Development of an automated acoustic calibration system for fisheries acoustic surveys

Current at-sea calibrations of ship mounted echosounders are based on the method of standard target calibration, which involves deploying one or more standard targets, or calibration spheres, to depths that are in the farfield of the transducers. The spheres are tethered with three lines fed through three downriggers which are mounted on the Port and Starboard sides of the vessel bracketing the centerboard, and controlled manually by an operator. The efficiency of the calibration depends heavily on the experience of the operator, especially when calibration of a hull-mounted multi-beam sonar is involved. To more effectively conduct the acoustic system calibration at sea, an automated calibration system has been built which includes three highly modified commercially available fishing downriggers, with each downrigger containing an integrated mechanical, electrical, and electronics control system. The mechanical component includes the commercial downrigger gear motor, line spool, and custom sensing head (line length in centimeters and line tension in fractions of kilograms). The electrical component consists of forward and reverse power relays, high power pulse width modulation motor speed control, and protection circuitry. The electronics control system consists of a microcontroller based panel with display, manual control buttons, RS-232 interface, and custom software. The microcontroller software allows the remote laptop computer to control the downrigger line movement at variable speeds in both the forward and reverse directions, keeps track of line length in centimeters, and observes line tension. The control software is written in MATLAB, with an easy-to-use Graphic User Interface (GUI). The control and response commands are sent and received from the laptop to the micro-controller via a USB to serial link RS232 port expander. The software can control each of the three downriggers independently to within one centimeter of accuracy. The locations of the spheres - can be determined by the software so that the calibration spheres can be moved to any desired locations in three dimensions within the acoustically insonified volume. The MATLAB control software calculates the desired line movement for each of the downriggers to control the sphere movement horizontally while maintaining proper depth under the transducers of the ship. Line tension is constantly monitored to prevent line fouling, excessive strain on the components, and to detect possible line snagging on the ship´s hull. Such an automated system can: 1) improve the efficiency of the calibration, especially for mapping the beampattern of transducers mounted on the ship´s centerboard; 2) be operated by an inexperienced staff; and 3) control the positions of standard calibration spheres more precisely and with much less fluctuation compared with the conventional manually operated at-sea calibration. This allows for more repeatable measurements and maintains the sphere positions to approximately the same ranges.