While Sentry primarily uses USBL, it  also uses a subset of a long baseline positioning system to determine the slant range to the vehicle. The ship sends a ping at 10.5khz, and Sentry replies with a 11.5khz ping that the ship decodes to display the acoustic signature of the sound in the water. During Sentry deployments, a waterfall display of the return ping is plotted, which gives the watchstander an idea of what the vehicle is doing —  i.e. moving away from the ship, moving toward the ship, descending, ascending, or making a turn in a survey.

While Sentry primarily uses USBL (ultra-short baseline), it also uses a subset of a long baseline positioning system to determine the slant range to the vehicle. The ship sends a ping at 10.5khz, and Sentry replies with a 11.5khz ping that the ship decodes to display the acoustic signature of the sound in the water. During Sentry deployments, a waterfall display of the return ping is plotted, which gives the watchstander an idea of what the vehicle is doing—i.e., moving away from the ship, moving toward the ship, descending, ascending, or making a turn in a survey. Image courtesy of AUV Sentry, Woods Hole Oceanographic Institution.

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