Three dimensional average target strength of fish by spheroid model

As the fish target strength (TS) shows three-dimensional (3D) variation, the TS requires 3D averaging for scanning sonar surveying. By connecting the coordinates of the ship, the sonar beam, and the fish body, we derived the relationship between the apparent orientation of the fish viewed by the sonar beam and the actual fish angle. We used this relationship to develop a method to average the TS in 3D. A prolate spheroid mode series scattering model was used to explain the characteristics of the 3D average TS for various parameters, such as the fish angle, fish length, and sonar frequency.