Publications

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2010
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.

2005
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.

2003
Osada, Y, Fujimoto H, Miura S, Sweeney A, Kanazawa T, Nakao S, Sakai SI, Hildebrand JA, Chadwell CD.  2003.  Estimation and correction for the effect of sound velocity variation on GPS/Acoustic seafloor positioning: An experiment off Hawaii Island. Earth Planets and Space. 55:E17-E20. AbstractWebsite

A GPS/Acoustic experiment on the southeastern slope of Hawaii Island presented precise seafloor positioning in the condition of large water depth (2.5-4.5 km) and large velocity variations. We estimated sound velocity variations from acoustic ranging, and found that temperature variation can well explain the velocity variation. The effect of daily variation in the sound velocity amounted to +/- 0.7 m on acoustic ranging of 4-7 km with a fixed velocity structure. CTD data observed about every 3 hours could decrease the range residuals to +/- 0.4 m. These large residuals were fairly well canceled in the positioning of the array center of three acoustic transponders. The estimated precision of the array center positioning was about 3 cm in latitude and longitude.