Sea Technology

JUL 2013

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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about the underwater environment. A large challenge faced when deploying underwater sensor networks is the acoustic communications equipment required. There are no inexpensive, short- to medium-range underwater modems and/or hydrophones available in the market today. Several research projects at universities and their industrial partners aim to change this, but the technology has not become widely available. Another challenge for those that deploy underwater sensor net- works are the algorithms and protocols required to convey data across the network. Since the propagation properties of acoustic signals underwater differ so drastically from radio frequency (RF) in the air, sensor network algorithms and protocols must be redesigned to take into account the unique properties of underwater acoustic communications. Underwater Acoustic Sensor Nodes Sensor network nodes are typically built with low-cost embedded systems. SOUTH BAY CABLES are the uncommon connection for tough jobs around the world. Uncommon in our industry, South Bay Cable has been owned and operated by the same management since 1957! We've designed and manufactured over 60,000 different cables meeting our customers most demanding requirements. Contact the uncommon leader. SOUTH BAY CABLE CORP Idyllwild, CA 92549 USA • Phone: (951) 659-2183 • Fax: (951) 659-3958 sales@southbaycable.com • http://www.southbaycable.com 34 st / July 2013 www.sea-technology.com However, for the deployment discussed in this article, we used a lowcost, small-form-factor PC. In doing so, we were able to rapidly prototype and test our acoustic and signal-processing algorithms. This platform is made up of several off-the-shelf components: a smallform-factor PC 96-kilohertz audio recording and playback capabilities, hydrophones for sending and receiving acoustic signals, and a GPS receiver with GPS pulse-per-second signal. The current cost of such a setup is approximately $500. However, a large portion of that cost is the hydrophone. Unfortunately, this is a limiting factor to wider deployment of underwater sensor networks until hydrophone manufacturers develop and market a sub-$20 hydrophone capable of sending and receiving signals at distances on the order of 1 kilometer. For this deployment, we used Aquarian Audio Products (Anacortes, Washington) hydrophones for receiving and Benthowave Instrument Inc. (Collingwood, Canada) hydrophones for transmit. Using a general purpose PC allows for rapid development and experimentation in underwater signal processing and underwater sensor network algorithms. The CPU can be used for digital signal-like processing, without the diffculty of the digital signal processing (DSP) or feld-programmable gate array (FPGA) development cycle. Once successfully prototyped, these algorithms can be ported to an embedded DSP or FPGA. The built-in sound card provides the ability to collect high-quality acoustic data from the water. If the fnal application requires ultrasonic signals, the system can be prototyped and tested at lower frequencies before developing and testing specialized hardware that supports such frequencies. Our prototype system uses receive and transmit hydrophones, instead of a more typical switched TX/RX (transmitting/receiving) single hydrophone. Doing so has several advantages during research and development deployments. We can loop back the local acoustic signal with the receive hydrophone to monitor the quality and timing of a transmitted signal. Providing a GPS receiver with pulse per second allows the operating system clock on the small-form-factor PC to be synchronized globally with high accuracy, as well as providing a way to

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