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Autonomous Underwater Vehicles (AUVs) are robotic devices with a propulsion system for navigation and an onboard computer for decision making. AUV research is gaining popularity because of its extensive applications in fields from military to science. Robotic systems are need of the hour for exploration and environmental safety of the vast and deep oceans and water bodies. This paper presents current research trends in the field of AUVs and highlights future research directions. Here localization and navigation techniques such as inertial navigation to simultaneous localization and mapping being used in current AUVs are discussed in detail. Different optimal path planning and control methods are highlighted. Use of different sensor technology like sonar, laser, acoustic modems and stereo vision systems for localization, navigation and mapping is presented. Recent developments in underwater wireless communication along with the commercially available devices are discussed. • Review of AUVs based on structural design and propulsion technique are presented.. • Different adaptive control and path-planning techniques have been reviewed.. • Underwater localization techniques suitable for different applications are discussed.. • Developments of relatively new Intervention AUVs are presented.. • Underwater wireless communication methods and available systems are discussed.. [ABSTRACT FROM AUTHOR]

The existing technologies using electromagnetic waves or lasers are not very efficient due to the large attenuation in the aquatic environment. Ultrasound reveals a lower attenuation, and thus has been used in underwater long-distance communications. For high data-rates and real-time applications it is necessary to use frequencies in the MHz range, allowing communication distances of hundreds of meters with a delay of milliseconds. To achieve this goal, it is necessary to develop ultrasound transducers able to work at high frequencies and wideband, with suitable responses to digital modulations. This work shows how the acoustic impedance influences the performance of an ultrasonic emitter transducer when digital modulations are used and operating at frequencies between 100 kHz and 1 MHz. The study includes a Finite Element Method and a MATLAB/Simulink simulation with an experimental validation to evaluate two types of piezoelectric materials: one based on ceramics (high acoustic impedance) with a resonance design and the other based in polymer (low acoustic impedance) designed to optimize the performance when digital modulations are used. The transducers performance for Binary Amplitude Shift Keying, On–Off Keying, Binary Phase Shift Keying and Binary Frequency Shift Keying modulations with a 1 MHz carrier at 125 kbps baud rate are compared. [ABSTRACT FROM AUTHOR]