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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Page 34 of 71

NOW AVAILABLE The new MultiNet Mammoth: 1 m² opening (Top) Time-of-fight measurements between two nodes. (Bottom) Water temperature probe measurements. time-stamp acoustic signals precisely. These features are required to properly time-stamp collected environmental data and perform acoustic tomography. Precise Timing Acoustic tomography is possible in water because the speed of sound for an acoustic wave in water is approximately proportional to the water temperature. Thus, the precision with which we can measure the speed at which an acoustic signal travels directly relates to the precision to which we can measure the water temperature. The question then is: how precise do we need to be? Using the MacKenzie equation to approximate the speed of sound in water, we can work out the precision attainable by our acoustic tomography system. At a nominal speed of sound of 1,500 meters per second, the travel time between two nodes spaced 50 meters apart is 33 milliseconds. If the water temperature changes by 1o C, the travel time will change by ap- proximately 100 microseconds. A temperature precision of 0.1o C will thus require time-of-fight measurements precise to 10 microseconds. Obtaining such precision is nontrivial, and a naive implementation would use the system clock and timing facilities provided by the operating system running on an underwater sensor node. Such timing techniques are known to suffer large errors (up to milliseconds) due to the nondeterministic nature of interrupts and process scheduling on commodity hardware. To overcome this challenge, we developed a novel self-listening technique that utilizes the simultaneous send and receive capabilities provided by having both transmit and receive hydrophones available. We sample the GPS pulse-per-second signal simultaneously with the signal from the receive hydrophone. Doing so allows us to assign a globally accurate (to 1 microsecond) time stamp to each audio sample recorded by the sound card. Since we loop back the local channel, See us at 2013, OI China ooth C 05 anghai, B Sh July 2013 / st 35

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