1.
Metre
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The metre or meter, is the base unit of length in the International System of Units. The metre is defined as the length of the path travelled by light in a vacuum in 1/299792458 seconds, the metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole. In 1799, it was redefined in terms of a metre bar. In 1960, the metre was redefined in terms of a number of wavelengths of a certain emission line of krypton-86. In 1983, the current definition was adopted, the imperial inch is defined as 0.0254 metres. One metre is about 3 3⁄8 inches longer than a yard, Metre is the standard spelling of the metric unit for length in nearly all English-speaking nations except the United States and the Philippines, which use meter. Measuring devices are spelled -meter in all variants of English, the suffix -meter has the same Greek origin as the unit of length. This range of uses is found in Latin, French, English. Thus calls for measurement and moderation. In 1668 the English cleric and philosopher John Wilkins proposed in an essay a decimal-based unit of length, as a result of the French Revolution, the French Academy of Sciences charged a commission with determining a single scale for all measures. In 1668, Wilkins proposed using Christopher Wrens suggestion of defining the metre using a pendulum with a length which produced a half-period of one second, christiaan Huygens had observed that length to be 38 Rijnland inches or 39.26 English inches. This is the equivalent of what is now known to be 997 mm, no official action was taken regarding this suggestion. In the 18th century, there were two approaches to the definition of the unit of length. One favoured Wilkins approach, to define the metre in terms of the length of a pendulum which produced a half-period of one second. The other approach was to define the metre as one ten-millionth of the length of a quadrant along the Earths meridian, that is, the distance from the Equator to the North Pole. This means that the quadrant would have defined as exactly 10000000 metres at that time. To establish a universally accepted foundation for the definition of the metre, more measurements of this meridian were needed. This portion of the meridian, assumed to be the length as the Paris meridian, was to serve as the basis for the length of the half meridian connecting the North Pole with the Equator
2.
Extremely high frequency
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Extremely high frequency is the International Telecommunications Union designation for the band of radio frequencies in the electromagnetic spectrum from 30 to 300 gigahertz. It lies between the high frequency band, and the far infrared band which is also referred to as the terahertz gap. Radio waves in this band have wavelengths from ten to one millimetre, giving it the name millimetre band or millimetre wave, millimetre-length electromagnetic waves were first investigated in the 1890s by Indian scientist Jagadish Chandra Bose. Compared to lower bands, radio waves in this band have high atmospheric attenuation, therefore, they have a short range and can only be used for terrestrial communication over about a kilometer. Absorption by humidity in the atmosphere is significant except in desert environments, however the short propagation range allows smaller frequency reuse distances than lower frequencies. The short wavelength allows modest size antennas to have a beam width. Millimeter waves propagate solely by line-of-sight paths, they are not reflected by the ionosphere nor do they travel along the Earth as ground waves as lower frequency radio waves do, at typical power densities they are blocked by building walls and suffer significant attenuation passing through foliage. The high free space loss and atmospheric absorption limits useful propagation to a few kilometers, thus they are useful for densely packed communications networks such as personal area networks that improve spectrum utilization through frequency reuse. They show optical propagation characteristics and can be reflected and focused by small metal surfaces around 1 ft. diameter, at millimeter wavelengths, surfaces appear rougher so diffuse reflection increases. Multipath propagation, particularly reflection from indoor walls and surfaces, causes serious fading, doppler shift of frequency can be significant even at pedestrian speeds. In portable devices, shadowing due to the body is a problem. Since the waves penetrate clothing and their small wavelength allows them to reflect from small metal objects they are used in millimeter wave scanners for security scanning. This band is used in radio astronomy and remote sensing. Ground-based radio astronomy is limited to high altitude such as Kitt Peak. Satellite-based remote sensing near 60 GHz can determine temperature in the atmosphere by measuring radiation emitted from oxygen molecules that is a function of temperature and pressure. The ITU non-exclusive passive frequency allocation at 57-59 and it is used commonly in flat terrain. The 71-76, 81-86 and 92–95 GHz bands are used for point-to-point high-bandwidth communication links. These higher frequencies do not suffer from oxygen absorption, but require a license in the US from the Federal Communications Commission
3.
Extremely low frequency
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Extremely low frequency is the ITU designation for electromagnetic radiation with frequencies from 3 to 30 Hz, and corresponding wavelengths of 100,000 to 10,000 kilometers, respectively. In atmospheric science, a definition is usually given, from 3 Hz to 3 kHz. In the related magnetosphere science, the lower frequency oscillations are considered to lie in the ULF range. ELF radio waves are generated by lightning and natural disturbances in Earths magnetic field, because of the difficulty of building antennas that can radiate such long waves, ELF frequencies have been used in only a very few human-made communication systems. ELF waves can penetrate seawater, which makes them useful in communication with submarines, the US, Russia, and India are the only nations known to have constructed ELF communication facilities. The U. S. facilities were used between 1985 and 2004 but are now decommissioned, ELF waves can also penetrate significant distances into earth or rock, and through-the-earth underground mine communication systems use frequencies of 300 to 3000 Hz. The frequency of alternating current flowing in electric grids,50 or 60 Hz, also falls within the ELF band. Some medical peer reviewed journal articles refer to ELF in the context of low frequency magnetic fields with frequencies of 50 Hz. United States Government agencies, such as NASA, describe ELF as non-ionizing radiation with frequencies between 0 and 300 Hz. Due to their long wavelength, ELF waves can diffract around large obstacles. ELF and VLF waves propagate long distances by an Earth-ionosphere waveguide mechanism, the Earth is surrounded by a layer of charged particles in the atmosphere at an altitude of about 60 km at the bottom of the ionosphere, called the D layer which reflects ELF waves. ELF waves have extremely low attenuation of 1 –2 dB per 1000 km. giving a single transmitter the potential to communicate worldwide, ELF waves can also travel considerable distances through lossy media like earth and seawater, which would absorb or reflect higher frequency radio waves. At certain frequencies these oppositely directed waves are in phase and add, in other words, the closed spherical Earth-ionosphere cavity acts as a huge cavity resonator, enhancing ELF radiation at its resonant frequencies. These are called Schumann resonances after German physicist Winfried Otto Schumann who predicted them in 1952, the United States Navy utilized extremely low frequencies as radio band and radio communications. The Submarine Integrated Antenna System was a research and development effort to communicate with submerged submarines, the Soviet/Russian Navy also utilized ELFs for submarine communications system, ZEVS. The Indian Navy has an operational ELF communication facility at the INS Kattabomman naval base to communicate with its Arihant class, because of its electrical conductivity, seawater shields submarines from most higher frequency radio waves, making radio communication with submerged submarines at ordinary frequencies impossible. Signals in the ELF frequency range, however, can penetrate much deeper, generally, ELF signals were used to order a submarine to rise to a shallow depth where it could receive some other form of communication. One of the difficulties posed when broadcasting in the ELF frequency range is antenna size, simply put, a 3 Hz signal would have a wavelength equal to the distance EM waves travel through a given medium in one third of a second
