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Over-the-horizon
Over-the-horizon radar (OTH), sometimes called beyond the horizon radar (BTH), is a type of radar system with the ability to detect targets at very long ranges, typically hundreds to thousands of kilometres, beyond the radar horizon, which is the distance limit for ordinary radar. Several OTH radar systems were deployed starting in the 1950s and 1960s as part of early warning radar systems, but these have generally been replaced by airborne early warning systems. OTH radars have recently been making a comeback, as the need for accurate long-range tracking becomes less important with the ending of the Cold War, and less-expensive ground-based radars are once again being considered for roles such as maritime reconnaissance and drug enforcement. Technology The frequency of radio waves used by most radars, in the form of microwaves, travel in straight lines. This generally limits the detection range of radar systems to objects on their horizon (generally referred to as "line of ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
ROTHR USNavy A
Over-the-horizon radar (OTH), sometimes called beyond the horizon radar (BTH), is a type of radar system with the ability to detect targets at very long ranges, typically hundreds to thousands of kilometres, beyond the radar horizon, which is the distance limit for ordinary radar. Several OTH radar systems were deployed starting in the 1950s and 1960s as part of early warning radar systems, but these have generally been replaced by airborne early warning systems. OTH radars have recently been making a comeback, as the need for accurate long-range tracking becomes less important with the ending of the Cold War, and less-expensive ground-based radars are once again being considered for roles such as maritime reconnaissance and drug enforcement. Technology The frequency of radio waves used by most radars, in the form of microwaves, travel in straight lines. This generally limits the detection range of radar systems to objects on their horizon (generally referred to as "line of sig ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
OTH-B
Over-the-horizon radar (OTH), sometimes called beyond the horizon radar (BTH), is a type of radar system with the ability to detect targets at very long ranges, typically hundreds to thousands of kilometres, beyond the radar horizon, which is the distance limit for ordinary radar. Several OTH radar systems were deployed starting in the 1950s and 1960s as part of early warning radar systems, but these have generally been replaced by airborne early warning systems. OTH radars have recently been making a comeback, as the need for accurate long-range tracking becomes less important with the ending of the Cold War, and less-expensive ground-based radars are once again being considered for roles such as maritime reconnaissance and drug enforcement. Technology The frequency of radio waves used by most radars, in the form of microwaves, travel in straight lines. This generally limits the detection range of radar systems to objects on their horizon (generally referred to as "line of sig ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
ROTHR USNavy B
Over-the-horizon radar (OTH), sometimes called beyond the horizon radar (BTH), is a type of radar system with the ability to detect targets at very long ranges, typically hundreds to thousands of kilometres, beyond the radar horizon, which is the distance limit for ordinary radar. Several OTH radar systems were deployed starting in the 1950s and 1960s as part of early warning radar systems, but these have generally been replaced by airborne early warning systems. OTH radars have recently been making a comeback, as the need for accurate long-range tracking becomes less important with the ending of the Cold War, and less-expensive ground-based radars are once again being considered for roles such as maritime reconnaissance and drug enforcement. Technology The frequency of radio waves used by most radars, in the form of microwaves, travel in straight lines. This generally limits the detection range of radar systems to objects on their horizon (generally referred to as "line of sig ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Radio Wave
Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz (GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (shorter than a grain of rice); at 30 Hz the corresponding wavelength is (longer than the radius of the Earth). Like all electromagnetic waves, radio waves in a vacuum travel at the speed of light, and in the Earth's atmosphere at a close, but slightly lower speed. Radio waves are generated by charged particles undergoing acceleration, such as time-varying electric currents. Naturally occurring radio waves are emitted by lightning and astronomical objects, and are part of the blackbody radiation emitted by all warm objects. Radio waves are generated artificially by an electronic device called a transmitter, which is connected to an antenna which radiates the waves. They are received by another antenna connected to a radio receiver, which p ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Shortwave
Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (100 to 10 metres); above the medium frequency band (MF), to the bottom of the VHF band. Radio waves in the shortwave band can be reflected or refracted from a layer of electrically charged atoms in the atmosphere called the ionosphere. Therefore, short waves directed at an angle into the sky can be reflected back to Earth at great distances, beyond the horizon. This is called skywave or "skip" propagation. Thus shortwave radio can be used for communication over very long distances, in contrast to radio waves of higher frequency, which travel in straight lines (line-of-sight propagation) and are limited by the visual horizon, about 64 km (40 miles). Shortwave broadcasts of radio programs played an important role in the early days of radio ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Radar
Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. A radar system consists of a transmitter producing electromagnetic waves in the radio or microwaves domain, a transmitting antenna, a receiving antenna (often the same antenna is used for transmitting and receiving) and a receiver and processor to determine properties of the objects. Radio waves (pulsed or continuous) from the transmitter reflect off the objects and return to the receiver, giving information about the objects' locations and speeds. Radar was developed secretly for military use by several countries in the period before and during World War II. A key development was the cavity magnetron in the United Kingdom, which allowed the creation of relatively small systems with sub-meter resolution. Th ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Early Warning Radar
An early-warning radar is any radar system used primarily for the long-range detection of its targets, i.e., allowing defences to be alerted as ''early'' as possible before the intruder reaches its target, giving the air defences the maximum time in which to operate. This contrasts with systems used primarily for tracking or gun laying, which tend to offer shorter ranges but offer much higher accuracy. EW radars tend to share a number of design features that improve their performance in the role. For instance, EW radar typically operates at lower frequencies, and thus longer wavelengths, than other types. This greatly reduces their interaction with rain and snow in the air, and therefore improves their performance in the long-range role where their coverage area will often include precipitation. This also has the side-effect of lowering their optical resolution, but this is not important in this role. Likewise, EW radars often use much lower pulse repetition frequency to maximi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Radar Horizon
The radar horizon is a critical area of performance for aircraft detection systems that is defined by the distance at which the radar beam rises enough above the Earth's surface to make detection of a target at low level impossible. It is associated with the low elevation region of performance, and its geometry depends on terrain, radar height, and signal processing. This is associated with the notions of ''radar shadow'', the ''clutter zone'', and the ''clear zone''. Airborne objects can exploit the radar shadow zone and clutter zone to avoid radar detection by using a technique called nap-of-the-earth navigation. Definition Without taking into account the refraction through the atmosphere, the radar horizon would be the geometrical distance D_h from the radar to the horizon only taking into account the height H of the radar above sea-level, and the radius of the earth R_e (approximately 6.4·103 km): :D_h = \sqrt When H is small compared to R_e, this can be approxima ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Naval Research Laboratory
The United States Naval Research Laboratory (NRL) is the corporate research laboratory for the United States Navy and the United States Marine Corps. It was founded in 1923 and conducts basic scientific research, applied research, technological development and prototyping. The laboratory's specialties include plasma physics, space physics, materials science, and tactical electronic warfare. NRL is one of the first US government scientific R&D laboratories, having opened in 1923 at the instigation of Thomas Edison, and is currently under the Office of Naval Research. As of 2016, NRL was a Navy Working Capital Fund activity, which means it is not a line-item in the US Federal Budget. Instead of direct funding from Congress, all costs, including overhead, were recovered through sponsor-funded research projects. NRL's research expenditures were approximately $1 billion per year. Research The Naval Research Laboratory conducts a wide variety of basic research and applied r ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Chirp Transmitter
An ionosonde, or chirpsounder, is a special radar for the examination of the ionosphere. The basic ionosonde technology was invented in 1925 by Gregory Breit and Merle A. Tuve and further developed in the late 1920s by a number of prominent physicists, including Edward Victor Appleton. The term '' ionosphere'' and hence, the etymology of its derivatives, was proposed by Robert Watson-Watt. Components An ionosonde consists of: * A high frequency (HF) radio transmitter, automatically tunable over a wide range. Typically the frequency coverage is 0.5–23 MHz or 1–40 MHz, though normally sweeps are confined to approximately 1.6–12 MHz. * A tracking HF receiver which can automatically track the frequency of the transmitter. * An antenna with a suitable radiation pattern, which transmits well vertically upwards and is efficient over the whole frequency range used. * Digital control and data analysis circuits. The transmitter sweeps all or part of the HF freque ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Radar Horizon
The radar horizon is a critical area of performance for aircraft detection systems that is defined by the distance at which the radar beam rises enough above the Earth's surface to make detection of a target at low level impossible. It is associated with the low elevation region of performance, and its geometry depends on terrain, radar height, and signal processing. This is associated with the notions of ''radar shadow'', the ''clutter zone'', and the ''clear zone''. Airborne objects can exploit the radar shadow zone and clutter zone to avoid radar detection by using a technique called nap-of-the-earth navigation. Definition Without taking into account the refraction through the atmosphere, the radar horizon would be the geometrical distance D_h from the radar to the horizon only taking into account the height H of the radar above sea-level, and the radius of the earth R_e (approximately 6.4·103 km): :D_h = \sqrt When H is small compared to R_e, this can be approxima ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Skywave
In radio communication, skywave or skip refers to the propagation of radio waves reflected or refracted back toward Earth from the ionosphere, an electrically charged layer of the upper atmosphere. Since it is not limited by the curvature of the Earth, skywave propagation can be used to communicate beyond the horizon, at intercontinental distances. It is mostly used in the shortwave frequency bands. As a result of skywave propagation, a signal from a distant AM broadcasting station, a shortwave station, or – during sporadic E propagation conditions (principally during the summer months in both hemispheres) a distant VHF FM or TV station – can sometimes be received as clearly as local stations. Most long-distance shortwave (high frequency) radio communication – between 3 and 30 MHz – is a result of skywave propagation. Since the early 1920s amateur radio operators (or "hams"), limited to lower transmitter power than broadcast stations, have ta ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
