Low Frequency Tank Calibration by Comparison

In order to check the performance of Low Frequency Sonar Systems (1 kHz and below) there is a need for Calibrated Sources that can be arranged at different distances and in different environments. For that purpose Saab has developed a Signature Generation Module that fits in an AUV, and makes these performance checks easy to perform with low personal effort. In order to calibrate the source itself, it would be feasible to use a medium-sized test tank, but because of the small dimensions in relation to the wave-length there will be errors due to multi-path. This paper describes a method where the Source Level of one Calibrated Source is first determined in absolute terms in free-water, where after it is checked in the indoor Test Tank using a calibrated hydrophone. The difference in Estimated Source Level is then used for characterization of the Test Tank. This means that subsequent Calibrated Sources of the same design will only need to be checked in the Test Tank, in order to Estimate the True Source Level. The paper shows an example of such a calibration scheme, together with the corresponding results and a discussion around possible sources of error.