Calibrating multibeam, wideband sonar with reference targets

The new multi-beam sonar, Simrad MS70, can transmit over the frequency band 70 - 120 kHz, and calibration of each individual beam becomes slightly more difficult than for conventional split beam systems. Two new tungsten carbide spheres have been specially designed and manufactured to enable an accurate calibration over the specified frequency band. They are also heavy enough to stabilize the calibration rig, and have sufficiently high target strength for discrimination against biological targets within the measurement volume during sea calibrations. Calibration results from the first trials of the new spheres will be shown. This includes tests made with the sonar itself to verify that the backscattering of the spheres was correct and according to the numerical computations. A procedure for standard calibration of the MS70 multibeam system is suggested and tested. Additionally, results from detailed inspections of individual beams in the 500-beam array of the MS70 sonar will be shown and discussed.