Long-range Ocean Acoustic Propagation Experiment (LOAPEX)

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LOAPEX obtained a bevy of environmental measurements (including the use of an underway CTD, full ocean depth CRDs, XBTs, and the deployment of two autonomous Seagliders. In addition, a suite of measurements was obtained to provide the absolute position and velocity of the acoustic source during transmissions. The transmissions consisted mostly of 20- and 80-minute M-sequences, but CW and 'prescription' FM slides were also used. Totaled, 5,650 minutes of acoustic transmission were completed, not counting the engineering transmissions to ensure the proper source level.

Following the LOAPEX cruise, a significant effort was put forth to determine the four-dimensional location and velocity of the acoustic source during transmission. This is important so that incoherence measured at the various receivers, due to source motion, is not falsely attributed to ocean variability. Although source motion was typically only 5 m during a lengthy transmission, this is still a significant portion of the 20-m wavelength. Utilizing several independent measurements and a finite element cable suspension model, we determined the source locations to approximately 1 m during transmission making the effect negligible. Work has also been done on a Doppler toolkit for NPAL researchers. The product uses the source position and velocity data to construct time-varying Doppler corrections to receivers at arbitrary bearings and ranges.

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