Radio waves and light are frequencies that occupy the electromagnetic (EM) spectrum. All frequencies of the EM spectrum comprise photons which have varying states of energy. Low-frequency radio waves are used for communicating with submarines specifically because they have properties which uniquely suit them to being conducted underwater.
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The EM spectrum is a range of frequencies that consists of radio, microwave, infrared, visible light, ultraviolet, X-rays and gamma rays. These are forms of electromagnetic radiation, all of which have specific wavelengths. Each wavelength is a packet of photons that has electrical and magnetic properties and is moving at the speed of light. Depending on the kind of frequency the electromagnetic radiation has, the photons will behave either as a wave or as a group of particles.
Radio Wave Frequencies
A radio wave is electromagnetic radiation which propagates out from its point of transmission. The length of radio waves varies from roughly 1 centimetre to 1 kilometre. Each radio wave has a specific number of oscillations per second and is measured in units of hertz--one megahertz is one million oscillations per second. The higher the number of oscillations in the wave, the more energy it has.
ELF Radio Waves
Extremely low frequency (ELF) radio waves lie in the 3 to 30 hertz range. The length of an ELF radio wave is between 100 to 10,000 kilometres. Because ELF waves are extremely long, they can travel great distances without losing their signal strength. Furthermore, the length of ELF waves enable them to penetrate up to 40 meters in water, depending on the salinity. However, the low frequency of the radio wave limits the amount of information it can carry. ELF waves must be generated using large transmission towers. The U.S. Navy transmits ELF waves using a dipole antenna 222 kilometres long.
ELF radio waves are ideal for communication with submerged submarines because of the vessel's ability to remain underwater and receive communication at the same time. A disadvantage to using ELF is that the submarine can only receive a signal; it cannot reply because it does not have the transmission capacity to generate an ELF radio wave. Submarines receive ELF transmissions by a radio wave being generated on land and reflected off the ionosphere toward the target location where the submarine is.
Other Submarine Communications
In order to facilitate two-way radio communication, submarines historically had to surface and extend an antenna above water. New technology allows two-way communication without the need for a submarine to surface. Floating antennas can be deployed to enable two-way communication by transmitting a radio signal to a submerged submarine. Floating radio transmission buoys can also be positioned to connect fibre-optic cables with submarines, allowing for high-volume two-way communications via satellite.
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- University of New South Wales: The Electromagnetic Spectrum
- Astrosurf.: Radio Propagation
- Government Executive: Navy Develops Two-Way Radio Communication for Submerged Submarines
- Janes: Extremely Low-Frequency (ELF) Communications Programme
- Federation of American Scientists: Extremely Low Frequency Communications Program