DEVELOPMENT OF A DIGITAL MULTIBEAM ECHOSOUNDER

We present a multibeam echosounder developed for fishing applications. The aim is to get a real time tomographic image in the vertical plane , orthogonal with respect to the ship axis. The aperture of the sector ial image is 32-degrees when the lateral resolution is about two degre es. The range can vary from 50 to 800 m. We explain the choices made t o achieve these specifications: circular antenna (radius : 1.1 m) made of 48 transducers made with 1-3 composite (bandwidth : 70-130 kHz) sp aced with a pitch of 18 mm. 16 preformed beams are numerically process ed in parallel after a heterodyn detection of the received signals. Th e roll is compensated for, to obtain a stabilized image displayed on a video monitor. This system has been tested on-board the oceanographic ship <<Thalassa>> and provided convenient images of the fishes and of the sea-bottom in various situations.