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Geostationary orbit

A geostationary orbit, also referred to as a geosynchronous equatorial orbit (GEO), is a circular geosynchronous orbit 35,786 km in altitude above Earth's equator, 42,164 km in radius from Earth's center, and following the direction of Earth's rotation.

An object in such an orbit has an orbital period equal to Earth's rotational period, one sidereal day, and so to ground observers it appears motionless, in a fixed position in the sky. The concept of a geostationary orbit was popularised by the science fiction writer Arthur C. Clarke in the 1940s as a way to revolutionise telecommunications, and the first satellite to be placed in this kind of orbit was launched in 1963.

Communications satellites are often placed in a geostationary orbit so that Earth-based satellite antennas do not have to rotate to track them but can be pointed permanently at the position in the sky where the satellites are located. Weather satellites are also placed in this orbit for real-time monitoring and data collection, and navigation satellites to provide a known calibration point and enhance GPS accuracy.

Geostationary satellites are launched via a temporary orbit, and placed in a slot above a particular point on the Earth's surface. The orbit requires some stationkeeping to keep its position, and modern retired satellites are placed in a higher graveyard orbit to avoid collisions.

History

In 1929, Herman Potočnik described both geosynchronous orbits in general and the special case of the geostationary Earth orbit in particular as useful orbits for space stations. The first appearance of a geostationary orbit in popular literature was in October 1942, in the first Venus Equilateral story by George O. Smith, but Smith did not go into details. British science fiction author Arthur C. Clarke popularised and expanded the concept in a 1945 paper entitled Extra-Terrestrial Relays – Can Rocket Stations Give Worldwide Radio Coverage?, published in Wireless World magazine. Clarke acknowledged the connection in his introduction to The Complete Venus Equilateral. The orbit, which Clarke first described as useful for broadcast and relay communications satellites, is sometimes called the Clarke orbit. Similarly, the collection of artificial satellites in this orbit is known as the Clarke Belt.

In technical terminology the orbit is referred to as either a geostationary or geosynchronous equatorial orbit, with the terms used somewhat interchangeably.

The first geostationary satellite was designed by Harold Rosen while he was working at Hughes Aircraft in 1959. Inspired by Sputnik 1, he wanted to use a geostationary satellite to globalise communications. Telecommunications between the US and Europe was then possible between just 136 people at a time, and reliant on high frequency radios and an undersea cable.

Conventional wisdom at the time was that it would require too much rocket power to place a satellite in a geostationary orbit and it would not survive long enough to justify the expense, so early efforts were put towards constellations of satellites in low or medium Earth orbit. The first of these were the passive Echo balloon satellites in 1960, followed by Telstar 1 in 1962. Although these projects had difficulties with signal strength and tracking, issues that could be solved using geostationary orbits, the concept was seen as impractical, so Hughes often withheld funds and support.

By 1961, Rosen and his team had produced a cylindrical prototype with a diameter of 76 cm, height of 38 cm, weighing 11.3 kg, light and small enough to be placed into orbit. It was spin stabilised with a dipole antenna producing a pancake shaped beam. In August 1961, they were contracted to begin building the real satellite. They lost Syncom 1 to electronics failure, but Syncom 2 was successfully placed into a geosynchronous orbit in 1963. Although its inclined orbit still required moving antennas, it was able to relay TV transmissions, and allowed for US President John F. Kennedy in Washington D.C., to phone Nigerian prime minister Abubakar Tafawa Balewa aboard the USNS Kingsport docked in Lagos on August 23, 1963.

The first satellite placed in a geostationary orbit was Syncom 3, which was launched by a Delta D rocket in 1964. With its increased bandwidth, this satellite was able to transmit live coverage of the Summer Olympics from Japan to America. Geostationary orbits have been in common use ever since, in particular for satellite television.

Today there are hundreds of geostationary satellites providing remote sensing and communications.

Although most populated land locations on the planet now have terrestrial communications facilities (microwave, fiber-optic), with telephone access covering 96% of the population and internet access 90%, some rural and remote areas in developed countries are still reliant on satellite communications.

Uses
Most commercial communications satellites, broadcast satellites and SBAS satellites operate in geostationary orbits.

Communications
Geostationary communication satellites are useful because they are visible from a large area of the earth's surface, extending 81° away in latitude and 77° in longitude. They appear stationary in the sky, which eliminates the need for ground stations to have movable antennas. This means that Earth-based observers can erect small, cheap and stationary antennas that are always directed at the desired satellite. However, latency becomes significant as it takes about 240 ms for a signal to pass from a ground based transmitter on the equator to the satellite and back again. This delay presents problems for latency-sensitive applications such as voice communication, so geostationary communication satellites are primarily used for unidirectional entertainment and applications where low latency alternatives are not available.

Geostationary satellites are directly overhead at the equator and appear lower in the sky to an observer nearer the poles. As the observer's latitude increases, communication becomes more difficult due to factors such as atmospheric refraction, Earth's thermal emission, line-of-sight obstructions, and signal reflections from the ground or nearby structures. At latitudes above about 81°, geostationary satellites are below the horizon and cannot be seen at all. Because of this, some Russian communication satellites have used elliptical Molniya and Tundra orbits, which have excellent visibility at high latitudes.

Meteorology
A worldwide network of operational geostationary meteorological satellites is used to provide visible and infrared images of Earth's surface and atmosphere for weather observation, oceanography, and atmospheric tracking. As of 2019 there are 19 satellites in either operation or stand-by. These satellite systems include:
 * the United States' GOES series, operated by NOAA
 * the Meteosat series, launched by the European Space Agency and operated by the European Weather Satellite Organization, EUMETSAT
 * the Republic of Korea COMS-1 and GK-2A multi mission satellites.
 * the Russian Elektro-L satellites
 * the Japanese Himawari series
 * Chinese Fengyun series
 * India's INSAT series

These satellites typically captures images in the visual and infrared spectrum with a spatial resolution between 0.5 and 4 square kilometres. The coverage is typically 70°, and in some cases less.

Geostationary satellite imagery has been used for tracking volcanic ash, measuring cloud top temperatures and water vapour, oceanography, measuring land temperature and vegetation coverage, facilitating cyclone path prediction, and providing real time cloud coverage and other tracking data. Some information has been incorporated into meteorological prediction models, but due to their wide field of view, full-time monitoring and lower resolution, geostationary weather satellite images are primarily used for short-term and real-time forecasting.

Navigation

Geostationary satellites can be used to augment GNSS systems by relaying clock, ephemeris and ionospheric error corrections (calculated from ground stations of a known position) and providing an additional reference signal. This improves position accuracy from approximately 5m to 1m or less.

Past and current navigation systems that use geostationary satellites include:
 * The Wide Area Augmentation System (WAAS), operated by the United States Federal Aviation Administration (FAA);
 * The European Geostationary Navigation Overlay Service (EGNOS), operated by the ESSP (on behalf of EU's GSA);
 * The Multi-functional Satellite Augmentation System (MSAS), operated by Japan's Ministry of Land, Infrastructure and Transport Japan Civil Aviation Bureau (JCAB);
 * The GPS Aided Geo Augmented Navigation (GAGAN) system being operated by India.
 * The commercial StarFire navigation system, operated by John Deere and C-Nav Positioning Solutions (Oceaneering);
 * The commercial Starfix DGPS System and OmniSTAR system, operated by Fugro.

Launch
Geostationary satellites are launched to the east into a prograde orbit that matches the rotation rate of the equator. The smallest inclination that a satellite can be launched into is that of the launch site's latitude, so launching the satellite from close to the equator limits the amount of inclination change needed later. Additionally, launching from close to the equator allows the speed of the Earth's rotation to give the satellite a boost. A launch site should have water or deserts to the east, so any failed rockets do not fall on a populated area.

Most launch vehicles place geostationary satellites directly into a geostationary transfer orbit (GTO), an elliptical orbit with an apogee at GEO height and a low perigee. On-board satellite propulsion is then used to raise the perigee, circularise and reach GEO.

Orbit allocation
Satellites in geostationary orbit must all occupy a single ring above the equator. The requirement to space these satellites apart, to avoid harmful radio-frequency interference during operations, means that there are a limited number of orbital slots available, and thus only a limited number of satellites can be operated in geostationary orbit. This has led to conflict between different countries wishing access to the same orbital slots (countries near the same longitude but differing latitudes) and radio frequencies. These disputes are addressed through the International Telecommunication Union's allocation mechanism under the Radio Regulations. In the 1976 Bogota Declaration, eight countries located on the Earth's equator claimed sovereignty over the geostationary orbits above their territory, but the claims gained no international recognition.

Statite proposal
A statite is a hypothetical satellite that uses radiation pressure from the sun against a solar sail to modify its orbit.

It would hold its location over the dark side of the Earth at a latitude of approximately 30 degrees. A statite is stationary relative to the Earth and Sun system rather than compared to surface of the Earth, and could ease congestion in the geostationary ring.

Retired satellites
Geostationary satellites require some station keeping to keep their position, and once they run out of thruster fuel they are generally retired. The transponders and other onboard systems often outlive the thruster fuel and by allowing the satellite to move naturally into an inclined geosynchronous orbit some satellites can remain in use, or else be elevated to a graveyard orbit. This process is becoming increasingly regulated and satellites must have a 90% chance of moving over 200 km above the geostationary belt at end of life.

Space debris

Space debris at geostationary orbits typically has a lower collision speed than at low Earth orbit (LEO) since all GEO satellites orbit in the same plane, altitude and speed; however, the presence of satellites in eccentric orbits allows for collisions at up to 4 km/s. Although a collision is comparatively unlikely, GEO satellites have a limited ability to avoid any debris.

At geosynchronous altitude, objects less than 10 cm in diameter cannot be seen from the Earth, making it difficult to assess their prevalence.

Despite efforts to reduce risk, spacecraft collisions have occurred. The European Space Agency telecom satellite Olympus-1 was struck by a meteoroid on August 11, 1993 and eventually moved to a graveyard orbit, and in 2006 the Russian Express-AM11 communications satellite was struck by an unknown object and rendered inoperable, although its engineers had enough contact time with the satellite to send it into a graveyard orbit. In 2017, both AMC-9 and Telkom-1 broke apart from an unknown cause.

Properties
A typical geostationary orbit has the following properties:
 * Inclination: 0°
 * Period: 1436 minutes (one sidereal day)
 * Eccentricity: 0
 * Argument of perigee: undefined
 * Semi-major axis: 42,164 km

Inclination
An inclination of zero ensures that the orbit remains over the equator at all times, making it stationary with respect to latitude from the point of view of a ground observer (and in the Earth-centered Earth-fixed reference frame).

Period
The orbital period is equal to exactly one sidereal day. This means that the satellite will return to the same point above the Earth's surface every (sidereal) day, regardless of other orbital properties. For a geostationary orbit in particular, it ensures that it holds the same longitude over time. This orbital period, T, is directly related to the semi-major axis of the orbit through the formula:

 * $$T = 2\pi\sqrt{a^3 \over \mu}$$

where:
 * $a$ is the length of the orbit's semi-major axis
 * $μ$ is the standard gravitational parameter of the central body

Eccentricity
The eccentricity is zero, which produces a circular orbit. This ensures that the satellite does not move closer or further away from the Earth, which would cause it to track backwards and forwards across the sky.

Stability
A geostationary orbit can be achieved only at an altitude very close to 35786 km and directly above the equator. This equates to an orbital speed of 3.07 km/s and an orbital period of 1,436 minutes, one sidereal day. This ensures that the satellite will match the Earth's rotational period and has a stationary footprint on the ground. All geostationary satellites have to be located on this ring.

A combination of lunar gravity, solar gravity, and the flattening of the Earth at its poles causes a precession motion of the orbital plane of any geostationary object, with an orbital period of about 53 years and an initial inclination gradient of about 0.85° per year, achieving a maximal inclination of 15° after 26.5 years. To correct for this perturbation, regular orbital stationkeeping maneuvers are necessary, amounting to a delta-v of approximately 50 m/s per year.

A second effect to be taken into account is the longitudinal drift, caused by the asymmetry of the Earth – the equator is slightly elliptical (equatorial eccentricity). There are two stable equilibrium points (at 75.3°E and 108°W) and two corresponding unstable points (at 165.3°E and 14.7°W). Any geostationary object placed between the equilibrium points would (without any action) be slowly accelerated towards the stable equilibrium position, causing a periodic longitude variation. The correction of this effect requires station-keeping maneuvers with a maximal delta-v of about 2 m/s per year, depending on the desired longitude.

Solar wind and radiation pressure also exert small forces on satellites: over time, these cause them to slowly drift away from their prescribed orbits.

In the absence of servicing missions from the Earth or a renewable propulsion method, the consumption of thruster propellant for station-keeping places a limitation on the lifetime of the satellite. Hall-effect thrusters, which are currently in use, have the potential to prolong the service life of a satellite by providing high-efficiency electric propulsion.

Derivation

For circular orbits around a body, the centripetal force required to maintain the orbit (Fc) is equal to the gravitational force acting on the satellite (Fg):

 * $$F_\text{c} = F_\text{g}$$

From Isaac Newton's universal law of gravitation,

 * $$F_\text{g} = G \frac{M_\text{E} m_\text{s}}{r^2}$$,

where Fg is the gravitational force acting between two objects, ME is the mass of the Earth, $0 Nm^{2}/kg^{2}$, ms is the mass of the satellite, r is the distance between the centers of their masses, and G is the gravitational constant, $5,980,000,000,000,000,000,000,000 kg$.

The magnitude of the acceleration, a, of a body moving in a circle is given by:

 * $$a = \frac{v^2}{r}$$

where v is the magnitude of the velocity (i.e. the speed) of the satellite. From Newton's second law of motion, the centripetal force Fc is given by:

 * $$F_\text{c} = m_\text{s}\frac{v^2}{r}$$.

As Fc = Fg,

 * $$m_\text{s}\frac{v^2}{r} = G \frac{M_\text{E} m_\text{s}}{r^2}$$,

so that

 * $$v^2 = G \frac{M_\text{E}}{r}$$

Replacing v with the equation for the speed of an object moving around a circle produces:

 * $$\left(\frac{2\pi r}{T}\right)^2 = G \frac{M_\text{E}}{r}$$

where T is the orbital period (i.e. one sidereal day), and is equal to $5,973,600,000,000,000,000,000,000 kg$. This gives an equation for r:

 * $$r = \sqrt[3]{\frac{GM_\text{E} T^2}{4\pi^2}}$$

The product GME is known with much greater precision than either factor alone; it is known as the geocentric gravitational constant μ = $6.67428e−11 m^{3} kg^{−1} s^{−2}$. Hence

 * $$ r = \sqrt[3]{\frac{\mu T^2}{4\pi^2}}$$

The resulting orbital radius is 42164 km. Subtracting the Earth's equatorial radius, 6378 km, gives the altitude of 35786 km.

The orbital speed is calculated by multiplying the angular speed by the orbital radius:

 * $$v = \omega r \quad  \approx 3074.6~\text{m/s}$$

In other planets
By the same method, we can determine the orbital altitude for any similar pair of bodies, including the areostationary orbit of an object in relation to Mars, if it is assumed that it is spherical (which it is not entirely). The gravitational constant GM (μ) for Mars has the value of $86,164.091 seconds$, its equatorial radius is $398,600.4418 km^{3} s^{−2}$ and the known rotational period (T) of the planet is $42,830 km^{3} s^{−2}$ ($3,389.5 km$). Using these values, Mars' orbital altitude is equal to $1.026 Earth days$.

title: Geostationary orbit


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Graded-index fiber

A graded-index fiber, or gradient-index fiber, is an optical fiber whose core has a refractive index that decreases continuously with increasing radial distance from the optical axis of the fiber, as opposed to a step-index fiber, which has a uniform index of refraction in the core, and a lower index in the surrounding cladding.

Because parts of the core closer to the fiber axis have a higher refractive index than the parts near the cladding, light rays follow sinusoidal paths down the fiber. The most common refractive index profile for a graded-index fiber is very nearly parabolic. The parabolic profile results in continual refocusing of the rays in the core, and minimizes modal dispersion.

Multi-mode optical fiber can be built with either a graded-index or a step-index profile. The advantage of graded-index multi-mode fiber compared to step-index fiber is a considerable decrease in modal dispersion. This means that the trip time of photons traversing the fiber is more consistent, allowing shorter and more frequent pulses of light to be discerned by the receiver. Modal dispersion can be further decreased by selecting a smaller core size (less than 10 μm) and forming a single-mode step index fiber.

This type of fiber is normalized by the International Telecommunication Union ITU-T in recommendation G.651.1.

Pulse dispersion
Pulse dispersion in a graded-index optical fiber is given by

$$\mathrm{Pulse~dispersion} = \frac{k \delta n\ n_1\ l}{c} \,\!,$$ where

$\delta n\,\!$ is the difference in refractive indices of core and cladding,

$n_1\,\!$ is the refractive index of the cladding,

$l\,\!$ is the length of the fiber taken for observing the pulse dispersion,

$c \approx 3\times 10^8~\mathrm{m/s}\,\!$ is the speed of light, and

$k\,\!$  is the constant of graded index profile.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Graded-index fiber


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Grade of service

In telecommunication engineering, and in particular teletraffic engineering, the quality of voice service is specified by two measures: the grade of service (GoS) and the quality of service (QoS).

Grade of service is the probability of a call in a circuit group being blocked or delayed for more than a specified interval, expressed as a vulgar fraction or decimal fraction. This is always with reference to the busy hour when the traffic intensity is the greatest. Grade of service may be viewed independently from the perspective of incoming versus outgoing calls, and is not necessarily equal in each direction or between different source-destination pairs. "Grade of Service" sometimes means a measure of inbound call center traffic to verify adherence to conditions to measure the success of customers served.

On the other hand, the quality of service which a single circuit is designed or conditioned to provide, e.g. voice grade or program grade is called the quality of service. Quality criteria for such circuits may include equalization for amplitude over a specified band of frequencies, or in the case of digital data transported via analogue circuits, may include equalization for phase. Criteria for mobile quality of service in cellular telephone circuits include the probability of abnormal termination of the call.

What is Grade of Service and how is it measured?
When a user attempts to make a telephone call, the routing equipment handling the call has to determine whether to accept the call, reroute the call to alternative equipment, or reject the call entirely. Rejected calls occur as a result of heavy traffic loads (congestion) on the system and can result in the call either being delayed or lost. If a call is delayed, the user simply has to wait for the traffic to decrease, however if a call is lost then it is removed from the system.

The Grade of Service is one aspect of the quality a customer can expect to experience when making a telephone call. In a Loss System, the Grade of Service is described as that proportion of calls that are lost due to congestion in the busy hour. For a Lost Call system, the Grade of Service can be measured using Equation 1.

 * $$\mbox{Grade of Service}=\frac{\mbox{number of blocked calls}}{\mbox{total offered calls}}\qquad(1)$$

For a delayed call system, the Grade of Service is measured using three separate terms:

The Grade of Service can be measured using different sections of a network. When a call is routed from one end to another, it will pass through several exchanges. If the Grade of Service is calculated based on the number of calls rejected by the final circuit group, then the Grade of Service is determined by the final circuit group blocking criteria. If the Grade of Service is calculated based on the number of rejected calls between exchanges, then the Grade of Service is determined by the exchange-to-exchange blocking criteria.
 * The mean delay $$t_d$$ – Describes the average time a user spends waiting for a connection if their call is delayed.
 * The mean delay $$t_o$$ – Describes the average time a user spends waiting for a connection whether or not their call is delayed.
 * The probability that a user may be delayed longer than time t while waiting for a connection. Time t is chosen by the telecommunications service provider so that they can measure whether their services conform to a set Grade of Service.
 * Where and when is Grade of Service measured?

The Grade of Service should be calculated using both the access networks and the core networks as it is these networks that allow a user to complete an end-to-end connection. Furthermore, the Grade of Service should be calculated from the average of the busy hour traffic intensities of the 30 busiest traffic days of the year. This will cater for most scenarios as the traffic intensity will seldom exceed the reference level.

The grade of service is a measure of the ability of a user to access a trunk system during the busiest hour. The busy is based upon customer demand at the busiest hour during a week month or year.

Class of Service
Different telecommunications applications require different Qualities of Service. For example, if a telecommunications service provider decides to offer different qualities of voice connection, then a premium voice connection will require a better connection quality compared to an ordinary voice connection. Thus different Qualities of Service are appropriate, depending on the intended use. To help telecommunications service providers to market their different services, each service is placed into a specific class. Each Class of Service determines the level of service required.

To identify the Class of Service for a specific service, the network's switches and routers examine the call based on several factors. Such factors can include:

 * The type of service and priority due to precedence
 * The identity of the initiating party
 * The identity of the recipient party

Quality of Service in broadband networks
In broadband networks, the Quality of Service is measured using two criteria. The first criterion is the probability of packet losses or delays in already accepted calls. The second criterion refers to the probability that a new incoming call will be rejected or blocked. To avoid the former, broadband networks limit the number of active calls so that packets from established calls will not be lost due to new calls arriving. As in circuit-switched networks, the Grade of Service can be calculated for individual switches or for the whole network.

Maintaining a Grade of Service
The telecommunications provider is usually aware of the required Grade of Service for a particular product. To achieve and maintain a given Grade of Service, the operator must ensure that sufficient telecommunications circuits or routes are available to meet a specific level of demand. It should also be kept in mind that too many circuits will create a situation where the operator is providing excess capacity which may never be used, or at the very least may be severely underutilized. This adds costs which must be borne by other parts of the network. To determine the correct number of circuits that are required, telecommunications service providers make use of Traffic Tables. An example of a Traffic Table can be viewed in Figure 1. It follows that in order for a telecommunications network to continue to offer a given Grade of Service, the number of circuits provided in a circuit group must increase (non-linearly) if the traffic intensity increases.

Erlang's lost call assumptions
To calculate the Grade of Service of a specified group of circuits or routes, Agner Krarup Erlang used a set of assumptions that relied on the network losing calls when all circuits in a group were busy. These assumptions are:

 * All traffic through the network is pure-chance traffic, i.e. all call arrivals and terminations are independent random events
 * There is statistical equilibrium, i.e., the average number of calls does not change
 * Full availability of the network, i.e., every outlet from a switch is accessible from every inlet
 * Any call that encounters congestion is immediately lost.

From these assumptions Erlang developed the Erlang-B formula which describes the probability of congestion in a circuit group. The probability of congestion gives the Grade of Service experienced.

Calculating the Grade of Service
To determine the Grade of Service of a network when the traffic load and number of circuits are known, telecommunications network operators make use of Equation 2, which is the Erlang-B equation.

 * $$\mbox{Grade of Service}=\frac{\left(\frac{A^N}{N!}\right)}{\left(\sum_{k=0}^N\frac{A^k}{k!}\right)}\qquad(2)$$

A = Expected traffic intensity in Erlangs, N = Number of circuits in group.

This equation allows operators to determine whether each of their circuit groups meet the required Grade of Service, simply by monitoring the reference traffic intensity.

(For delay networks, the Erlang-C formula allows network operators to determine the probability of delay depending on peak traffic and the number of circuits. )

title: Grade of service


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Graphic character

In ISO/IEC 646 (commonly known as ASCII) and related standards including ISO 8859 and Unicode, a graphic character, also known as printing character (or printable character), is any character intended to be written, printed, or otherwise displayed in a form that can be read by humans. In other words, it is any encoded character that is associated with one or more glyphs.

ISO/IEC 646
In ISO 646, graphic characters are contained in rows 2 through 7 of the code table. However, two of the characters in these rows, namely the space character SP at row 2 column 0 and the delete character DEL (also called the rubout character) at row 7 column 15, require special mention.

The space is considered to be both a graphic character and a control character in ISO 646. It can be considered as a character with a visible form or, in contexts such as teleprinters, a control character that advances the print head without printing a character.

The delete character is strictly a control character, not a graphic character. This is true not only in ISO 646, but also in all related standards including Unicode. However, many other character sets deviate from ISO 646, and as a result a graphic character might occupy the position originally reserved for the delete character.

Unicode
In Unicode, Graphic characters are those with General Category Letter, Mark, Number, Punctuation, Symbol or Zs=space. Other code points (General categories Control, Zl=line separator, Zp=paragraph separator) are Format, Control, Private Use, Surrogate, Noncharacter or Reserved (unassigned).

Spacing and non-spacing characters
Most graphic characters are spacing characters, which means that each instance of a spacing character has to occupy some area in a graphic representation. For a teletype or a typewriter this implies moving of the carriage after typing of a character. In the context of text mode display, each spacing character occupies one rectangular character box of equal sizes. Or maybe two adjacent ones, for non-alphabetic characters of East Asian languages. If a text is rendered using proportional fonts, widths of character boxes are not equal, but are positive.

There exist also non-spacing graphic characters. Most of non-spacing characters are modifiers, also called combining characters in Unicode, such as diacritical marks. Although non-spacing graphic characters are uncommon in traditional code pages, there are many such in Unicode. A combining character has its distinct glyph, but it applies to a character box of another character, a spacing one. In some historical systems such as line printers this was implemented as overstrike.

Note that not all modifiers are non-spacing – there exists Spacing Modifier Letters Unicode block.

title: Graphic character


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Ground (electricity)

In electrical engineering, ground or earth may be a reference point in an electrical circuit from which voltages are measured, a common return path for electric current, or a direct physical connection to the Earth.

Electrical circuits may be connected to ground for several reasons. Exposed conductive parts of electrical equipment are connected to ground, to protect users from electrical shock hazard. If internal insulation fails, dangerous voltages may appear on the exposed conductive parts. Connecting exposed conductive parts to a "Ground" wire which provides a low-impedance path for current to flow back to the incoming Neutral (which is also connected to Ground, close to the point of entry) will allow circuit breakers (or RCDs) to interrupt power supply in the event of a fault. In electric power distribution systems, a protective earth (PE) conductor is an essential part of the safety provided by the earthing system.

Connection to ground also limits the build-up of static electricity when handling flammable products or electrostatic-sensitive devices. In some telegraph and power transmission circuits, the ground itself can be used as one conductor of the circuit, saving the cost of installing a separate return conductor (see single-wire earth return and earth-return telegraph).

For measurement purposes, the Earth serves as a (reasonably) constant potential reference against which other potentials can be measured. An electrical ground system should have an appropriate current-carrying capability to serve as an adequate zero-voltage reference level. In electronic circuit theory, a "ground" is usually idealized as an infinite source or sink for charge, which can absorb an unlimited amount of current without changing its potential. Where a real ground connection has a significant resistance, the approximation of zero potential is no longer valid. Stray voltages or earth potential rise effects will occur, which may create noise in signals or produce an electric shock hazard if large enough.

The use of the term ground (or earth) is so common in electrical and electronics applications that circuits in portable electronic devices, such as cell phones and media players, as well as circuits in vehicles, may be spoken of as having a "ground" or chassis ground connection without any actual connection to the Earth, despite "common" being a more appropriate term for such a connection. That is usually a large conductor attached to one side of the power supply (such as the "ground plane" on a printed circuit board), which serves as the common return path for current from many different components in the circuit.

History
Long-distance electromagnetic telegraph systems from 1820 onwards used two or more wires to carry the signal and return currents. It was discovered by German scientist C.A. von Steinheil in 1836–1837, that the ground could be used as the return path to complete the circuit, making the return wire unnecessary. Steinheil was not the first to do this, but he was not aware of earlier experimental work, and he was the first to do it on an in-service telegraph, thus making the principle known to telegraph engineers generally. However, there were problems with this system, exemplified by the transcontinental telegraph line constructed in 1861 by the Western Union Company between St. Joseph, Missouri, and Sacramento, California. During dry weather, the ground connection often developed a high resistance, requiring water to be poured on the ground rod to enable the telegraph to work or phones to ring.

In the late nineteenth century, when telephony began to replace telegraphy, it was found that the currents in the earth induced by power systems, electric railways, other telephone and telegraph circuits, and natural sources including lightning caused unacceptable interference to the audio signals, and the two-wire or 'metallic circuit' system was reintroduced around 1883.

Building wiring installations
Electrical power distribution systems are often connected to earth ground to limit the voltage that can appear on distribution circuits. A distribution system insulated from earth ground may attain a high potential due to transient voltages caused by static electricity or accidental contact with higher potential circuits. An earth ground connection of the system dissipates such potentials and limits the rise in voltage of the grounded system.

In a mains electricity (AC power) wiring installation, the term ground conductor typically refers to two different conductors or conductor systems as listed below:

Equipment bonding conductors or equipment ground conductors (EGC) provide a low-impedance path between normally non-current-carrying metallic parts of equipment and one of the conductors of that electrical system's source. If any exposed metal part should become energized (fault), such as by a frayed or damaged insulator, it creates a short circuit, causing the overcurrent device (circuit breaker or fuse) to open, clearing (disconnecting) the fault. It is important to note this action occurs regardless of whether there is a connection to the physical ground (earth); the earth itself has no role in this fault-clearing process since current must return to its source; however, the sources are very frequently connected to the physical ground (earth). (see Kirchhoff's circuit laws). By bonding (interconnecting) all exposed non-current carrying metal objects together, as well as to other metallic objects such as pipes or structural steel, they should remain near the same voltage potential, thus reducing the chance of a shock. This is especially important in bathrooms where one may be in contact with several different metallic systems such as supply and drain pipes and appliance frames. When a conductive system is to be electrically connected to the physical ground (earth), one puts the equipment bonding conductor and the grounding electrode conductor at the same potential (for example, see §Metal water pipe as grounding electrode below).

A  (GEC) is used to connect the system grounded ("neutral") conductor, or the equipment to a grounding electrode, or a point on the grounding electrode system. This is called "system grounding" and most electrical systems are required to be grounded. The U.S. NEC and the UK's BS 7671 list systems that are required to be grounded. According to the NEC, the purpose of connecting an electrical system to the physical ground (earth) is to limit the voltage imposed by lightning events and contact with higher voltage lines. In the past, water supply pipes were used as grounding electrodes, but due to the increased use of plastic pipes, which are poor conductors, the use of a specific grounding electrode is often mandated by regulating authorities. The same type of ground applies to radio antennas and to lightning protection systems.

Permanently installed electrical equipment, unless not required to, has permanently connected grounding conductors. Portable electrical devices with metal cases may have them connected to earth ground by a pin on the attachment plug (see AC power plugs and sockets). The size of power grounding conductors is usually regulated by local or national wiring regulations.

Bonding
Strictly speaking, the terms grounding or earthing are meant to refer to an electrical connection to ground/earth. Bonding is the practice of intentionally electrically connecting metallic items not designed to carry electricity. This brings all the bonded items to the same electrical potential as a protection from electrical shock. The bonded items can then be connected to ground to eliminate foreign voltages.

Earthing systems
In electricity supply systems, an earthing (grounding) system defines the electrical potential of the conductors relative to that of the Earth's conductive surface. The choice of earthing system has implications for the safety and electromagnetic compatibility of the power supply. Regulations for earthing systems vary considerably between different countries.

A functional earth connection serves more than protecting against electrical shock, as such a connection may carry current during the normal operation of a device. Such devices include surge suppression, electromagnetic-compatibility filters, some types of antennas, and various measurement instruments. Generally the protective earth system is also used as a functional earth, though this requires care.

Impedance grounding
Distribution power systems may be solidly grounded, with one circuit conductor directly connected to an earth grounding electrode system. Alternatively, some amount of electrical impedance may be connected between the distribution system and ground, to limit the current that can flow to earth. The impedance may be a resistor, or an inductor (coil). In a high-impedance grounded system, the fault current is limited to a few amperes (exact values depend on the voltage class of the system); a low-impedance grounded system will permit several hundred amperes to flow on a fault. A large solidly grounded distribution system may have tens of thousands of amperes of ground fault current.

In a polyphase AC system, the instantaneous vector sum of the phases is zero. This neutral point is commonly used to refer the phase voltages to earth ground instead of connecting one of the phase conductors to earth. Any Δ-Y (delta-wye) connected transformer may be used for the purpose. A nine winding transformer (a "zig zag" transformer) may be used to balance the phase currents of a delta connected source with an unbalanced load.

Low-resistance grounding systems use a neutral grounding resistor (NGR) to limit the fault current to 25 A or greater. Low resistance grounding systems will have a time rating (say, 10 seconds) that indicates how long the resistor can carry the fault current before overheating. A ground fault protection relay must trip the breaker to protect the circuit before overheating of the resistor occurs.

High-resistance grounding (HRG) systems use an NGR to limit the fault current to 25 A or less. They have a continuous rating, and are designed to operate with a single-ground fault. This means that the system will not immediately trip on the first ground fault. If a second ground fault occurs, a ground fault protection relay must trip the breaker to protect the circuit. On an HRG system, a sensing resistor is used to continuously monitor system continuity. If an open-circuit is detected (e.g., due to a broken weld on the NGR), the monitoring device will sense voltage through the sensing resistor and trip the breaker. Without a sensing resistor, the system could continue to operate without ground protection (since an open circuit condition would mask the ground fault) and transient overvoltages could occur.

Ungrounded systems
Where the danger of electric shock is high, special ungrounded power systems may be used to minimize possible leakage current to ground. Examples of such installations include patient care areas in hospitals, where medical equipment is directly connected to a patient and must not permit any power-line current to pass into the patient's body. Medical systems include monitoring devices to warn of any increase of leakage current. On wet construction sites or in shipyards, isolation transformers may be provided so that a fault in a power tool or its cable does not expose users to shock hazard.

Circuits used to feed sensitive audio/video production equipment or measurement instruments may be fed from an isolated ungrounded technical power system to limit the injection of noise from the power system.

Power transmission
In single-wire earth return (SWER) AC electrical distribution systems, costs are saved by using just a single high voltage conductor for the power grid, while routing the AC return current through the earth. This system is mostly used in rural areas where large earth currents will not otherwise cause hazards.

Some high-voltage direct-current (HVDC) power transmission systems use the ground as second conductor. This is especially common in schemes with submarine cables, as sea water is a good conductor. Buried grounding electrodes are used to make the connection to the earth. The site of these electrodes must be chosen carefully to prevent electrochemical corrosion on underground structures.

A particular concern in design of electrical substations is earth potential rise. When very large fault currents are injected into the earth, the area around the point of injection may rise to a high potential with respect to points distant from it. This is due to the limited finite conductivity of the layers of soil in the earth of the substation. The gradient of the voltage (the change in voltage across the distance to the injection point) may be so high that two points on the ground may be at significantly different potentials. This gradient creates a hazard to anyone standing on the earth in an area of the electrical substation that is insufficiently insulated from ground. Pipes, rails, or communication wires entering a substation may see different ground potentials inside and outside the substation, creating a dangerous touch voltage for unsuspecting persons who might touch those pipes, rails, or wires. This problem is alleviated by creating a low-impedance equipotential bonding plane installed in accordance with IEEE 80, within the substation. This plane eliminates voltage gradients and ensures that any fault is cleared within three voltage cycles.

Electronics

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 * Signal ground
 * Chassis ground
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 * Earth ground

Signal grounds serve as return paths for signals and power (at extra-low voltages, less than about 50 V) within equipment, and on the signal interconnections between equipment. Many electronic designs feature a single return that acts as a reference for all signals. Power and signal grounds often get connected, usually through the metal case of the equipment. Designers of printed circuit boards must take care in the layout of electronic systems so that high-power or rapidly switching currents in one part of a system do not inject noise into low-level sensitive parts of a system due to some common impedance in the grounding traces of the layout.

Circuit ground versus earth
Voltage is defined as the difference of electric potentials between points in an electric field. A voltmeter is used to measure the potential difference between some point and a reference point. This common reference point is denoted "ground" and considered to have zero potential. Signals are defined with respect to signal ground, which may be connected to a power ground. A system where the system ground is not connected to another circuit or to earth (in which there may still be AC coupling between those circuits) is often referred to as a floating ground, and may correspond to Class 0 or Class II appliances.

Functional grounds
Some devices require a connection to the mass of earth to function correctly, as distinct from any purely protective role. Such a connection is known as a functional earth- for example some long wavelength antenna structures require a functional earth connection, which generally should not be indiscriminately connected to the supply protective earth, as the introduction of transmitted radio frequencies into the electrical distribution network is both illegal and potentially dangerous. Because of this separation, a purely functional ground should not normally be relied upon to perform a protective function. To avoid accidents, such functional grounds are normally wired in white, cream or pink cable, and not green or green/yellow.

Separating low signal ground from a noisy ground
In television stations, recording studios, and other installations where signal quality is critical, a special signal ground known as a "technical ground" (or "technical earth", "special earth", and "audio earth") is often installed, to prevent ground loops. This is basically the same thing as an AC power ground, but no general appliance ground wires are allowed any connection to it, as they may carry electrical interference. For example, only audio equipment is connected to the technical ground in a recording studio. In most cases, the studio's metal equipment racks are all joined with heavy copper cables (or flattened copper tubing or busbars) and similar connections are made to the technical ground. Great care is taken that no general chassis grounded appliances are placed on the racks, as a single AC ground connection to the technical ground will destroy its effectiveness. For particularly demanding applications, the main technical ground may consist of a heavy copper pipe, if necessary fitted by drilling through several concrete floors, such that all technical grounds may be connected by the shortest possible path to a grounding rod in the basement.

Radio frequency ground
Certain types of radio antennas (or their feedlines) require a connection to ground that functions adequately at radio frequencies. The required caliber of grounding system is called a radio frequency ground.

Since the radio frequencies of the current in antennas are far higher than the 50 or 60 Hz frequency of the power line, radio grounding systems use different principles than AC power grounding. The "protective earth" (PE) safety ground wires in AC utility building wiring were not designed for, and cannot be used as an adequate substitute for an RF ground. The long utility ground wires have high impedance at certain frequencies. In the case of a transmitter, the RF current flowing through the ground wires can radiate radio frequency interference and induce hazardous voltages on grounded metal parts of other appliances, so separate ground systems are used.

Monopole antennas operating at lower frequencies, below 20 MHz, use the surface of the Earth as a part of the antenna, as a conductive plane to reflect the radio waves and provide a return path for electric fields extending from the antenna. The monopoles include the mast radiator used by AM radio stations, and the 'T' and inverted 'L' antenna, and umbrella antenna. The feedline from the transmitter is connected between the antenna and ground, so it requires a grounding (earthing) system under the antenna to make contact with the soil to collect the return current. The ground system also functions as a capacitor plate, to receive the displacement current from the antenna and return it to the ground side of the transmitter's feedline, so it is preferably located directly under the antenna. In receivers and low efficiency / low power transmitters, the ground connection can be as simple as one or several metal rods or stakes driven into the soil, or an electrical connection to a building's metal water piping which extends into the earth. However, in transmitting antennas the ground system carries the full output current of the transmitter, so the resistance of an inadequate ground contact can be a major loss of transmitter power.

Medium to high power transmitters usually have an extensive ground system consisting of bare copper cables buried in the earth under the antenna, to lower resistance. Since for the omnidirectional antennas used on these bands the Earth currents travel radially toward the ground point from all directions, the grounding system usually consists of a radial pattern of buried cables extending outward under the antenna in all directions, connected together to the ground side of the transmitter's feedline at a terminal next to the base of the antenna. The transmitter power lost in the ground resistance, and so the efficiency of the antenna, depends on the soil conductivity. This varies widely; marshy ground or ponds, particularly salt water, provide the lowest resistance ground, while dry rocky or sandy soil are the highest. The power loss per square meter in the ground is proportional to the square of the transmitter current density flowing in the earth. The current density, and power dissipated, increases the closer one gets to the ground terminal at the base of the antenna, so the radial ground system can be thought of as providing a higher conductivity medium, copper, for the ground current to flow through, in the parts of the ground carrying high current density, to reduce power losses.

Design
A standard ground system widely used for mast radiator broadcasting antennas operating in the MF and LF bands consists of 120 equally-spaced, buried, radial ground wires extending out one quarter of a wavelength ($$\ \tfrac{ 1 }{ 4 } \lambda\ $$, or 90 electrical degrees) from the antenna base. AWG 8 to AWG 10 soft-drawn copper wire is typically used, buried 4–10 inches deep. For AM broadcast band antennas this requires a circular land area extending from the mast 47-136 meter. This is usually planted with grass, which is kept mowed short, as tall grass can increase power loss in certain circumstances. If the land area available is too limited for such long radials, they can in many cases be adequately replaced by a greater number of shorter radials, or a smaller number of longer radials.

In transmitting antennas a second cause of power wastage is dielectric power losses of the electric field (displacement current) of the antenna passing through the earth to reach the ground wires. For antennas near a half-wavelength high (180 electrical degrees) the antenna has a voltage maximum (antinode) near its base, which results in strong electric fields in the earth above the ground wires near the mast where the displacement current enters the ground. To reduce this loss these antennas often use a conductive copper ground screen under the antenna connected to the buried ground wires, either lying on the ground or elevated a few feet, to shield the ground from the electric field.

In a few cases where rocky or sandy soil has too high a resistance for a buried ground, a counterpoise is used. This is a radial network of wires similar to that in a buried ground system, but lying on the surface or suspended a few feet above the ground. It acts as a capacitor plate, capacitively coupling the feedline to conductive layers of the soil.

Electrically short antennas
At lower frequencies the resistance of the ground system is a more critical factor because of the small radiation resistance of the antenna. In the LF and VLF bands, construction height limitations require that electrically short antennas be used, shorter than the fundamental resonant length of one quarter of a wavelength ($$\ \tfrac{ 1 }{ 4 } \lambda\ $$). A quarter wave monopole has a radiation resistance of around 25~36 ohms, but below $$\ \tfrac{ 1 }{ 4 } \lambda\ $$ the resistance decreases with the square of the ratio of height to wavelength. The power fed to an antenna is split between the radiation resistance, which represents power emitted as radio waves, the desired function of the antenna, and the ohmic resistance of the ground system, which results in power wasted as heat. As the wavelength gets longer in relation to antenna height, the radiation resistance of the antenna decreases so the ground resistance constitutes a larger proportion of the input resistance of the antenna and consumes more of the transmitter power. Antennas in the VLF band often have a resistance of less than 1 ohm, and even with extremely low resistance ground systems 50% to 90% of the transmitter power may be wasted in the ground system.

Lightning protection systems
Lightning protection systems are designed to mitigate the effects of lightning through connection to extensive grounding systems that provide a large surface area connection to earth. The large area is required to dissipate the high current of a lightning strike without damaging the system conductors by excess heat. Since lightning strikes are pulses of energy with very high frequency components, grounding systems for lightning protection tend to use short straight runs of conductors to reduce the self-inductance and skin effect.

Ground (earth) mat
In an electrical substation a ground (earth) mat is a mesh of conductive material installed at places where a person would stand to operate a switch or other apparatus; it is bonded to the local supporting metal structure and to the handle of the switchgear, so that the operator will not be exposed to a high differential voltage due to a fault in the substation.

In the vicinity of electrostatic sensitive devices, a ground (earth) mat or grounding (earthing) mat is used to ground static electricity generated by people and moving equipment. There are two types used in static control: Static Dissipative Mats, and Conductive Mats.

A static dissipative mat that rests on a conductive surface (commonly the case in military facilities) are typically made of 3 layers (3-ply) with static dissipative vinyl layers surrounding a conductive substrate which is electrically attached to ground (earth). For commercial uses, static dissipative rubber mats are traditionally used that are made of 2 layers (2-ply) with a tough solder resistant top static dissipative layer that makes them last longer than the vinyl mats, and a conductive rubber bottom. Conductive mats are made of carbon and used only on floors for the purpose of drawing static electricity to ground as quickly as possible. Normally conductive mats are made with cushioning for standing and are referred to as "anti-fatigue" mats.

For a static dissipative mat to be reliably grounded it must be attached to a path to ground. Normally, both the mat and the wrist strap are connected to ground by using a common point ground system (CPGS).

In computer repair shops and electronics manufacturing, workers must be grounded before working on devices sensitive to voltages capable of being generated by humans. For that reason static dissipative mats can be and are also used on production assembly floors as "floor runner" along the assembly line to draw static generated by people walking up and down.

Isolation
Isolation is a mechanism that defeats grounding. It is frequently used with low-power consumer devices, and when engineers, hobbyists, or repairmen are working on circuits that would normally be operated using the power line voltage. Isolation can be accomplished by simply placing a "1:1 wire ratio" transformer with an equal number of turns between the device and the regular power service, but applies to any type of transformer using two or more coils electrically insulated from each other.

For an isolated device, touching a single powered conductor does not cause a severe shock, because there is no path back to the other conductor through the ground. However, shocks and electrocution may still occur if both poles of the transformer are contacted by bare skin. Previously it was suggested that repairmen "work with one hand behind their back" to avoid touching two parts of the device under test at the same time, thereby preventing a current from crossing through the chest and interrupting cardiac rhythms or causing cardiac arrest.

Generally every AC power line transformer acts as an isolation transformer, and every step up or down has the potential to form an isolated circuit. However, this isolation would prevent failed devices from blowing fuses when shorted to their ground conductor. The isolation that could be created by each transformer is defeated by always having one leg of the transformers grounded, on both sides of the input and output transformer coils. Power lines also typically ground one specific wire at every pole, to ensure current equalization from pole to pole if a short to ground is occurring.

In the past, grounded appliances have been designed with internal isolation to a degree that allowed the simple disconnection of ground by cheater plugs without apparent problem (a dangerous practice, since the safety of the resulting floating equipment relies on the insulation in its power transformer). Modern appliances however often include power entry modules which are designed with deliberate capacitive coupling between the AC power lines and chassis, to suppress electromagnetic interference. This results in a significant leakage current from the power lines to ground. If the ground is disconnected by a cheater plug or by accident, the resulting leakage current can cause mild shocks, even without any fault in the equipment. Even small leakage currents are a significant concern in medical settings, as the accidental disconnection of ground can introduce these currents into sensitive parts of the human body. As a result, medical power supplies are designed to have low capacitance.

Class II appliances and power supplies (such as cell phone chargers) do not provide any ground connection, and are designed to isolate the output from input. Safety is ensured by double-insulation, so that two failures of insulation are required to cause a shock.

title: Ground (electricity)


text:
Ground constants

In telecommunication, ground constants are the electrical parameters of earth: electrical conductivity, σ, electrical permittivity, ε, and magnetic permeability, μ.

The values of these parameters vary with the local chemical composition and density of the Earth. For a propagating electromagnetic wave, such as a surface wave propagating along the surface of the Earth, these parameters vary with frequency and direction.

title: Ground constants


text:
Ground loop

Ground loop may refer to:
 * Ground loop (electricity), an unwanted electric current that flows in a conductor connecting two points inadvertently having different potentials
 * Ground loop (aviation), the rapid circular rotation of an aircraft in the horizontal plane while on the ground
 * Ground-coupled heat exchanger, an underground heat exchanger loop that can capture or dissipate heat to or from the ground

title: Ground loop


text:
Ground plane

In electrical engineering, a ground plane is an electrically conductive surface, usually connected to electrical ground.

The term has two different meanings in separate areas of electrical engineering.

 * In antenna theory, a ground plane is a conducting surface large in comparison to the wavelength, such as the Earth, which is connected to the transmitter's ground wire and serves as a reflecting surface for radio waves.

 * In printed circuit boards, a ground plane is a large area of copper foil on the board which is connected to the power supply ground terminal and serves as a return path for current from different components on the board.

Radio antenna theory
In telecommunication, a ground plane is a flat or nearly flat horizontal conducting surface that serves as part of an antenna, to reflect the radio waves from the other antenna elements. The plane does not necessarily have to be connected to ground to be used as a reflecting surface for radio waves. Ground plane shape and size play major roles in determining its radiation characteristics including gain.

To function as a ground plane, the conducting surface must be at least a quarter of the wavelength ($1⁄4$ $λ$) of the radio waves in radius. In lower frequency antennas, such as the mast radiators used for broadcast antennas, the Earth itself (or a body of water such as a salt marsh or ocean) is used as a ground plane. For higher frequency antennas, in the VHF or UHF range, the ground plane can be smaller, and metal disks, screens and wires are used as ground planes. At upper VHF and UHF, the metal skin of a car or aircraft can serve as a ground plane for whip antennas projecting from it. In microstrip antennas and printed monopole antennas an area of copper foil on the opposite side of a printed circuit board serves as a ground plane. The ground plane need not be a continuous surface. In the ground plane antenna style whip antenna, the "plane" consists of several wires $1⁄4$ $λ$ long radiating from the base of a quarter-wave whip antenna.

The radio waves from an antenna element that reflect off a ground plane appear to come from a mirror image of the antenna located on the other side of the ground plane. In a monopole antenna, the radiation pattern of the monopole plus the virtual "image antenna" make it appear as a two element center-fed dipole antenna. So a monopole mounted over an ideal ground plane has a radiation pattern identical to a dipole antenna. The feedline from the transmitter or receiver is connected between the bottom end of the monopole element and the ground plane. The ground plane must have good conductivity; any resistance in the ground plane is in series with the antenna, and serves to dissipate power from the transmitter.

Printed circuit boards

A ground plane on a printed circuit board (PCB) is a large area or layer of copper foil connected to the circuit's ground point, usually one terminal of the power supply. It serves as the return path for current from many different components.

A ground plane is often made as large as possible, covering most of the area of the PCB which is not occupied by circuit traces. In multilayer PCBs, it is often a separate layer covering the entire board. This serves to make circuit layout easier, allowing the designer to ground any component without having to run additional traces; component leads needing grounding are routed directly through a hole in the board to the ground plane on another layer. The large area of copper also conducts the large return currents from many components without significant voltage drops, ensuring that the ground connection of all the components are at the same reference potential.

In digital and radio frequency PCBs, the major reason for using large ground planes is to reduce electrical noise and interference through ground loops and to prevent crosstalk between adjacent circuit traces. When digital circuits switch state, large current pulses flow from the active devices (transistors or integrated circuits) through the ground circuit. If the power supply and ground traces have significant impedance, the voltage drop across them may create noise voltage pulses that disturb other parts of the circuit (ground bounce). The large conducting area of the ground plane has much lower impedance than a circuit trace, so the current pulses cause less disturbance.

In addition, a ground plane under printed circuit traces can reduce crosstalk between adjacent traces. When two traces run parallel, an electrical signal in one can be coupled into the other through electromagnetic induction by magnetic field lines from one linking the other; this is called crosstalk. When a ground plane layer is present underneath, it forms a transmission line with the trace. The oppositely-directed return currents flow through the ground plane directly beneath the trace. This confines most of the electromagnetic fields to the area near the trace and consequently reduces crosstalk.

A power plane is often used in addition to a ground plane in a multilayer circuit board, to distribute DC power to the active devices. The two facing areas of copper create a large parallel plate decoupling capacitor that prevents noise from being coupled from one circuit to another through the power supply.

Ground planes are sometimes split and then connected by a thin trace. This allows the separation of analog and digital sections of a board or the inputs and outputs of amplifiers. The thin trace has low enough impedance to keep the two sides very close to the same potential while keeping the ground currents of one side from coupling into the other side, causing ground loop.

title: Ground plane


text:
Ground wave

Ground waves are radio waves propagating parallel to and adjacent to the surface of the Earth, following the curvature of the Earth beyond the visible horizon. This radiation is known as Norton surface wave, or more properly Norton ground wave, because ground waves in radio propagation are not confined to the surface.

The normal line-of-sight distance to the horizon is about 80 to 100 kilometers (50 to 60 miles) at best, for an antenna on a tall tower. But signals at medium or low frequency are observed to travel several hundred miles beyond the horizon, up to several thousand kilometers in some cases. This "hugging the ground" effect is called ground wave, as opposed to direct wave or sky wave. This effect does not occur for signals in the VHF or UHF frequencies, which typically propagate only about 100 km (60 miles) to the horizon.

AM radio stations rely on ground wave propagation to cover their listening areas, which may be a several hundred miles radius.

Overview
Lower frequency radio waves, below 3 MHz, travel efficiently as ground waves. In ITU nomenclature, this includes (in order): medium frequency (MF), low frequency (LF), very low frequency (VLF), ultra low frequency (ULF), super low frequency (SLF), extremely low frequency (ELF) waves.

Ground propagation works because lower-frequency waves are more strongly diffracted around obstacles due to their long wavelengths, allowing them to repeatedly bend downward to follow the Earth's curvature.

Do not confuse with refraction. Refraction also causes radio waves to bend toward the ground, slightly extending propagation distance, but refraction alone generally only extends the radio horizon by about 15% (4/3), not enough to explain the much longer distances that ground waves are observed to propagate. Refraction decreases at lower frequencies, whereas diffraction increases at lower frequencies. Medium-wave and Long-wave signals are of such long wavelength that mountain ridges and low hills act as a knife edge, diffracting signals downward into the valleys behind such obstacles. The horizon itself may act as the obstacle around which long waves diffract, as when low-frequency waves are observed to travel thousands of kilometers across open oceans, where no sharp-edged obstacles exist.

Ground waves propagate in vertical polarization, with their magnetic field horizontal and electric field (close to) vertical.

Conductivity of the surface affects the propagation of ground waves, with more conductive surfaces such as sea water providing better propagation. Increasing the conductivity in a surface results in less dissipation. The refractive indices are subject to spatial and temporal changes. Since the ground is not a perfect electrical conductor, ground waves are attenuated as they follow the earth's surface. The wavefronts initially are vertical, but the ground, acting as a lossy dielectric, causes the wave to tilt forward as it travels. This directs some of the energy into the earth where it is dissipated, so that the signal decreases exponentially.

Applications
Most long-distance LF "longwave" radio communication (between 30 kHz and 300 kHz) is a result of groundwave propagation. Mediumwave radio transmissions (frequencies between 300 kHz and 3000 kHz), including AM broadcast band, travel both as groundwaves and, for longer distances at night, as skywaves. Ground losses become lower at lower frequencies, greatly increasing the coverage of AM stations using the lower end of the band. The VLF and LF frequencies are mostly used for military communications, especially with ships and submarines. The lower the frequency the better the waves penetrate sea water. ELF waves (below 3 kHz) have even been used to communicate with deeply submerged submarines.

Ground waves have been used in over-the-horizon radar, which operates mainly at frequencies between 2–20 MHz over the sea, which has a sufficiently high conductivity to convey them to and from a reasonable distance (up to 100 km or more; over-horizon radar also uses skywave propagation at much greater distances). In the development of radio, ground waves were used extensively. Early commercial and professional radio services relied exclusively on long wave, low frequencies and ground-wave propagation. To prevent interference with these services, amateur and experimental transmitters were restricted to the high frequencies (HF), felt to be useless since their ground-wave range was limited. Upon discovery of the other propagation modes possible at medium wave and short wave frequencies, the advantages of HF for commercial and military purposes became apparent. Amateur experimentation was then confined to only authorized frequencies in the range.

Related terms
Mediumwave and shortwave reflect off the ionosphere at night, which is known as skywave. During daylight hours, the lower D layer of the ionosphere forms and absorbs lower frequency energy. This prevents skywave propagation from being very effective on mediumwave frequencies in daylight hours. At night, when the D layer dissipates, mediumwave transmissions travel better by skywave. Ground waves do not include ionospheric and tropospheric waves.

title: Ground wave


text:
Group alerting and dispatching system

In telecommunication, a group alerting and dispatching system is a service feature that (a) enables a controlling telephone to place a call to a specified number of telephones simultaneously, (b) enables the call to be recorded, (c) if any of the called lines is busy, enables the equipment to camp on until the busy line is free, and (d) rings the free line and plays the recorded message.

title: Group alerting and dispatching system


text:
Talk:Chronometer

Does not Rolex mass-produce chronometers?

Not that I wouldn't take a fake Rolex's timekeeping over a genuine one's.

Rolex makes 400,000 watches a year. I think that statement is simply WRONG. Most mechanical watches are NOT custom made.

What is mass-production? what is custom made? Ever since Waltham Watch Company started showing the way to produce very precise interchangeable watch parts the whole watch production has been based on increasingly sophisticated machinery, everywhere. However, the assembly of a standard mechanical movement (including chronometer quality) is still made by hand, with the assistance of a "chain of assembly", rationalising the work, by trained previously unskilled (no watchmaker) crafts. The watchmakers concentrate their efforts to the final regulating of the movement and final control of the watch and overseeing the whole assembling procedure.

Today's artisan Master Watchmaker shall show his skills in developing new complications, respectively simplify existing ones on the drawing board (all computerised) and confy the manufacturing of the necessary pieces to machinery. Of course, those very complicated watches shall be singly assembled one by one by very experienced watchmakers.

I hope to have brought some new light in your perception and stay at your disposal for any further information.

I also feel that the conclusion of the last paragraph of with regards to quartz and atomic clocks is false: even if in U.S.A. all those techniques have been lost, since no watch company could survive there, those techniques are being applied, used & perfected daily in the other watch producing countries like Switzerland, Japan, India, Germany, and Greater China, I shall think about and propose a modification. Claude Girardin

I have made now some modification on the page CHRONOMETER, please check also COSC and you shall almost know everything about today's wristwatch chronometers. Claude girardin

Thanks for the insight. Glad to have you watching over this page.

Comatose51 04:06, 19 Apr 2005 (UTC)

Today's Chronometer Manufacturing: Rolex and all others
Please check on CHRONOMETER, last paragraph and click or search COSC and you shall have more information about today's criterias and checking instution delivering the COSC Officially Certified Chronometer Certificates ---Claude girardin---

Offensive?
"Used by the ugly navigator Tianyang Sun the chronometer has been used for centuries in history."

This line could be considered offensive to some, including Asian minorities where they are a minority in the world, like in North America. What do you mean by ugly? I'm sure ther's a reason for this but This sentence is redundant and grammatically incorrect anyway; this should be edited to explain what ugly is or change the word so that it less ambiguous. Does the author mean "crude" instead of "ugly"? Even "crude" is offensive. Why does ugly modify Tianyang? Should it be a modifier for "navigator"? If so, perhaps this sentence should read:

"The chronometer was first used by the navigator Tianyang Sun in a primive way, and for centuries since the chronometer has been improved and perfected."

Regardless, that kind of statement is highly subjective and should not be in an encyclopedia. It is borderline vandalism.

Comatose51 04:08, 19 Apr 2005 (UTC)

Does the section on complications belong in the chronograph article?

I also have a problem with the section on the balance wheel--My understanding is that for the most part, a temprature-compensated balance is primarily to compensate for the hairspring rather than the balance itself. Invar, Elinvar and Nivarox are alloys used for hairsprings to reduce temprature errors, usually combined with glucydur alloy for the balance wheel--I'm not familiar with Invar as balance material, not that it hasn't been used.

Pqdave 20:53, 11 January 2006 (UTC)

No, complications don't belong here. Chonometers don't use any of them, for the simple reason that chronometers are designed to excel at one job: precision timekeeping. The only exception I can think of is "power reserve" if by that the writer meant an indicator that shows the amount of running time left on the mainspring. All chronometers have that, for obvious reasons. It's not a complication really, it's mechanically trivial. Paul Koning

Cornhill
In the late 18th and early nineteenth century the main centre for the production of chronometers was Cornhill, London. Makers with an address on that street include James McCabe, Barraud, N Hart and Sons, J Sewill, RD Webster, John Alexander Lund and unexpectedly, a female chronometer maker, Eliza Andrews.

Complications
The term chronometer, despite how it may have been employed by the marketing departments of various watch companies, implies a timepiece pure and simple with its only purpose being to keep as steady a rate as possible. The section on 'Complications' is out of place under this heading and should be transferred to watches. Peter R Hastings 13:24, 13 February 2007 (UTC)

Split it?
I think this article currently mixes two topics: 1: marine chronometers, 2: watches called "chronometer". They are very different beasts. For example, you will never see "complications" on a marine chronometer, but they are common on a fancy watch. Also, marine chronometers are (were) a tool; "chronometer" watches are luxury goods. Paul Koning 17:57, 11 March 2007 (UTC)

 * I created marine chronometer from the portion of this article that covers that topic. I added a few words there to mention maintaining power and power reserve (not a "complication" but a required feature in a marine chronometer).  I also took out the sentence about the use of gold and other exotic materials; I don't believe that applies to marine chronometers.  And I flagged the claim that diamond is used in jewel bearings -- as I understand it diamond is in fact unsuited for that purpose and is not used. Paul Koning 14:56, 9 April 2007 (UTC)
 * I also created chronometer watch in the same way. It came out rather short; there probably is an opportunity to flesh it out. Paul Koning 15:13, 9 April 2007 (UTC)

title: Talk:Chronometer