Contents Introduction
Underwater wireless communications can enable many civilian and military applications such as oceanographic data collection, scientific ocean sampling, pollution and environmental monitoring, climate recording, offshore exploration, disaster prevention, assisted navigation, distributed tactical surveillance, and mine reconnaissance. Some of these applications can be supported by underwater acoustic sensor networks (UW-ASNs) [1], which consist of devices with sensing, processing, and communication capabilities that are deployed to perform collaborative monitoring tasks (Fig. 1). Wireless signal transmission is also crucial to remotely control instruments in ocean observatories and to enable coordination of swarms of autonomous underwater vehicles (AUVs) and robots, which will play the role of mobile nodes in future ocean observation networks by virtue of their flexibility and reconfig-urability. To make underwater applications viable, real-time communication protocols among underwater devices must be enabled. Wireless acoustic networking is the enabling technology for underwater applications to cover distances in excess of one hundred meters, whereas shorter distances can be covered using electro-magnetic waves. Radio frequency (RF) waves, in fact, propagate through conductive salty water only at extra-low frequencies (30–300 Hz), which require large antennae and high transmission power. Optical waves do not suffer from such high attenuation but are affected by scattering. Moreover, transmission of optical signals requires high precision in pointing the narrow laser beams.
Scenario of a UW-ASN composed of underwater and surface vehicles.
