Standard target calibration of broad-band active sonar systems in a laboratory tank

High-frequency, scientific active sonar systems are frequently calibrated using standard target spheres. Spot frequency calibration accuracies (using carefully optimized sphere diameters) of 0.1 dB have been previously established as theoretically and technically feasible. The premise of this work is that a tungsten-carbide sphere may be used to calibrate an arbitrary sonar system in a laboratory water tank. In order to achieve an accurate calibration it is necessary to obtain exact knowledge of the characteristics of the immersion medium and the target. With this aim, the temperature and salinity of the water was accurately determined, resulting in an estimate of the sound speed of the water. The density and composition of the spheres were examined using laboratory equipment. Moreover, various other sources of variability have been addressed. For the purposes of this work, seven tungsten carbide spheres of different sizes were used to perform inter-comparison calibration estimates. Measured and modeled target strength curves were compared across the bandwidth of 50 to 150 kHz. Comparative results are presented for all the spheres, with the best-case average precision of 0.22 dB being obtained for the 25 mm sphere.