NDLI: Acoustic Positioning and Tracking in Portsmouth Harbor, New Hampshire

Content Provider	IEEE Xplore Digital Library
Author	Weirathmueller, M. Weber, T.C. Schmidt, V. McGillicuddy, G. Mayer, L. Huff, L.
Copyright Year	2007
Description	Author affiliation: Univ. of New Hampshire Durham, Durham (Weirathmueller, M.; Weber, T.C.; Schmidt, V.; McGillicuddy, G.; Mayer, L.; Huff, L.)
Abstract	Portsmouth Harbor, New Hampshire, is frequently used as a testing area for multibeam and sidescan sonars, and is the location of numerous ground-truthing studies. Having the ability to accurately position underwater sensors is an important aspect of this type of work. However, underwater positioning in Portsmouth Harbor is challenging. It is relatively shallow, approximately one kilometer wide with depths of less than 25 meters. There is mixing between fresh river water and seawater, which is intensified by high currents and strong tides. This causes a very complicated spatial and temporal sound speed structure. Solutions that use the time-of-arrival of an acoustic pulse to estimate range will require very precise knowledge of the travel paths of the signal in order to separate out issues of multipath arrivals. An alternative solution is to use the phase measurements between closely spaced hydrophones to measure the bearing of an acoustic pinger. By using two bearing measurement devices that are widely separated, the intersection of the two bearings can be used to position the pinger. The advantage of this approach is that the sound speed only needs to be known at the location of the phase measurements. Both time-of-arrival and phase difference systems may encounter difficulties arising from horizontal refraction due to spatially varying sound speed. To ascertain which solution would be optimal in Portsmouth Harbor, the time-of-arrival and phase measurement approaches are being examined individually. Initial field tests have been conducted using a 40 kHz signal to look at bearing accuracy. Using hydrophones that are spaced 2/3 wavelengths apart, the bearing accuracy was found to be 1.25deg for angles up to 20deg from broadside with signal to noise ratios (SNR) greater than 15 dB. The results from the closely spaced hydrophones were used to resolve phase ambiguities, allowing finer bearing measurements to be made between hydrophones spaced 5 wavelengths apart. The fine bearing measurements resulted in a bearing accuracy of 0.3deg for angles up to 20deg from broadside with SNR greater than 15 dB. Field tests planned for summer 2007 will include a more detailed investigation of how the environmental influences affect each of the measurement types including range, signal to noise ratio, currents, and sound speed structure.
Starting Page	1
Ending Page	4
File Size	2883416
Page Count	4
File Format	PDF
ISBN	9780933957350
DOI	10.1109/OCEANS.2007.4449246
Language	English
Publisher	Institute of Electrical and Electronics Engineers, Inc. (IEEE)
Publisher Date	2007-09-29
Publisher Place	Canada
Access Restriction	Subscribed
Rights Holder	MTS
Subject Keyword	Phase measurement Sonar equipment Signal to noise ratio Wavelength measurement Acoustic testing Acoustic measurements Underwater acoustics Underwater tracking Acoustic sensors Rivers
Content Type	Text
Resource Type	Article

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