The over-the-horizon radar , or OTH (sometimes called outside the horizon , or BTH ), is a type a radar system with the ability to detect targets over a very long range, usually hundreds to thousands of kilometers, beyond the radar horizon, which is the distance limit for ordinary radar. Some OTH radar systems were deployed from the 1950s and 1960s as part of an early warning radar system, but these have been largely superseded by airborne early warning systems. OTH radars have recently made a comeback, because the need for accurate remote tracking becomes less important with the end of the Cold War, and cheaper ground-based radar is once again being considered for roles such as surveillance and maritime drug implementation.
Video Over-the-horizon radar
Technology
The super high-frequency radio waves used by most radars, called microwaves, move in a straight line. This generally limits the range of detection of radar systems to objects in their horizons (commonly referred to as "line of sight" because the aircraft must be at least theoretically visible to a person at the location and elevation of the radar transmitter) due to curvature on earth. For example, a radar mounted on a 10 m (33 ft) pole has a range to the horizon about 13 kilometers (8.1 mi), taking into account the effects of atmospheric refraction. If the target is above the surface, this range will increase accordingly, so the target 10 m (33 ft) high can be detected by the same radar at 26 km (16 mi). Placement of antennas on high mounts can increase the range somewhat; but, in general, it is not practical to build a radar system with a range of sight lines beyond a few hundred kilometers.
OTH radar uses a variety of techniques to look beyond that limit. Two of the most commonly used techniques; shortwave systems that reflect their signals from the ionosphere for very long distance detection, and surface wave systems, which use high frequency radio waves which, due to diffraction, follow the curvature of the Earth to reach beyond the horizon. The system reaches a detection range of a hundred kilometers from a small conventional radar installation. They can scan a series of microwave frequencies using a chirp transmitter.
Skywave System
The most common types of OTH radar use sky wave propagation or "skip", where shortwave radio waves are reflected from ionized layers in the atmosphere, the ionosphere. Under certain conditions in the atmosphere, radio signals transmitted at an angle to the sky will be reflected to the ground by the ionosphere, allowing them to return to Earth beyond the horizon. A small number of these signals will be scattered from the desired target back to the sky, reflecting back the ionosphere, and returning to the receiving antenna with the same path. Only one frequency range regularly exhibits this behavior: high frequency (HF) or short-wave portion of the spectrum from 3-30 MHz. The best frequency to use depends on current atmospheric conditions and sunspot cycles. For this reason, systems using skywaves typically use real-time monitoring of backscattered signal reception to continuously adjust the frequency of transmitted signals.
Any radar resolution depends on beam width and range to target. As an example; radar with a 1-degree ray width and a target in the range of 120 km (75 mi) will show a target of 2 km (1.2 mi). To produce a 1 degree beam at the most common frequency, a 1.5-kilometer (0.93Ã, mi) wide antenna is required. Because of the physics of the reflection process, actual accuracy is even lower, with a range resolution of 20 to 40 kilometers (12-25 mi) and bearing accuracy of 2 to 4 kilometers (1.2-2.5 mi) recommended. Even 2 km accuracy is only useful for early warning, not for firearms.
Another problem is that the reflection process depends greatly on the angle between the signal and the ionosphere, and is generally limited to about 2-4 degrees from the local horizon. Creating blocks at this angle generally requires a very large array of antennas and highly reflective soil along the transmitted signal path, often enhanced by wire mesh lengthening of 3 kilometers (1.9 mi) in front of the antenna. The OTH system is thus very expensive to build, and is essentially immobile.
Given the losses on each reflection, the "backscatter" signal is very small, which is one of the reasons why the OTH radar is not practical until the 1960s, when very low sound amplifiers were first designed. Since the reflected signal from the ground, or the ocean, will be very large compared to the reflected signal of the "target", some systems need to be used to distinguish the targets from background noise. The easiest way to do this is to use the Doppler effect, which uses a frequency shift created by moving objects to measure its speed. By filtering all backscatter signals close to the original transmission frequency, the moving target becomes visible. Even a small amount of movement can be seen using this process, speeds as low as 1.5 knots (2.8 km/h).
This basic concept is used in almost all modern radars, but in the case of the OTH system it becomes much more complex because of the similar effects introduced by the ionosphere movement. Most systems use both transmitter broadcasts directly in the ionosphere to measure their movement and adjust the primary radar returns in real-time. Doing so required computer use, another reason the OTH system did not become completely practical until the 1960s, with the introduction of a high-performance solid-state system.
The ground wave system
The second type of OTH radar uses a much lower frequency, in long-wave bands. Radio waves at these frequencies can diffract around obstacles and follow the contours of the earth's arch, walking beyond the horizon. Echos bounces off the target back to the transmitter location with the same path. This ground wave has the longest distance over the sea. Like the high-frequency ionosphere system, the signals received from this ground wave system are very low, and demand highly sensitive electronics. Because these signals move closer to the surface, and lower frequencies result in lower resolutions, low frequency systems are generally used to track ships, rather than airplanes. However, the use of bistatik techniques and computer processing can result in higher resolution, and has been used since the 1990s.
Maps Over-the-horizon radar
History
Engineers in the Soviet Union are known to have developed what appeared to be the first operational OTH system in 1949, called "Veyer". However, little information about the system is available in western sources, and no details of its operations are known. It is well known that no further research was undertaken by the Soviet team until the 1960s and 1970s.
Much preliminary research into an effective OTH system was performed under Dr. William J. Thaler at the Naval Research Laboratory. The work was dubbed "Teepee Project" (for "Project Thaler"). Their first experimental system, MUSIC ( Storage, Integration, and Multiple Correlation ), began operation in 1955 and was able to detect the rocket launch of 600 miles (970 km) on Cape Canaveral , and a nuclear explosion in Nevada at 1,700 miles (2,700 km). The highly improved system, testbed for operational radar, was built in 1961 as a MADRE ( Magnetic-Drum Radar Equipment ) at Chesapeake Bay. The aircraft detects aircraft as far as 3,000 kilometers (1,900 mi) using 50 kW of broadcast energy.
As the name suggests, both NRL systems depend on the ratio of the returned signal stored on the magnetic drum. In an attempt to remove the clutter from the radar screen, many end-war and post-war radar systems add an acoustic delay line that stores the received signal for the right amount of time for the next signal pulse to arrive. By adding a newly arrived signal to the inverse version of the signal stored in the delay line, the output signal only includes changes from one pulse to the next pulse. It removes all static reflections, such as nearby hills or other objects, leaving only a moving object, such as an airplane. This basic concept will work for long range radar as well, but has the problem that the delay line must be mechanically measured by the frequency of pulse repetition from the radar, or PRF. For long-term use, PRF is very long to start, and is intentionally changed to make different ranges appear. For this role, delay lines can not be used, and magnetic drums, recently introduced, provide easy and easily controlled variable delay systems.
Another early shortwave OTH system was built in Australia in the early 1960s. It consists of several antennas positioned into four separate wavelengths, allowing the system to use a phase-shifting beam shift to direct the direction of sensitivity and adjust it to cover Singapore, Calcutta and the UK. The system consumes 25 miles (40 km) of power lines in the antenna array.
OTH System
UK/US Cobra Mist
The first operational development really is the Anglo-American system known as the Cobra Mist, which began construction in the late 1960s. Cobra Mist uses a very large 10MW transmitter and can detect aircraft in the western Soviet Union from its location in Suffolk. When system testing started in 1972, an unexpected sound source made it largely unusable. They left the site in 1973, a noise source never identified.
Other early British/US systems of the same era include:
- installations at RAF Akrotiri in Cyprus and Okinawa.
- Sugar Tree radar system.
AS. Air Force
The United States Air Force Laboratory of Rome had the first complete success with their AN/FPS-118 OTH-B . The prototype with 1 MW transmitter and separate receiver installed in Maine, offering coverage of more than 60 degrees between 900 and 3,300 km arcs. A permanent transmission facility was later built at AFS Moscow, a receiving facility at Columbia Falls AFS, and an operational center between them in Bangor, Maine. Coverage can be extended with additional recipients, providing a complete coverage of more than 180 degrees arc (each 60 degree section known as "sector").
GE Aerospace was awarded a development contract, expanding the existing eastern coastal system with two additional sectors, while building another three-sector system on the west coast, a two-sector system in Alaska, and a south-facing one sector system. In 1992, the Air Force contracted to extend the range of 15 degrees clockwise in the southern part of the three east coastal sectors to be able to cover the southeastern US border. In addition, the range is extended to 3,000 miles (4,800 km), across the equator. It's operated 40 hours a week randomly. Radar data fed to US Customs/Coast Guard C3I Center, Miami; Joint Working Center 4 Operations Centers, Key West; US Southern Command Operations Center, Key West; and the US Southern Command Operations Center, Panama.
With the end of the Cold War, the influence of two senators from Maine was not enough to save the operation and Alaska as well as the south-facing sites were canceled, two west sectors so far completed and that the east had been turned off and placed in "warm storage," allowing them to used again if needed. In 2002, the west coast facilities were downgraded to "cold storage" status, meaning only minimal maintenance was done by the caretaker.
Research begins to the feasibility of facility removal. Following the period of public input and environmental studies, in July 2005, the US Air Force Airborne Command published "Final Environmental Assessment for Elimination of Equipment on the Radar Backscatter Over-the-Horizon - West Coast Facility". The final decision was made to remove all radar equipment on the west coast sector transmitting sites outside of Christmas Valley, Oregon and receiving locations near Tulelake, California. This work was completed in July 2007 with the dismantling and removal of the antenna arrangement, leaving the building, fence and utility infrastructure at each site intact.
The US Navy created their own system, AN/TPS-71 ROTHR ( Relocatable Over-the-Horizon Radar ), which includes a 64-degree spike area in the range of 500 up to 1,600 nautical miles (925 to 3,000 km). ROTHR was originally intended to monitor the movement of ships and aircraft over the Pacific, and thus enable the movement of the fleet well coordinated before the engagement. The ROTHR prototype system was installed on the isolated Aleutian Amchitka Island, Alaska, monitoring Russia's east coast, in 1991 and used until 1993. The equipment was then transferred to storage. The first production system was installed at the test site in Virginia for acceptance testing, but was later diverted to counter illegal drug trafficking, which included Central America and the Caribbean. The second production of ROTHR was later established in Texas, covering much of the same region in the Caribbean, but also providing coverage in the Pacific as far south as Colombia. It also operates in the role of anti-drug trade. The third, and final, production system is installed in Puerto Rico, extending anti-drug surveillance over the equator, deep into South America.
USSR/Russia
Beginning in the early 1950s, the Soviets also studied the OTH system. Their first experimental model seems to be Veyer (Hand Fan), built in 1949. The next serious Soviet project is Duga , built outside Nikolayev on the shore of the Black Sea. near Odessa. Aimed eastwards, Duga first ran on November 7, 1971, and successfully used to track missile launches from the Far East and the Pacific Ocean to the test site at Novaya Zemlya.
This was followed by their first operating system Duga-1 , known in the west as Steel Yard , first broadcast in 1976. Built outside Gomel, near Chernobyl, it addressed to the north and covered the continent of the United States. Its loud and repeated pulsations in the midst of shortwave radio waves cause it to be known as "Russian Woodpecker" by amateur radio operators (ham). The Soviets eventually shifted the frequencies they used, without recognizing them even the source, largely because of their interference with certain fixed air-to-ground communications used by commercial aircraft. The second system established in Siberia, also includes the continents of the United States and Alaska.
In early 2014, Russia announced a new system, called the Container , which saw more than 3000 km.
Podsolnuh (Sunflower) - shortwave short-range radar station-shoreline. Designed to detect surface and air targets at a distance of 450 km. Designed for use in surface and air control systems of coastal situations within 200-mile economic zone. "Sunflower" allows operators to automatically transcend radio horizons simultaneously detect, track and classify up to 300 offshore objects and 100 air, determine their coordinates and provide them with complex targeting and weapons systems of ships and air defense systems. Radar has passed the state test in 2008. It is currently in charge of three stations - in the Sea of ​​Okhotsk, Sea of ​​Japan, and Caspian Sea.
Australia
The more recent addition is the Jindalee Operational Radar Network developed by the Australian Department of Defense in 1998 and completed in 2000. It is operated by the No. 1 Radar Surveillance Unit of the Royal Australian Air Force. Jindalee is a multistatic radar system (multi-receiver) using OTH-B, which allows it to have both remote and anti-stealth capabilities. It has an official range of 3,000 kilometers (1,900 mi), but in 1997 the prototype was able to detect missile launch by China over 5,500 kilometers (3,400 miles) away.
Jindalee uses 560 kW compared to 1 MW OTH-B United States, but offers a much better range than the US system in the 1980s, due to much better electronic and signal processing.
French
France developed an OTH radar called NOSTRADAMUS during the 1990s (NOSTRADAMUS stands for New Transhorizon Decametric System Applying the Studio Method (French: nouveau systÃÆ'¨me transhorizon dÃÆ' Â © camÃÆ' Â © trique appliquant les In March 1999, the OTH NOSTRADAMUS radar was said to have detected two Northrop B2 spirits flying into Kosovo. It entered service for the French army in 2005, and is still under development. It is based on the field of star-shaped antennas, used for emission and reception (monostatic), and can detect aircraft over distances of 2,000 kilometers, in 360 degree arcs. The frequency range used is 6 to 30 MHz.
Officially launched in 2009, the French STRADIVARIUS research project is developing a new High-Frequency Surface Wave Radar (HFSWR) capable of monitoring maritime traffic up to 200 nautical miles offshore. A demonstration site operates since January 2015 on the Mediterranean coast of France to showcase the 24/7 capabilities of the system now offered for sale by DIGINEXT.
China
A number of OTH-B and OTH-SW radar are reported to operate in China. Some details are known about this system. However, the transmission of these radars caused many disruptions to other licensed users.
A set of Chinese OTH-B radar is found on Google maps for transmitter and receiver.
Iran
Iran is working on an OTH radar called Sepehr , with a reported range of 3,000 kilometers. Currently operational.
Alternative OTH App
Another common radar over-horizon application uses surface waves, also known as ground waves. The ground wave provides a propagation method for intermediate wave AM broadcasting below 1.6 MHz and other transmissions at lower frequencies. Wind wave propagation provides a rapidly decaying signal at distances that are farther above the ground and many broadcast stations have limited coverage. However, seawater, with its high conductivity, supports ground waves up to 100 kilometers (62 mi) or more. This type of radar, OTH surface wave, is used for surveillance, and operates most commonly between 4 and 20 MHz. Lower frequencies enjoy better propagation but radar reflections are worse than small targets, so there is usually an optimum frequency that depends on the target type.
A different approach to radar over the horizon is to use propagating waves or electromagnetic surface waves at much lower frequencies. The creeping wave is scattering to the back of the object due to diffraction, which is the reason both ears can hear sounds on one side of the head, for example, and how initial communication and radio broadcasting are achieved. In the radar role, a questionable wave of wonder is around the Earth, although processing the returned signal is difficult. The development of the system became practical in the late 1980s due to the rapidly increasing processing power. Such a system is known as OTH-SW , for Surface Waves .
The first OTH-SW system used seems to be a Soviet system positioned to monitor traffic in the Sea of ​​Japan. The newer system was recently used for coastal surveillance in Canada, and is now offered for sale by Thales as a Coast Watcher . Australia has also deployed High Frequency Surface Radar Radar.
Note
References
- Quote
- References
External links
- Radar Systems in Shortwave, extensive list of OTH and similar radar systems compiled by Wolf Hadel, August 2013
- Canadian Perspective on High Frequency Over-the-Horizon Radar - paper by R. J. Riddolls, Defense R & amp; D Canada, Ottawa
- Development of Over-the-Horizon Radar in Australia - paper by D.H. Sinnott on the Australian Department of Defense website
- Google maps links - Russia's "Steel-Yard" radar near Chernobyl.
Source of the article : Wikipedia
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