Nonetheless, before choosing a simulator/emulator, an accurate emulation of sound propagation and channel model is a significant choice to provide an accurate picture of UAN. Various novel techniques and standards have been proposed recently that play a crucial role in building nextgeneration underwater networks BIB008 , BIB009 , BIB011 . Likewise, terrestrial communication systems, interoperability issues due to several underwater communication approaches based on different proprietary protocols have been aroused in underwater communication systems. Underwater cognitive acoustic network (UCAN) and software-defined underwater network (SDUN) are two emerging technologies that resolve the interoperability issues in underwater communication systems. Moreover, network function virtualization (NFV) along with cloud and fog computing, the components of internet of underwater things (IoUT), are becoming popular to explore numerous underwater applications and resources and to resolve various issues (especially energy consumption and latency) in this challenging domain. The key contributions of this paper are as follows: i A thorough review of unmanned underwater vehicles utilized for target detection and tracking and analysis of their characteristics along with existing issues are presented. ii We discuss several ray tracing models with the purpose of their development, features, and the open challenges for the designer to modify the existing issues in their codes. iii A novel next-generation underwater target detection and tracking technique is proposed to open several doors for research community to excel in this area. The proposed solution considers several acoustic devices at different control layers to integrate SDN, NFV, and fog computing in underwater networks to reduce complexities in the existing infrastructure. The remainder of the paper is organized as follows. An overview of unmanned underwater vehicles used for detecting and tracking the targets is presented in Section II. In Section III, we discuss the ray-tracing models essential in target detection and tracking, while Section IV first gives a brief introduction of each of the novel terminologies discussed in Section I, the literature using these notions to improve underwater communications, and lastly provides the solution for next-generation underwater target detection and tracking. Finally, Section V concludes the paper.