Acoustic navigation of a large-aperture array

Abstract

Acoustic travel time measurements were used to navigate the elements of a large-aperture (900 m) acoustic array. Array navigation system performance was evaluated during a vertical deployment in the northeast Pacific from the Research Platform FLIP. A network of bottom-moored acoustic transponders were interrogated from FLIP and their 12-kHz replies were detected by receivers at 75-m intervals along the array. A nonlinear least-squares algorithm was used to estimate FLIP and array element positions from the travel time measurements. The FLIP positions derived from this procedure agreed with positions obtained from global positioning system (GPS) satellite navigation to within a 10-m rms error. Navigated positions for FLIP were internally consistent with a 0.5-m mean rms error and standard deviation of 1.1 m, and, for an array element, were consistent with a 2.8-m mean rms error and standard deviation of 0.8 m. The resulting time series of array and FLIP motions were analyzed with respect to wind, tidal, and internal wave forcing functions. Wind and tidal forcing had the greatest influence on FLIP motion, whereas array motion was governed by FLIP movement, tides, and higher frequency sources. Low-frequency array motion, with periods on the order of hours, was a result of FLIP towing the array over a horizontal range of 300 m in response to the wind and the semidiurnal tidal oscillations; the array remained within a 30-m horizontal range of FLIP’s position. Higher frequency array motion had apparent internal wave and surface-coupled components. The array shape was primarily straight and nearly vertical, to within approximately a 2° tilt, responding as a simple pendulum with small displacements.