TIVA A SELF CONTAINED IMAGE/DATA ACOUSTIC TRANSMISSION SYSTEM FOR UNDERWATER APPLICATION

Abstract

Underwater exploration and exploitation programs have early expressed the need for pictures and images. Still cameras and TV cameras have been used to take photographs and films of the sea bottom, of the living resources, of undersea man built equipments. Sophisticated instruments have recently been designed with enhanced characteristics (high definition and sensitivity, color. ... ). Unfortunately, because of the needed bandwidth to transmit the signal, the only way to get real-time data was to use cables. This technique is however limited to a few hundred meters of water. Another mean to transmit image data over long distances was needed. Within the framework of its programs related to underwater intervention, IFREMER (Institut Francais de Recherche pour l'Exploitation de la Mer) and ORCA Instrumentation have designed a self-contained system to transmit images from the sea bottom to a surface vessel : TIVA. The use of acoustic waves enables the source to be completely separated from the receiver. This technique is bandwidth limited and the maximum baud rate is far beyond the ones which can be achieved in the labs. The quantity of information within an image is so large that a long time is needed to transmit the whole set of data. Compression algorithms have been studied and applied to actual data in order to decrease this transmission time. The modulation and the associated demodulation equipments were specifically designed for the application. The specific constraints of the equipment (self-contained instrument, memory, space, power limitations. ... ) and the desire to process the data in a very short time before the transmission, led to a trade-off between the power of the compression algorithm and the other characteristics such as current consumption, memory size ... The equipment has been successfully used to send images in real time during the survey of the TITANIC'S wreck. Scientific at-sea campains (penetrometry, shear vane meter deployment ...) have been optimized with the use of TIVA and its capacity to send images and data to the user at the surface. The transmission of data gathered by different underwater instruments on control systems was first done by using electrical and/or electromechanical cables which linked the source to the receiver. However these technics are limited to a few hundred meters of water because of logistic problem and price. Then, acoustic transmission is the only way to break the material link between the source (the camera or other instruments) and the receiver (the display) if long distances have to be reached. IFREMER and ORCA Instrumentation have designed a self contained equipment which achieves the transmission of image data through more than 4000 meters of water over an acoustic data link.