Sea Technology

JUL 2017

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36 st / July 2017 www.sea-technology.com Calibration, System Performance To ensure consistent measures of the magnitude of back- scatter that are comparable across other fishery acoustic sys- tems, the glider AZFP was calibrated using a 38.1-mm-diam- eter tungsten carbide sphere as a standard acoustic target. During a short mission in December 2016, the glider was mote and autonomous mooring deployments. The resulting low-power and small form factor echosounder allows for extended operations at acoustic frequencies of 38 kHz to 2 MHz. The AZFP is equipped with onboard data storage and software (AZFPLink) that allows for flexible mission control, including pulse length, pulse transmission rate, data-logging range, duty cycle and mission duration limited only by pow- er supply. An AZFP transceiver and control module was adapted for installation in a science bay of a Slocum electric glider man- ufactured by TWR. The module can handle up to four fre- quency channels, though for this initial evaluation a single channel operating at 200 kHz was selected. A custom-built 7°-beam-width transducer was constructed to use an exist- ing downward "ECO Puck" port commonly available in Slo- cum gliders. The single-beam transducer was pitched 22.5° forward to account for angle of attack by the glider during descents through the water column, resulting in downward vertical transducer beam orientation during glider descent. The glider power management system supplies 12 VDC power to the AZFP. The glider science computer uses a sig- nal trigger to initiate the echosounder transmission and log- ging during descents and suspends operations on glider as- cent. Acquisition parameters for the AZFP were set prior to the glider mission, but could be changed during the mission by conveying a parameter set as a text file via satellite com- munication to the AZFP by serial connection. This allows for periodic parameter changes such as pulse length, transmis- sion rate and logging range when operational or environ- mental conditions dictate a need to optimize echosounder functionality. During normal glider surfacings, in addition to glider performance, CTD and optical sensor data, the status and pulse transmission counts of the AZFP are conveyed to provide feedback to the user on AZFP functionality. Due to Iridium satellite bandwidth limitations, raw echosounder data currently must be downloaded from the glider after- ward. (Top and Bottom) Slocum glider on a pole alongside a Simrad EK60 120-kHz split-beam transducer during calibration tests. (Photo Credit: J. Christopher Taylor, NOAA) Glider path along Gulfstream Pipeline in northeast Gulf of Mexico with red circles indicating relative acoustic biomass as area backscatter coeffi- cient. Inset: AZFP echogram showing glider descent, detected seafloor and biological features such as plankton and fish de- tected in the water column.

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