Sidescan sonar calibration by bottom arrays of standard targets

Many sidescan sonars are used for imaging rather than quantitative amplitude measurement per se. To improve imaging, the attempt is often made to enhance features by internal signal or image processing. Applied gains often deviate from the quantitative gain functions of range compensation, e.g., time-varied gain for spherical spreading. Affected sonars may still benefit from calibration, although this may not be achieved in the amplitude domain. In the case of sonar operations for which calibration at a special facility or with a special configuration is not practicable, performance measurement in situ may be necessary. If this is done with a standard target, how should the target be marked to distinguish it from other interesting targets? This question is answered for sidescan sonar through use of distinctive geometric arrays of standard targets resting on the seafloor. Two examples of arrays are given for use in calibrating a sidescan sonar, with nominal center frequencies at 550 and 1600 kHz. These are seven-target arrays, one L-shaped, with equal arms and common vertex; the other, centered-hexagonal. The standard target is a 79.375-mm-daimeter solid sphere composed of the aluminum alloy 2024. Results are shown in the form of echo visualizations, measurements, and analyses of standard-target positions.