Figure 2 illustrates the propagation paths that can be exploited for communications. The seabed is an alternative low-loss, low-noise, covert communications path. A similar effect is seen at the seabed, where conductivity is much lower than the water. In comparison, acoustic signals cannot cross the water-to-air boundary, so 1km through-water loss would apply. For example, if two divers are 1km apart at 2m below the surface, attenuation will be significantly less than anticipated from the 1km through-water loss. This effect aids communication from a submerged station to land and between shallow submerged stations without the need for surface repeater buoys. The large refraction angle produced by the high permittivity launches a signal almost parallel with the water surface. Propagation losses and the refraction angle are such that an electromagnetic signal crosses the air-to-water boundary and appears to radiate from a patch of water directly above the transmitter. Doppler shift is inversely proportional to propagation velocity, so is much smaller for electromagnetic signals.Īnother important consideration is the effect of the air to water interface. This has import-ant advantages for command latency and networking protocols, where many signals have to be exchanged. Propagating waves continually cycle energy between the electric and magnetic fields hence conduction leads to strong attenuation of electromagnetic propagating wavesĪbove 10kHz, electromagnetic propagation is more than a hundred times faster than acoustic. Loss is largely due to the effect of conduction on the electric field component. Relative permeability is approximately 1, so there is little direct effect on the magnetic field component. At-tenuation of em signals is much lower in fresh water than in seawater, but fresh water has a similar permittivity. With a relative permittivity of 80, water has among the highest permittivity of any material and this has a significant impact on the angle of refraction at the air/water interface.Ĭonductivity of seawater is typically around 4S/m, while nominally 'fresh' water conductivity is quite variable but typically in the mS/m range. Plane wave attenuation is high compared to air, and increases rapidly with frequency. An initial investigation revealed that electromagnetic signalling, coupled with digital technology and signal-compression techniques, had many advantages that made it suitable for niche underwater applications.Įlectromagnetic propagation through water is very different from propagation through air because of water's high permittivity and electrical conductivity. At the same time, the oil industry and military operations have changing requirements that have created demand for reliable, connectorless short-range data links. ![]() In the digital era we have become familiar with the benefits of short-range, high-bandwidth communications systems such as Bluetooth. Through water, full-bandwidth, long-range, analogue voice communications were found to be impractical and there rapidly developed a 'perceived wisdom' that electromagnetic signals had no applications in the underwater environment. It implemented a one way 'bell ring' to call an individual submarine to the surface for higher bandwidth communications using terrestrial radio. This system operated at 76Hz for the US system and 82Hz in the Russian system and allowed transmission of a few characters per minute across the globe. In fact, the Extremely Low Frequency (ELF) submarine communications system is believed to be the only successfully deployed subsea electromagnetic application. Then terrestrial radio typically delivered manual digital communications (Morse code) or full bandwidth analogue voice communications over long range, and research was aimed at delivering these types of service in the underwater environment. Underwater electromagnetic communications have been investigated since the very early days of radio, and again received considerable attention during the 1970s. Given modern operational requirements and digital communications technology, the time is now ripe for re-evaluating the role of electromagnetic signals in the underwater environment. ![]() Although underwater radio links were experimentally evaluated in the pioneering days of radio, they did not meet the requirements of the time. Optical links have proved impractical for many applications. ![]() Underwater wireless communications links have almost exclusively been implemented using acoustic systems.
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