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Underwater surveillance has traditionally been carried out by means of surface and undersea manned vessels equipped with advanced sensor systems. This approach is often costly and manpower intensive. Marine robotics is an emerging technological area that enables the development of advanced networks for underwater surveillance applications. In contrast with the use of standard assets, these advanced networks are typically composed of small, low-power, and possibly mobile robots, which have limited endurance, processing and wireless communication capabilities. When deployed in a region of interest, these robots can cooperatively form an intelligent network achieving high performance with significant features of scalability, adaptability, robustness, persistence and reliability. Such networks of robots can be the enabling technology for a wide range of applications in the maritime domain. However, they also introduce new challenges for underwater distributed sensing, data processing and analysis, autonomy and communications. The main thrust of this study is to review the underwater surveillance scenario within a framework of four research areas: (i) underwater robotics, (ii) acoustic signal processing, (iii) tracking and distributed information fusion, and (iv) underwater communications networks. Progress in each of these areas as well as future challenges is presented.

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A high bit rate acoustic link for video transmission over an underwater channel is investigated. The key to achieving this objective lies in two approaches: use of efficient data compression algorithms and use of high-level bandwidthefficient modulation methods. Currently available video encoding standards allow video transmission at bit rates as low as 64 kbps. While this rate is still above the limit of commercially available acoustic modems, prototype acoustic modems based on phase coherent modulation / detection have demonstrated successful transmission up to 30 kbps over a deepwater vertical path. To bridge the final gap and provide acoustic transmission capability needed for near real-time video, we focus on the use of high-level bandwidth-efficient modulation methods. An experimental system, based on discrete cosine transform and Huffman entropy coding for video compression, and variable rate M-ary QAM was implemented. Phasecoherent detection is accomplished by decision-directed synchronization and adaptive equalization. System performance is demonstrated experimentally, using 25000 symbols/sec at a carrier frequency of 75 kHz over a short vertical path. Excellent results were obtained using modulation methods of 16, 32 and 64-QAM, thus achieving bit rates as high as 150 kbps, which are sufficient for real-time transmission of compressed video.

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