INTRODUCTION

The transmission of data, gathered by different underwater instruments or control systems, was first done using electrical and/or electromechanical cable which linked the source to the receiver. The use of acoustic waves to transmit these data enables the source to become a self-contained instrument and eliminates the drawbacks of the cables. Many scientific institutes and industrial firms tried to design such systems. IFREMER, in the framework of its programmes related to under-contained system to transmit pictures from the sea-bottom to a surface vessel.

The use of recently designed components such as CCD cameras low power consumption microprocessors and high-efficiency wide-band amplifiers led to a compact instrument which can be used in a wide range of applications and which can be installed in manned or unmanned submersibles or in deep sea structures to control underwater operations in real time.

The frequency is the limitation factor on the range. IFREMER chose a low frequency (26 kHz) to get ranges as long as 3000m the data rate depends on the modulation scheme and on the bandwidth of the transmission system, including the transducer. The number of images that can be transmitted per second also depends on the algorithms which are used to reduce the data.

Fig 1a Fish Epaulard and camera equipment and housing (available in full paper)

Fig 1b (available in full paper)

Very good results have been obtained in several configurations with the Epaulard acoustically remote-controlled autonomous submersible (Fig 1a). The system can be divided into two main parts : the underwater acquisition, processing and transmitting subsystem; and the receiving and decoding station aboard the vessel.

The Underwater Equipment

Besides the batter pack and the light, which were existing Epaulard equipment, the underwater equipment consists of a single housing (see Fig 1b)

(Table available in full paper)

Figure 2 presents the block diagram of the equipment which includes:

  • a CCD camera and the associated circuits (144 lines X 208 points);

  • acquisition and processing circuits based on the IFREMER-designed card set (NSC 800 microprocessor);

  • a modulator;

  • a wide-band power amplifier; and

  • power supplies

Fig 2 Schematic diagram of emitter (available in full paper)

The CCS camera

Manufactured by 12S (Bordeaux, France), the CCD camera has been designed to withstand a hard environment. The sensing head is removable and can be separated from the two specific printed circuit boards. The low-weight compact camera is a cylinder of length 80 mm and diameter 60 mm.

The CCS sensor is the TH 7852 manufactured by Thomson, and gives a 144 lines X 208 points image. It is fitted with an anti-blooming circuit. Its opto-electronic characteristics are :

Table (available in full paper)

As it was to be used in sea water, the CCD camera had to be placed behind a special optic system. This designed by CERCO (Paris, France) and is made of removable corrective lenses to take the characteristics of the sea water into account. Its overall specifications are :

Table (available in full paper)

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