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Line Of Sight
The line of sight, also known as visual axis or sightline (also sight line), is an imaginary line between a viewer/observer/spectator's eye(s) and a subject of interest, or their relative direction. The subject may be any definable object taken note of or to be taken note of by the observer, at any distance more than least distance of distinct vision. In optics, refraction of a ray due to use of lenses can cause distortion. Shadows, patterns and movement can also influence line of sight interpretation (as in optical illusions). The term "line" typically presumes that the light by which the observed object is seen travels as a straight ray, which is sometimes not the case as light can take a curved/angulated path when reflected from a mirror, refracted by a lens or density changes in the traversed media, or deflected by a gravitational field. Fields of study feature specific targets, such as vessels in navigation, marker flags or natural features in surveying, celestial object ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Imaginary Line
In general, an imaginary line is usually any sort of geometric line that has only an abstract definition and does not physically exist. In fact, they are used to properly identify places on a map. Some outside geography do exist, such as the Mendoza Line, which in baseball divides below-average hitters and extremely poor hitters. A ''centerline'' is a nautical term for a line down the center of a vessel lengthwise. Examples Geography As a geographical concept, an imaginary line may serve as an arbitrary division, such as * Antarctic Circle, * Arctic Circle, * Border, * International Date Line, * Latitude, including the Equator, * Longitude, the prime meridian, the Tropic of Capricorn and the Tropic of Cancer. Any axis about which an object spins is an imaginary line. * Mason–Dixon line, which informally marks pieces of the borders of four U.S. states: Delaware, Maryland, Pennsylvania, and West Virginia, once part of Virginia. Symbolically, the line separates the Ame ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematically, density is defined as mass divided by volume: : \rho = \frac where ''ρ'' is the density, ''m'' is the mass, and ''V'' is the volume. In some cases (for instance, in the United States oil and gas industry), density is loosely defined as its weight per unit volume, although this is scientifically inaccurate – this quantity is more specifically called specific weight. For a pure substance the density has the same numerical value as its mass concentration. Different materials usually have different densities, and density may be relevant to buoyancy, purity and packaging. Osmium and iridium are the densest known elements at standard conditions for temperature and pressure. To simplify comparisons of density across different s ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Double Star
In observational astronomy, a double star or visual double is a pair of stars that appear close to each other as viewed from Earth, especially with the aid of optical telescopes. This occurs because the pair either forms a binary star (i.e. a binary system of stars in mutual orbit, gravitationally bound to each other) or is an ''optical double'', a chance line-of-sight alignment of two stars at different distances from the observer. Binary stars are important to stellar astronomers as knowledge of their motions allows direct calculation of stellar mass and other stellar parameters. The only (possible) case of "binary star" whose two components are separately visible to the naked eye is the case of Mizar and Alcor (though actually a multiple-star system), but it is not known for sure whether Mizar and Alcor are gravitationally bound. Since the beginning of the 1780s, both professional and amateur double star observers have telescopically measured the distances and angles between d ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Line-of-sight Velocity
The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the rate of change of the distance or range between the two points. It is equivalent to the vector projection of the target-observer relative velocity onto the relative direction connecting the two points. In astronomy, the point is usually taken to be the observer on Earth, so the radial velocity then denotes the speed with which the object moves away from the Earth (or approaches it, for a negative radial velocity). Formulation Given a differentiable vector \mathbf \in \mathbb^3 defining the instantaneous position of a target relative to an observer. Let with \mathbf \in \mathbb^3, the instantaneous velocity of the target with respect to the observer. The magnitude of the position vector \mathbf is defined as The quantity range rate is the time derivative of the magnitude (norm) of \mathbf, expressed as Substituting () into () : \fra ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Line-of-sight Fire
Direct fire or line-of-sight fire refers to firing of a ranged weapon whose projectile is launched directly at a target within the line-of-sight of the user. The firing weapon must have a sighting device and an unobstructed view to the target, which means no obstacles or friendly units can be between it and the target. A weapon engaged in direct fire conversely exposes itself to direct return fire from the target.p.49, Bailey This is in contrast to indirect fire, which refers to firing a projectile on a curved ballistic trajectory or delivering self-accelerated munitions capable of long range and various degrees of homing abilities to alter the flight path. Indirect fire does not need a direct line-of-sight to the target because the shots are normally directed by a forward observer. As such, indirect-fire weapons can shoot over obstacles or friendly units and the weapons can be concealed from counter-battery fire. Description Examples of direct-fire weapons include most anci ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Non-line-of-sight Propagation
Non-line-of-sight (NLOS) radio propagation occurs outside of the typical line-of-sight (LOS) between the transmitter and receiver, such as in ground reflections. Near-line-of-sight (also NLOS) conditions refer to partial obstruction by a physical object present in the innermost Fresnel zone. Obstacles that commonly cause NLOS propagation include buildings, trees, hills, mountains, and, in some cases, high voltage electric power lines. Some of these obstructions reflect certain radio frequencies, while some simply absorb or garble the signals; but, in either case, they limit the use of many types of radio transmissions, especially when low on power budget. Lower power levels at a receiver reduce the chance of successfully receiving a transmission. Low levels can be caused by at least three basic reasons: low transmit level, for example Wi-Fi power levels; far-away transmitter, such as 3G more than away or TV more than away; and obstruction between the transmitter and the re ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Line-of-sight Propagation
Line-of-sight propagation is a characteristic of electromagnetic radiation or acoustic wave propagation which means waves travel in a direct path from the source to the receiver. Electromagnetic transmission includes light emissions traveling in a straight line. The rays or waves may be diffracted, refracted, reflected, or absorbed by the atmosphere and obstructions with material and generally cannot travel over the horizon or behind obstacles. In contrast to line-of-sight propagation, at low frequency (below approximately 3 MHz) due to diffraction, radio waves can travel as ground waves, which follow the contour of the Earth. This enables AM radio stations to transmit beyond the horizon. Additionally, frequencies in the shortwave bands between approximately 1 and 30 MHz, can be refracted back to Earth by the ionosphere, called skywave or "skip" propagation, thus giving radio transmissions in this range a potentially global reach. However, at frequencies above ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Line-of-sight (missile)
In missile guidance, line of sight (LOS) is the line directly between the launcher/tacker and the target, which must be in view. If the target is moving the missile is constantly manoeuvred to be on the line of sight as it changes, this results in the missile flying a curved path to the target (see command to line-of-sight). The same path is flown as in pursuit guidance Pursuit guidance, or a pursuit course, is a form of guidance widely used in older guided missiles. In pursuit guidance, the missile is steered so that the velocity vector of the missile always points at the target, i.e. it has always the directio ... but without the missile having a seeker. No ranger data is used in the control and no prediction of intercept location made. Literature * Tactical and Strategic Missile Guidance, Paul Zarchan, American Institute of Aeronautics and Astronautics Inc. References * Missile operation Missile technology Tracking Targeting (warfare) {{Missile-stub ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Line-of-sight Range
In radio electronics, especially radar terminology, slant range or slant distance is the distance along the relative direction between two points. If the two points are at the same level (relative to a specific datum), the slant distance equals the horizontal distance. An example of slant range is the distance to an aircraft flying at high altitude with respect to that of the radar antenna. The slant range (1) is the hypotenuse of the triangle represented by the altitude of the aircraft and the distance between the radar antenna and the aircraft's ground track (point (3) on the earth directly below the aircraft). In the absence of altitude information, for example from a height finder, the aircraft location would be plotted farther (2) from the antenna than its actual ground track. See also *Ranging *Spherical range The great-circle distance, orthodromic distance, or spherical distance is the distance along a great circle. It is the shortest distance between two points on ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Sightline (architecture)
In architecture, sightlines are a particularly important consideration in the design of civic structures, such as a stage, arena, or monument. They determine the configuration of such items as theater and stadium design, road junction layout and urban planning. In cities such as London, construction within sightlines is restricted to protect the key views of famous landmarks Subjects that have a line of sight with one another are said to be intervisible, where intervisibility is the ability of viewers at separate places to see each other without any landform blocking their view. C-Value Good sightlines allow spectators to see all areas of a venue stage or field of play. To ensure this designers utilize the C-value, defined as the vertical distance from a spectator's eyes to sightline of the spectator directly behind. The C-value is determined in part by the rake, that is, the upward slope of the seating. The stadium bowl rake if based on consistent C-values will follow half ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Gravitational Lens
A gravitational lens is a distribution of matter (such as a cluster of galaxies) between a distant light source and an observer that is capable of bending the light from the source as the light travels toward the observer. This effect is known as gravitational lensing, and the amount of bending is one of the predictions of Albert Einstein's general theory of relativity. Treating light as corpuscles travelling at the speed of light, Newtonian physics also predicts the bending of light, but only half of that predicted by general relativity. Although Einstein made unpublished calculations on the subject in 1912, Orest Khvolson (1924) and Frantisek Link (1936) are generally credited with being the first to discuss the effect in print. However, this effect is more commonly associated with Einstein, who published an article on the subject in 1936. Fritz Zwicky posited in 1937 that the effect could allow galaxy clusters to act as gravitational lenses. It was not until 1979 that this ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Optical Medium
An optical medium is material through which light and other electromagnetic waves propagate. It is a form of transmission medium. The permittivity and permeability of the medium define how electromagnetic waves propagate in it. Properties The optical medium has an '' intrinsic impedance'', given by ::\eta = where E_x and H_y are the electric field and magnetic field, respectively. In a region with no electrical conductivity, the expression simplifies to: ::\eta = \sqrt\ . For example, in free space the intrinsic impedance is called the characteristic impedance of vacuum, denoted ''Z''0, and ::Z_0 = \sqrt\ . Waves propagate through a medium with velocity c_w = \nu \lambda , where \nu is the frequency and \lambda is the wavelength of the electromagnetic waves. This equation also may be put in the form : c_w = \ , where \omega is the angular frequency of the wave and k is the wavenumber of the wave. In electrical engineering, the symbol \beta, called the ''phase constant'', ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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