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Kussat, NH, Chadwell CD, Zimmerman R.  2005.  Absolute positioning of an autonomous underwater vehicle using GPS and acoustic measurements. Ieee Journal of Oceanic Engineering. 30:153-164.   10.1109/joe.2004.835249   AbstractWebsite

Kinematic global positioning system (GPS) positioning and underwater acoustic ranging can combine to locate an autonomous underwater vehicle (AUV) with an accuracy of +/- 30 cm (2-sigma) in the global International Terrestrial Reference Frame 2000 (ITRF2000). An array of three precision transponders, separated by approximately 700 m, was established on the seafloor in 300-m-deep waters off San Diego. Each transponder's horizontal position was determined with an accuracy of +/- 8 cm (2-sigma) by measuring two-way travel times with microsecond resolution between transponders and a shipboard transducer, positioned to +/- 10 cm (2-sigma) in ITRF2000 coordinates with GPS, as the ship circled each seafloor unit. Travel times measured from AUV to ship and from AUV to transponders to ship were differenced and combined with AUV depth from a pressure gauge to estimate ITRF2000 positions of the AUV to +/- 1 m (2-sigma). Simulations show that +/- 30 cm (2-sigma) absolute positioning of the AUV can be realized by replacing the time-difference approach with directly measured two-way travel times between AUV and seafloor transponders. Submeter absolute positioning of underwater vehicle!; in water depths up to several thousand meters is practical. The limiting factor is knowledge of near-surface sound speed which degrades the precision to which transponders can be located in the ITRF2000 frame.

Chadwell, CD, Sweeney AD.  2010.  Acoustic Ray-Trace Equations for Seafloor Geodesy. Marine Geodesy. 33:164-186.   10.1080/01490419.2010.492283   AbstractWebsite

One goal of seafloor geodesy is to measure horizontal deformation of the seafloor with millimeter resolution. A common technique precisely times an acoustic signal propagating between two points to estimate distance and then repeats the measurement over time. The accuracy of the distance estimate depends upon the travel time resolution, sound speed uncertainty, and the degree to which the path computed from propagation equations replicates the actual path traveled by the signal. In this paper, we address the error from ray propagation equations by comparing three approximations to Snell's Law with ellipsoidal geometry.

D'Spain, GL, Terrill E, Chadwell CD, Smith JA, Lynch SD.  2006.  Active control of passive acoustic fields: Passive synthetic aperture/Doppler beamforming with data from an autonomous vehicle. Journal of the Acoustical Society of America. 120:3635-3654.   10.1121/1.2346177   AbstractWebsite

The maneuverability of autonomous underwater vehicles (AUVs) equipped with hull-mounted arrays provides the opportunity to actively modify received acoustic fields to optimize extraction of information. This paper uses ocean acoustic data collected by an AUV-mounted two-dimensional hydrophone array, with overall dimension one-tenth wavelength at 200-500 Hz, to demonstrate aspects of this control through vehicle motion. Source localization is performed using Doppler shifts measured at a set of receiver velocities by both single elements and a physical array. Results show that a source in the presence of a 10-dB higher-level interferer having exactly the same frequency content (as measured by a stationary receiver) is properly localized and that white-noise-constrained adaptive beamforming applied to the physical aperture data in combination with Doppler bearnforming provides greater spatial resolution than physical-aperture-alone bearnforming and significantly lower sidelobes than single element Doppler beamforming. A new broadband beamformer that adjusts for variations in vehicle velocity on a sample by sample basis is demonstrated with data collected during a high-acceleration maneuver. The importance of including the cost of energy expenditure in determining optimal vehicle motion is demonstrated through simulation, further illustrating how the vehicle characteristics are an integral part of the signal/array processing structure. (c) 2006 Acoustical Society of America.

Blum, JA, Chadwell CD, Driscoll N, Zumberge MA.  2010.  Assessing slope stability in the Santa Barbara Basin, California, using seafloor geodesy and CHIRP seismic data. Geophysical Research Letters. 37   10.1029/2010gl043293   AbstractWebsite

Seafloor slope instability in the Santa Barbara Basin, California, poses risk to the region. Two prominent landslides, the Goleta and Gaviota slides, occupy the northern flank, with a scarp-like crack extending east from the headwall of the Gaviota slide towards the Goleta complex. Downslope creep across the crack might indicate an imminent risk of failure. Sub-bottom CHIRP profiles with <1 m accuracy across the crack exhibit no evidence of internal deformation. Daily seafloor acoustic range measurements spanning the crack detected no significant motion above a 99% confidence level of +/- 7 mm/yr over two years of monitoring. These disparate data over different timescales suggest no active creep and that the crack is likely a relict feature that formed concomitantly with the Gaviota slide. Citation: Blum, J. A., C. D. Chadwell, N. Driscoll, and M. A. Zumberge (2010), Assessing slope stability in the Santa Barbara Basin, California, using seafloor geodesy and CHIRP seismic data, Geophys. Res. Lett., 37, L13308, doi: 10.1029/2010GL043293.