Optimizing Two Targets for Calibrating a Broadband Multibeam Sonar

The new Simrad MS70 multibeam sonar has a nominal total bandwidth of 70-120 kHz. Scientific applications of this, as in fisheries and other ecological research, require calibration. The standard-target method is suitable. Design criteria for the standard target are that this be robust and dense, with a mass of order several kilograms and with target strength in the nominal range from -35 to -30 dB. Candidate targets necessarily contain resonances in their backscattering spectra. Two targets are therefore proposed to cover the total band in piecewise fashion. For the selected material of tungsten carbide with 6% cobalt binder, the optimized sphere diameters are 75 and 84 mm. Detailed computations of the respective target strength spectra and sensitivity to variations in medium temperature are given with respect to the reference ambient temperature 8 deg C and salinity 33 ppt