The accuracy of length measurements made using imaging SONAR is inversely proportional to the beam width

Multibeam imaging SONARs have been used for a range of measurement applications, such as measurements of fish lengths. This study aimed to quantify the accuracy of imaging SONAR systems, that varied in frequency and beam geometry, to measure the length of synthetic targets positioned perpendicularly. Blueprint Oculus imaging SONAR systems, with four different (centre) frequencies (750 kHz, 1.2 MHz, 2.1 MHz, and 3 MHz), were used to measure the length of three targets of nominal lengths: 10 cm, 20 cm, and at ranges between 1 m and 15.5 m. The effect of beam geometry on measurement error was then examined using regression analysis. This study found that there was an overestimation of the actual length of the target for all measurements that was inversely proportional to the horizontal beam width. The measurement error can be reduced by normalising for beam width. However, the variation of measurements (i.e. the precision), was found to also increase with range, which was attributed to the increasing beam separation. It is important that the effect of beam width on the accuracy of target length measurements, by imaging SONARs is acknowledged in future studies. One approach to mitigating this problem, is to limit the range at which length measurements are made to a beam width that produces an estimated level of error that is acceptable for that study. Keywords: acoustic camera, imaging SONAR, underwater acoustics, underwater measurements.