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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fornia and is located directly across the street from a small boat marina. To develop our sensor network acoustic tomography system, we deployed fve nodes across various locations in the marina. The nodes have a secure wireless uplink to ISI, allowing for remote confguration, monitoring and data collection. The nodes are protected from the environment inside the dock boxes found at each slip, while the hydrophones and other sensors are deployed into the water. The internode distances vary from 50 to 200 meters. Acoustic Tomography Experiment During this deployment, the transmit and receive hydrophones, along with a water temperature probe, were placed at a depth of 1 meter. Five chirp signals of length 6,173 bits were binary phase-shift keying modulated with a carrier frequency of 12 kilohertz, giving a modulated length of 500 milliseconds and a spreading gain of approximately 35 decibels. The fve signals were assigned as one per node. Over the course of the deployment of 10 days, the signals were transmitted 10 times at the start of each minute. The raw acoustic data were captured, compressed with a loss-less compression algorithm and then stored. Over the deployment, the data were also transmitted over the wireless uplink and stored on a server. First, we processed the data to evaluate 1D acoustic tomography. This is similar to the tomography performed by inverted echosounders in shallow water. The correlation between calculated time of fight and water temperature is clear. Statistical tests validated our ability to measure precisely changes in time of fight, such as those caused by water temperature fuctuations. Next, we processed the data to perform 2D acoustic tomography. To do so, we calculate the time of fight for all pair-wise node combinations and determine the average water temperature along each path. Given this set of average temperatures we can use a tomographic reconstruction algorithm to reconstruct the 2D temperature feld in the marina. We set a baseline water temperature measurement in the morning of the ffth deployment day. We performed the reconstruction after three hours of heating (caused by the sun), and after six hours of heating, we plotted the relative temperature change. The shallower, eastern end of the marina heats up faster and to a higher overall temperature, while the deeper channel in the center of the marina remains cooler. The results show that we can measure water temperature with high spatial resolution across a small body of water using an underwater sensor network. Conclusion Developing sensor network acoustic tomography is a novel and exciting endeavor that we think will fnd many applications and deployment scenarios in the feld of undersea exploration. In this article we have discussed the system design and deployment of a sensor network that can perform acoustic tomography between the nodes. Using off-the-shelf components and simple yet powerful signal processing, we have shown how this is possible. We deployed our prototype system in an underwater test bed and presented 1D and 2D water temperature measurements. Going forward, we are developing and testing a new data-encoding algorithm that will reduce the length of the chirps. On a remote or deepwater deployment, reducing the length of the chirps will increase power effciency, thus extending battery life. We are also actively seeking partners to deploy our technology at additional sites. n Andrew Goodney is a graduate research assistant at the University of Southern California's Information Sciences Institute. His research interests are sensing and sensor networks, health and wellness technology (mobile), motion picture/VFX/multimedia technology, computer network interconnects and performance, software-defned networking, SAN disk architectures, distributed and cloud computing, database performance and scaling, and computer science/electrical engineering education. BA-SDA14 WiFi remote Sound & GPS Buoy "Listen and receive data in real-time at over 3km distance" GPS positioning & synchronization 4 hydrophone inputs Up to 2TB storage Rechargeable batteries Easy to deploy & to recover Young H. Cho is an electrical engineer/computer scientist at the University of Southern California's (USC) Information Sciences Institute (ISI). In 2008, he started his position as a research scientist at ISI to continue his research in computer networks, wireless sensor networks and other distributed computing systems. He also has a joint appointment as research assistant professor in the Department of Computer Science and the Department of Electrical Engineering at USC. info@rtsys.eu +33 297 898 580 www.sea-technology.com July 2013 / st 37

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