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.

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Underwater acoustic sensor networks: research challenges

This paper explores applications and challenges for underwater sensor networks. We highlight potential applications to off-shore oilfields for seismic monitoring, equipment monitoring, and underwater robotics. We identify research directions in short-range acoustic communications, MAC, time synchronization, and localization protocols for high-latency acoustic networks, long-duration network sleeping, and application-level data scheduling. We describe our preliminary design on short-range acoustic communication hardware, and summarize results of high-latency time synchronization

Research challenges and applications for underwater sensor networking

Environmental Sensor Networks (ESNs) facilitate the study of fundamental processes and the development of hazard response systems. They have evolved from passive logging systems that require manual downloading, into ‘intelligent’ sensor networks that comprise a network of automatic sensor nodes and communications systems which actively communicate their data to a Sensor Network Server (SNS) where these data can be integrated with other environmental datasets. The sensor nodes can be fixed or mobile and range in scale appropriate to the environment being sensed. ESNs range in scale and function and we have reviewed over 50 representative examples. Large Scale Single Function Networks tend to use large single purpose nodes to cover a wide geographical area. Localised Multifunction Sensor Networks typically monitor a small area in more detail, often with wireless adhoc systems. Biosensor Networks use emerging biotechnologies to monitor environmental processes as well as developing proxies for immediate use. In the future, sensor networks will integrate these three elements (Heterogeneous Sensor Networks). The communications system and data storage and integration (cyberinfrastructure) aspects of ESNs are discussed, along with current challenges which need to be addressed. We argue that Environmental Sensor Networks will become a standard research tool for future Earth System and Environmental Science. Not only do they provide a ‘virtual’ connection with the environment, they allow new field and conceptual approaches to the study of environmental processes to be developed. We suggest that although technological advances have facilitated these changes, it is vital that Earth Systems and Environmental Scientists utilise them.

Environmental Sensor Networks: A revolution in Earth System Science?

A video surveillance system extracts video primitives and extracts event occurrences from the video primitives using event discriminators. The system can undertake a response, such as an alarm, based on extracted event occurrences.

Video surveillance system employing video primitives

There has been a growing interest in underwater acoustic communications over the past 30 years because of its defense, offshore oil industry, marine commercial operations, oceanography, and marine research applications. As compared to initial communication systems, improved performance and robustness have resulted from continued research over the years. The authors aim to provide an overview, spanning this decade, of key underwater networking protocol and point-to-point communication technique developments. Insight into some of the open challenges and problems researchers in this field will face in the near future is also provided by the authors.

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Underwater Acoustic Communications and Networking: Recent Advances and Future Challenges

Results are reported from field tests of networked acoustic modems used for wireless real-time delivery of oceanographic measurements from a distributed array of subsurface instruments in coastal waters. The network demonstrated consists of sensor nodes, repeater nodes, gateway nodes, and a shore-based control center. Sensors are oceanographic instruments interfaced with acoustic modems, deployed in trawl-resistant bottom frames with azimuthally omnidirectional acoustic signaling needed for flexible network rerouting. Repeaters are individual acoustic modems to relay data so the array covers a larger area; only these relatively low-cost nodes are suited for deployment unprotected from trawlers. Gateways are buoys with acoustic modems interfaced to cellular telephone modems for communication between the underwater network and the shore. The experiment site is the inner continental shelf off Montauk Point, New York, and Block Island, Rhode Island, with a U.S. Coast Guard navigation buoy equipped as...

Networked Acoustic Modems for Real-Time Data Delivery from Distributed Subsurface Instruments in the Coastal Ocean: Initial System Development and Performance

Acoustic communication is an attractive option for networking distributed assets, such as UUVs, autonomous sensors, and other systems that must talk to one another in the ocean. The ocean is often a complex multipath channel and impressive progress has been made in developing equalization algorithms to overcome this. Unfortunately, many of these algorithms are computationally demanding and not as power-efficient as one would like; in many applications it is better to trade bit rate for longer operational life. For example, a wakeup function is intrinsically low rate but must be low power and highly reliable. We discuss a fundamentally different modulation scheme to address these requirements. An added benefit is that it leads to a receiver that is much less complex than an adaptive equalizer.

https://www.hlsresearch.com/personnel/porter/papers/Oceans/Oceans2001%20PPM.pdf

Passive phase-conjugate signaling using pulse-position modulation

Although work on digital underwater acoustic communications dates back to the early 70s, the mid-/late-1990s have seen a strong renewed interest. The signaling schemes include a variety of noncoherent and coherent schemes including MFSK, DPSK, and QAM and a variety of manufacturers provide such modems 'off-the-shelf'. Meanwhile, the R&D community has demonstrated the impressive potential of the modems as the key connector in undersea networks. While the signaling schemes have become much more sophisticated, the role of the ocean environment on their performance remains poorly understood. As a corollary, predictive models of modem performance are not readily available. The SignalEx tests are designed to address specifically these issues, drawing upon the significant navy experience with acoustic propagation models originally developed largely for ASW applications. A variety of different test sites have been selected with the goal of understanding better how modems respond to multipath and variability induced by a changing ocean. This paper summarizes the lessons learned from SignalEx tests conducted on the Loma Shelf near San Diego and on the New England Shelf near Long Island.

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SignalEx: linking environmental acoustics with the signaling schemes

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.

http://ieeexplore.ieee.org/iel5/6465/17369/00805010.pdf

Performance measurements of a diverse collection of undersea acoustic communications signals

A novel system to use networked acoustic modems for real-time wireless delivery of oceanographic measurements from a distributed array of subsurface instruments in coastal waters is presented. The network consists of sensor nodes, repeater nodes, gateway nodes, and a server. Gateway nodes are buoys equipped with acoustic modems interfaced to Internet-accessible cellular modems for two-way communication between server and subsurface network. Network data flow is remotely re-configurable with multiple paths to the server from each sensor so loss or failure of an individual node can be accommodated. Oceanographic instruments are housed in bottom-mounted frames to limit biofouling and minimize fishing and shipping interference. A trawl-resistant frame enables azimuthally omnidirectional acoustic signaling as required for the network. Gateways serve multiple sensors distributed across wide areas, minimizing the number of instruments with surface exposure. A low-bandwidth data format for acoustic Doppler current profilers is devised to reduce network throughput and power consumption. Gateway equipment is engineered for a Coast Guard buoy. Tests in winter and spring indicate wind limits acoustic communications range by roughening the sea surface. A preliminary network deployment demonstrates gateway control, data delivery across multiple repeaters, and route reconfiguration.

Joseph Rice

Papers

Networked Acoustic Modems for Real-Time Data Delivery from Distributed Subsurface Instruments in the Coastal Ocean: Initial System Development and Performance

Passive phase-conjugate signaling using pulse-position modulation

SignalEx: linking environmental acoustics with the signaling schemes

Performance measurements of a diverse collection of undersea acoustic communications signals

Real-time delivery of subsurface coastal circulation measurements from distributed instruments using networked acoustic modems