Sensor networks provide services to a broad range of applications ranging from intelligence service surveillance to weather forecasting. While most of the sensor networks are terrestrial, Underwater Sensor Networks (USN) are an emerging area. One of the unavoidable and increasing challenges for modern USN technology is tolerating faults, i.e., accepting that hardware is imperfect, and coping with it. Fault Tolerance tends to have more impact in underwater than in terrestrial environment as the latter is generally more forgiving. Moreover, reaching the malfunctioning devices for replacement and maintenance under water is harder and more costly. The current paper is the first to provide an overview of fault-tolerant, particularly cross-layer fault-tolerant, techniques in USNs. In the paper, we present a systematic survey of the techniques, introduce a taxonomy of the Fault Tolerance tasks, present a categorized list of articles, and list the open research issues within the area.

Monitoring and assessment of coastal and river velocities plays a key role in both scientific and industry applications. Field measurements are key for decision making for resource management and protection, as well as for validation of numerical models and climate change studies. In this paper, a Hall effect sensor-based cost effective novel device is proposed for measuring velocities and direction of near-bed currents, and water-level. This device, called the Hydromast, provides instantaneous measurements in real-world conditions, and is equipped with communication capabilities to allow near real-time data transfer and monitoring. The validation of the device is performed in real-world steady and unsteady flow conditions. Within the device measurement range it is shown that the root-mean-square-error of the time averaged flow measurements is under 0.1m/s.

Sensor networks provide services to a broad range of applications ranging from intelligence service surveillance to weather forecasting. Most of the sensor networks are terrestrial, however much of our planet is covered by water and Underwater Sensor Networks (USN) are an emerging research area. One of the unavoidable increasing challenge for modern technology is tolerating faults - accepting that hardware is imperfect and cope with it. Fault tolerance may have more impact underwater than in terrestrial environment as terrestrial environment is more forgiving, reaching the malfunctioning devices for replacement underwater is harder and may be more costly. Current paper is the first to investigate fault tolerance, particularly cross layer fault tolerance, in USN-s.