Over 70% of the Earth’s surface is covered by water, while the majority of the underwater areas are unexplored yet. Owing to growing underwater activities, e.g. undersea oil & gas exploration, and Internet of underwater things (IoUT), which are expected to connect various underwater sensors and autonomous underwater vehicles (AUVs) to the outside world, reliable water-to-air wireless communications are urgently needed by the industry, military, and scientific communities. However, it is difficult to communicate across a water-air interface using a single type of wireless signal, e.g. acoustic, radio frequency (RF), or optical. RF signals decay exponentially in water. Although acoustic and optical signals can travel over long distances underwater, they both suffer from severe reflections off the water surface and hence cannot transmit smoothly across the water-air interface. To overcome the water-air barrier, most existing water-to-air communications rely on sonobuoys, which are floating devices that receive acoustic signals from underwater transmitters and then RF-transmit them to receivers on/above the water surface. Presently, battery-powered sonobuoys use magnesium/silver chloride or lithium primary batteries, which are full of toxic metals and tend to vent toxic fumes during discharging. As they might not be retrieved from remote, hostile areas of the ocean, battery-powered sonobuoys are posing a great threat to the marine environment. Although energy-harvesting technologies (e.g. solar, wave, or seawater semi-fuel cell) have been developed for sonobuoys to charge onboard energy storage systems, the challenge of miniaturizing and incorporating these technologies into the sonobuoy volume constraint remains a dominant issue. Large sonobuoys would require complex mooring systems, thus limiting their wide deployment for water-to-air communications.
Miniature/lightweight battery-free sonobuoys, which can be flexibly deployed (e.g. air-dropped by unmanned aerial vehicles (UAVs)) where and when needed and are disposable with a minimum impact on the marine environment, are in high demand for many ocean applications, such as underwater search and rescue (SAR), IoUT, oil spill monitoring, and scientific exploration. Since RF communication is the major source of energy consumption for sonobuoys, a battery-free design of sonobuoy can leverage backscatter communications, which allow a device to communicate at near-zero power by reflecting an existing RF signal, thus leading to novel water-to-air backscatter communications (WABCom).
The project validates the feasibility of the proposed WABCom system through a comprehensive approach that includes mathematical simulations, prototyping, and extensive experimentation. The results demonstrate the potential of the proposed system, and the innovation activities of the project can lead to the development of new products, services, reference materials, processes, or methods that can be launched into the market. These developments have significant benefits for various industries and government agencies involved in underwater IoT, underwater detection, marine biology research, oil exploration, and emergency rescue.
