Very long-range underwater acoustic communication (UAC) is crucial for long cruising (>1000 km) autonomous underwater vehicles (AUVs). Very long-range UAC source for AUV must exhibit high electro-acoustic efficiency (>60%) and compactness. This paper describes the Janus-Hammer Bell (JHB) transducer that has been designed for this purpose and meets those requirements. The transducer works on the 450-550 Hz bandwidth and reaches source level above 200 dB (ref. 1 μPa at 1 m) with 1 kW excitation and full immersion capability. JHB source has been used for communication experiments by the Japanese institute for marine technology (Japan Agency for Marine-Earth Science and Technology) achieving a baud rate of 100 bits/s at 1000 km.
The synthetic aperture imaging is a very promising solution in the well-known compromise between contrast and frame rate. Indeed this method leads to the measurement of each transmitter/receiver impulse response of the system. From this fact, synthetic aperture imaging reach the transmit/receive focus imaging quality for the cost in frame rate of the number of antenna's elements. The main inconvenient of this method is the very low signal to noise ratio provided. Indeed, using only one transmitter per sequence leads to a very poor penetration. To correct this, a method using spatial Hadamard sequences has been introduced. For each of this Hadamard sequence, a Hadamard beam is generated in the medium. By a temporal approach, some interesting properties of those beams are highlighted and a method using those properties is proposed. Some experiments have been done using those properties and the results show an important improvement of the frame rate for a very small cost in contrast.
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