A technique has been developed to determine the geographic position and depth of acoustic transponders or releases that are fixed to bottom-moored oceanographic instruments. An interrogation transducer, operating at 10 kHz, is towed by the research vessel. It receives slant range information from the individual transponders, at distinct frequencies from 9 to 12 kHz. The interrogation command unit is controlled remotely by software written in the G programming language (LabVIEW/sup TM/). Slant range to the acoustic transponders and the geographic position of the interrogation transducer are then used in real-time or playback modes to localize the acoustic transponders. Two localization techniques have been developed and compared. The first is an exhaustive grid search that compares the measured slant range with the slant range calculated from the center of each grid cell to the position of the interrogation transducer. The second is a minimization of a function-the summed squared deviation between known interrogator positions and estimated transponder positions using the Levenberg-Marquardt technique. This objective function is approximated by a linear function of the parameters to be determined (geographic position, depth, and sound speed). In both cases, the estimated position of each transponder is revised as each additional slant range measurement is introduced. To improve the depth estimate, slant range data can be filtered and weighted to remove outliers and to emphasize shorter slant ranges (steeper grazing angles). A simulation program has been written to compare different search patterns, in particular, the rate they converge to stable estimates of the position and the error associated with these estimates.
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