Two mechanical rigs for field calibration of multi-beam fishery sonars

Two calibration rigs for controlling the movement of a reference target during a field calibration of multi-beam fishery sonars are described. The first rig was designed to be firmly mounted on the vessel hull and position the reference target inside a single beam, or a few selected sonar beams, with a specified spatial precision. This rig was also used for developing within-beam positioning algorithms, based upon the split-beam principle, using data from individual transducer elements. The size and weight of this rig limited its capacity to calibrate multiple sonar beams. A second rig was therefore designed for swifter movement of the target through multiple beams from each rig-mounting location. The position of the reference target inside each beam was now directly computed from the measured target echo. The rig designs, operation and the experiences of using them, along with comparative performance tests are presented along with some examples of field calibrations.