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Over-the-horizon radar (OTH), sometimes called beyond the horizon radar (BTH), is a type of
radar Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...
system with the ability to detect targets at very long ranges, typically hundreds to thousands of kilometres, beyond the
radar horizon The radar horizon is a critical area of performance for air traffic, aircraft detection systems, defined by the distance at which the radar beam rises enough above the Earth's surface to make detection of a target at the lowest level possible. I ...
, 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 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 tim ...
systems, but
airborne early warning An airborne early warning and control (AEW&C) system is an airborne radar early warning system designed to detect aircraft, ships, vehicles, missiles and other incoming projectiles at long ranges, as well as performing command and control of t ...
systems have generally replaced these. OTH radars have recently been making a comeback, as the need for accurate long-range tracking has become less important since the ending of the
Cold War The Cold War was a period of global Geopolitics, geopolitical rivalry between the United States (US) and the Soviet Union (USSR) and their respective allies, the capitalist Western Bloc and communist Eastern Bloc, which lasted from 1947 unt ...
, 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 wave Radio waves (formerly called Hertzian waves) are a type of electromagnetic radiation with the lowest frequencies and the longest wavelengths in the electromagnetic spectrum, typically with frequencies below 300 gigahertz (GHz) and wavelengths g ...
s used by most radars, in the form of
microwave Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequency, frequencies between 300&n ...
s, propagate in straight lines. This generally limits the detection range of radar systems to objects on their
horizon The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all viewing directions based on whethe ...
(generally referred to as "line of sight" since the aircraft must be at least theoretically visible to a person at the location and elevation of the radar transmitter) due to the curvature of the Earth. For example, a radar mounted on top of a mast has a range to the horizon of about , considering atmospheric refraction effects. If the target is above the surface, this range will be increased accordingly, so a target high can be detected by the same radar at . Siting the antenna on a high mountain can increase the range somewhat; but, in general, it is impractical to build radar systems with line-of-sight ranges beyond a few hundred kilometres. OTH radars use various techniques to see beyond that limit. Two techniques are most commonly used; shortwave systems that refract their signals off the
ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...
for very long-range detection, and
surface wave In physics, a surface wave is a mechanical wave that propagates along the Interface (chemistry), interface between differing media. A common example is gravity waves along the surface of liquids, such as ocean waves. Gravity waves can also occu ...
systems, which use low-frequency radio waves that, due to
diffraction Diffraction is the deviation of waves from straight-line propagation without any change in their energy due to an obstacle or through an aperture. The diffracting object or aperture effectively becomes a secondary source of the Wave propagation ...
, follow the curvature of the Earth to reach beyond the horizon. These systems achieve detection ranges of the order of a hundred kilometres from small, conventional radar installations. They can scan a series of high frequencies using a
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 ph ...
.


Skywave systems

The most common type of OTH radar, OTH-B (backscatter), uses
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 curvatur ...
or "skip" propagation, in which
shortwave Shortwave radio is radio transmission using radio frequencies in the shortwave bands (SW). There is no official definition of the band range, but it always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (app ...
radio waves are refracted off an
ionized Ionization or ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule i ...
layer in the atmosphere, the
ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...
, and return to Earth some distance away. A small amount of this signal will be scattered off desired targets back towards the sky, refracted off the ionosphere again, and return to the receiving antenna by the same path. Only one range of frequencies regularly exhibits this behaviour: the high frequency (HF) or
shortwave Shortwave radio is radio transmission using radio frequencies in the shortwave bands (SW). There is no official definition of the band range, but it always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (app ...
part of the
spectrum A spectrum (: spectra or spectrums) is a set of related ideas, objects, or properties whose features overlap such that they blend to form a continuum. The word ''spectrum'' was first used scientifically in optics to describe the rainbow of co ...
from 3–30 MHz. The best frequency to use depends on the conditions of the atmosphere and the
sunspot cycle The Solar cycle, also known as the solar magnetic activity cycle, sunspot cycle, or Schwabe cycle, is a periodic 11-year change in the Sun's activity measured in terms of variations in the number of observed sunspots on the Sun's surface. Ov ...
. For these reasons, systems using skywaves typically employ real-time monitoring of the reception of backscattered signals to continuously adjust the frequency of the transmitted signal. The resolution of any radar depends on the width of the beam and the range to the target. For example; a radar with 1 degree beam width and a target at range will show the target as wide. To produce a 1-degree beam at the most common frequencies, an antenna wide is required. Due to the physics of the refraction process, actual accuracy is even lower, with range resolution on the order of and bearing accuracy of being suggested. Even a 2 km accuracy is useful only for early warning, not for weapons fire. Another problem is that the refraction process is highly dependent on the angle between the signal and the ionosphere, and is generally limited to about 2–4 degrees off the local horizon. Making a beam at this angle generally requires enormous antenna arrays and highly reflective ground along the path the signal is being sent, often enhanced by the installation of wire mesh mats extending as much as in front of the antenna. OTH systems are thus very expensive to build, and essentially immobile. Given the losses at each refraction, this "backscatter" signal is extremely small, which is one reason why OTH radars were not practical until the 1960s, when extremely low-noise amplifiers were first being designed. Since the signal refracted from the ground, or sea, will be very large compared to the signal refracted from a "target", some system needs to be used to distinguish the targets from the background noise. The easiest way to do this is to use the
Doppler effect The Doppler effect (also Doppler shift) is the change in the frequency of a wave in relation to an observer who is moving relative to the source of the wave. The ''Doppler effect'' is named after the physicist Christian Doppler, who described ...
, which uses frequency shift created by moving objects to measure their velocity. By filtering out all the backscatter signal close to the original transmitted frequency, moving targets become visible. Even a small amount of movement can be seen using this process, speeds as low as . This basic concept is used in almost all modern radars, but in the case of OTH systems it becomes considerably more complex due to similar effects introduced by movement of the ionosphere. Most systems used a second transmitter broadcasting directly up at the ionosphere to measure its movement and adjust the returns of the main radar in real-time. Doing so required the use of
computer A computer is a machine that can be Computer programming, programmed to automatically Execution (computing), carry out sequences of arithmetic or logical operations (''computation''). Modern digital electronic computers can perform generic set ...
s, another reason OTH systems did not become truly practical until the 1960s, with the introduction of solid-state high-performance systems.


Ground wave systems

A second type of OTH radar, known as OTH-SW (surface wave), uses much lower frequencies, in the
longwave In radio, longwave (also spelled long wave or long-wave and commonly abbreviated LW) is the part of the radio spectrum with wavelengths longer than what was originally called the medium-wave (MW) broadcasting band. The term is historic, dati ...
bands. Radio waves at these frequencies can diffract around obstacles and follow the curving contour of the earth, traveling beyond the horizon. Echos reflected off the target return to the transmitter location by the same path. These ''
ground wave Ground wave is a mode of radio propagation that consists of currents traveling through the earth. Ground waves propagate parallel to and adjacent to the surface of the Earth, and are capable of covering long distances by diffracting around the E ...
s'' have the longest range over the sea. Like the ionospheric high-frequency systems, the received signal from these ground wave systems is very low, and demands extremely sensitive electronics. Because these signals travel close to the surface, and lower frequencies produce lower resolutions, low-frequency systems are generally used for tracking ships, rather than aircraft. However, the use of bistatic techniques and computer processing can produce higher resolutions, and has been used beginning in the 1990s.


Limitations

Radar clutter can degrade the ability of OTH to detect targets. Such clutter can be caused by atmospheric phenomena such as disturbances in the
ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...
caused by
geomagnetic storm A geomagnetic storm, also known as a magnetic storm, is a temporary disturbance of the Earth's magnetosphere that is driven by interactions between the magnetosphere and large-scale transient Plasma (physics), plasma and magnetic field structur ...
s or other
space weather Space weather is a branch of space physics and aeronomy, or heliophysics, concerned with the varying conditions within the Solar System and its heliosphere. This includes the effects of the solar wind, especially on the Earth's magnetosphere, ion ...
events. This phenomenon is especially apparent near the
geomagnetic pole The geomagnetic poles are antipodal points where the axis of a best-fitting dipole intersects the surface of Earth. This ''theoretical'' dipole is equivalent to a powerful bar magnet at the center of Earth, and comes closer than any other poi ...
s, where the action of the
solar wind The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...
on the earth’s
magnetosphere In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior Dynamo ...
produces convection patterns in the ionospheric plasma.


History

Engineers in the
Soviet Union The Union of Soviet Socialist Republics. (USSR), commonly known as the Soviet Union, was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 until Dissolution of the Soviet ...
are known to have developed what appears to be the first operational OTH system in 1949, called "Veyer". However, little information on this system is available in Western sources, and no details of its operation are known. It is known that no further research was carried out by Soviet teams until the 1960s and 70s. Much of the early research into effective OTH systems was carried out under the direction of Dr. William J. Thaler at the
United States 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. Located in Washington, DC, it was founded in 1923 and conducts basic scientific research, appl ...
(NRL). The work was dubbed "Project Teepee" (for "Thaler's Project"). Their first experimental system, MUSIC (''Multiple Storage, Integration, and Correlation''), became operational in 1955 and was able to detect rocket launches away at
Cape Canaveral Cape Canaveral () is a cape (geography), cape in Brevard County, Florida, in the United States, near the center of the state's Atlantic Ocean, Atlantic coast. Officially Cape Kennedy from 1963 to 1973, it lies east of Merritt Island, separated ...
, and nuclear explosions in
Nevada Nevada ( ; ) is a landlocked state in the Western United States. It borders Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. Nevada is the seventh-most extensive, th ...
at . A greatly improved system, a testbed for an operational radar, was built in 1961 as MADRE (''Magnetic-Drum Radar Equipment'') at
Chesapeake Bay The Chesapeake Bay ( ) is the largest estuary in the United States. The bay is located in the Mid-Atlantic (United States), Mid-Atlantic region and is primarily separated from the Atlantic Ocean by the Delmarva Peninsula, including parts of the Ea ...
. It detected aircraft as far as using as little as 50 kW of broadcast energy.Laurie states two ranges for MADRE against aircraft, , on the same page. The former appears to be correct from a comparison with other sources. To add to the confusion, ''Signals'' describes MADRE as having an ''average'' power of 100 kW and a peak of 5 MW, much more powerful than suggested by Laurie. See ''Signals'', Vol 31, Issue 1, p. 7. As the names imply, both of the NRL systems relied on the comparison of returned signals stored on magnetic drums. In an attempt to remove clutter from radar displays, many late-war and post-war radar systems added an
acoustic delay line Delay-line memory is a form of computer memory, mostly obsolete, that was used on some of the earliest digital computers, and is reappearing in the form of optical delay lines. Like many modern forms of electronic computer memory, delay-line mem ...
that stored the received signal for exactly the amount of time needed for the next signal pulse to arrive. By adding the newly arrived signal to an inverted version of the signals stored in the delay line, the output signal included just the ''changes'' from one pulse to the next. This removed any static reflections, like nearby hills or other objects, leaving only the moving objects, such as aircraft. This basic concept would work for a long-range radar as well but had the problem that a delay line has to be mechanically sized to the
pulse-repetition frequency The pulse-repetition frequency (PRF) is the number of pulses of a repeating signal in a specific time unit. The term is used within a number of technical disciplines, notably radar. In radar, a radio signal of a particular carrier frequency is tu ...
of the radar, or PRF. For long-range use, the PRF was very long to start, and deliberately changed to make different ranges come into view. For this role, the delay line was not usable, and the magnetic drum, recently introduced, provided a convenient and easily controlled variable-delay system. Another early shortwave OTH system was built in
Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
in the early 1960s. This consisted of several antennas positioned four wavelengths apart, allowing the system to use phase-shift
beamforming Beamforming or spatial filtering is a signal processing technique used in sensor arrays for directional signal transmission or reception. This is achieved by combining elements in an antenna array in such a way that signals at particular angles ...
to steer the direction of sensitivity and adjust it to cover Singapore, Calcutta, and the UK. This system consumed of electrical cable in the antenna array.


Systems


Australia

A more recent addition is the
Jindalee Operational Radar Network The Jindalee Operational Radar Network (JORN) is an over-the-horizon radar (OHR) network operated by the Royal Australian Air Force (RAAF) that can monitor air and sea movements across . It has a normal operating range of . The network is used ...
developed by the Australian Department of Defence in 1998 and completed in 2000. It is operated by No. 1 Radar Surveillance Unit of the
Royal Australian Air Force The Royal Australian Air Force (RAAF) is the principal Air force, aerial warfare force of Australia, a part of the Australian Defence Force (ADF) along with the Royal Australian Navy and the Australian Army. Constitutionally the Governor-Gener ...
. Jindalee is a
multistatic radar A multistatic radar system contains multiple spatially diverse monostatic radar or bistatic radar components with a shared area of coverage. An important distinction of systems based on these individual radar geometries is the added requirement f ...
(multiple-receiver) system using OTH-B, allowing it to have both long range as well as anti-
stealth Stealth may refer to: Military *Stealth technology, technology used to conceal ships, aircraft, and missiles **Stealth aircraft, aircraft which use stealth technology ** Stealth ground vehicle, ground vehicles which use stealth technology ** Ste ...
capabilities. It has an official range of , but in 1997 the prototype was able to detect missile launches by
China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...
over distant. Jindalee uses 560  kW compared to the United States' OTH-B's 1 MW, yet offers far better range than the U.S. 1980s system, due to the considerably improved electronics and signal processing.


Brazil

The OTH 0100 Radar is capable of monitoring vessels beyond away from shore, exceeding the direct line of sight of conventional radars.


Canada

Canada has been investigating the use of High Frequency Surface Wave Radar (HFSWR) for surveillance of the 200 nautical mile Exclusive Economic Zone (EEZ) for more than 30 years. Research was initiated in 1984 with the re-purposing of a decommissioned LORAN-A navigation beacon for undertaking experimentation in aircraft, vessel and iceberg tracking. Research continued for the next decade and in 1999, Canada’s installed two SWR503 HFSWR systems at Cape Race and Cape Bonavista, Newfoundland. The sites underwent a technology evaluation in 2000 and were subsequently upgraded and operationally evaluated in 2002. The following is a quote from the October 2002 Operational Evaluation (OPEVAL) performed by Canadian Department of National Defence: "HFSWR is a beneficial addition to the Recognized Maritime Picture (RMP). Of all the data sources evaluated, it was the only sensor offering near real-time information updates. It provided frequent reporting and generally demonstrated reliable tracking of surface targets in its area of coverage. When the HFSWR system was combined with other data sources, there was a synergistic effect that improved the overall quality of the RMP. Furthermore, from the analysis of the potential contribution to the surveillance-related Force Planning Scenarios, it was evident that the RMP would benefit from the addition of the HFSWR as a new data source." International sales of the SWR503 radar followed with operational systems installed in Asia (2008) and Europe (2009). In 2007 operation of the Canadian systems was halted due to concerns over the potential for harmful interference with primary spectrum users. In 2010 the unique capability of HFSWR to provide low cost surveillance of the EEZ resulted in a re-evaluation of the technology and subsequent development of a 3rd Generation, (3rd Gen) HFSWR system based on the principle of sense-and-adapt technology that enabled operation on a non-allocated, non-interference basis through the use of Dynamic Spectrum Management . Additional developments included improved range performance, better positional accuracy and reduction of false tracks and earlier track initiation. In June 2019, MAEROSPACE was granted a global license to design,, manufacture, and internationally market the Canadian HFSWR System and its derivatives. On 18 March 2025, Canadian prime minister
Mark Carney Mark Joseph Carney (born March 16, 1965) is a Canadian politician and economist who has served as the 24th and current Prime Minister of Canada, prime minister of Canada since 2025. He has served as Leader of the Liberal Party of Canada, lead ...
announced that Canada would purchase JORN radar technology from Australia, for deployment over the Arctic.


China

A number of OTH-B and OTH-SW radars are reportedly in operation in China. However, transmission from these radars causes much
interference Interference is the act of interfering, invading, or poaching. Interference may also refer to: Communications * Interference (communication), anything which alters, modifies, or disrupts a message * Adjacent-channel interference, caused by extra ...
to other international licensed users.Over-the-Horizon Backscatter Radar #91; OTH-B #93
/ref> One set of Chinese OTH-B radars in Wenzhou is found on Google Maps for th
transmitter
an
receiver
An OTH-B set was added in Inner Mongolia following the deployment of THAAD to South Korea.


France

France developed an OTH radar called NOSTRADAMUS during the 1990s (NOSTRADAMUS stands for New Transhorizon Decametric System Applying Studio Methods (French: ''nouveau système transhorizon décamétrique appliquant les méthodes utilisées en studio'').) In March 1999, the OTH radar NOSTRADAMUS was said to have detected two Northrop B-2 Spirits flying to Kosovo. It entered service for the French army in 2005 and is still in development. It is based on a star-shaped antenna field, used for emission and reception (monostatic), and can detect aircraft at a range of more than , in a 360-degree arc. The frequency range used is from 6 to 30 MHz. Launched officially in 2009, the Frenc
STRADIVARIUS research project
developed a new over-the-horizon radar (High Frequency Surface Wave Radar – HFSWR) capable of monitoring maritime traffic up to offshore. A demonstration site is operational since January 2015 on the French Mediterranean coast to showcase the 24/7 capabilities of the system that is now offered for sale by DIGINEXT.


India

India has developed a variety of long and short range radars. Although it doesn't have an operational over the horizon radar at present, India's Swordfish Long Range Tracking Radar, a part of India's missile defence system has a maximum range of currently being upgraded for .
DRDO The Defence Research and Development Organisation (DRDO) is an agency under the Department of Defence Research and Development in the Ministry of Defence of the Government of India, charged with the military's research and development, headqu ...
's LRDE is working on a prototype OTH radar. The design work of system is already complete and a prototype OTH is expected to realised by late 2021. The prototype will have two different types of arrays and itself will determine best frequency to use to track objects. After successful trials of existing system, India is expected to develop a large OTH radar based on same design.


Iran

Iran is working on an OTH radar called Sepehr, with a reported range of . It is currently operational.


Soviet Union/Russia

Beginning as early as the 1950s, the Soviet Union had also studied OTH systems. The first experimental model appears to be the Veyer (Hand Fan), which was built in 1949. The next serious Soviet project was Duga, built outside
Mykolaiv Mykolaiv ( ), also known as Nikolaev ( ) is a List of cities in Ukraine, city and a hromada (municipality) in southern Ukraine. Mykolaiv is the Administrative centre, administrative center of Mykolaiv Raion (Raions of Ukraine, district) and Myk ...
on the
Black Sea The Black Sea is a marginal sea, marginal Mediterranean sea (oceanography), mediterranean sea lying between Europe and Asia, east of the Balkans, south of the East European Plain, west of the Caucasus, and north of Anatolia. It is bound ...
coast near
Odessa ODESSA is an American codename (from the German language, German: ''Organisation der ehemaligen SS-Angehörigen'', meaning: Organization of Former SS Members) coined in 1946 to cover Ratlines (World War II aftermath), Nazi underground escape-pl ...
. Aimed eastward, Duga first ran on 7 November 1971, and was successfully used to track missile launches from the far east and Pacific Ocean to the testing ground on
Novaya Zemlya Novaya Zemlya (, also , ; , ; ), also spelled , is an archipelago in northern Russia. It is situated in the Arctic Ocean, in the extreme northeast of Europe, with Cape Flissingsky, on the northern island, considered the extreme points of Europe ...
. This was followed by the first operational system Duga-1, known in the west as Steel Yard, which first broadcast in 1976. Built outside Gomel, near
Chernobyl Chernobyl, officially called Chornobyl, is a partially abandoned city in Vyshhorod Raion, Kyiv Oblast, Ukraine. It is located within the Chernobyl Exclusion Zone, to the north of Kyiv and to the southwest of Gomel in neighbouring Belarus. ...
, it was aimed northward and covered the continental United States. Its loud and repetitive pulses in the middle of the shortwave radio bands led to its being known as the "Russian Woodpecker" by
amateur radio Amateur radio, also known as ham radio, is the use of the radio frequency radio spectrum, spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emer ...
(ham) operators. The Soviet Union eventually shifted the frequencies they used, without admitting they were even the source, largely due to its interference with certain long-range air-to-ground communications used by commercial airliners. A second system was set up near
Komsomolsk-on-Amur Komsomolsk-on-Amur ( rus, Комсомольск-на-Амуре, r=Komsomolsk-na-Amure, p=kəmsɐˈmolʲsk nɐ‿ɐˈmurʲə) is a types of inhabited localities in Russia, city in Khabarovsk Krai, Russia, located on the west bank of the Amur R ...
in the Russian Far East, also covering the continental United States and Alaska. In early 2014, Russia announced a new system, called
Container A container is any receptacle or enclosure for holding a product used in storage, packaging, and transportation, including shipping. Things kept inside of a container are protected on several sides by being inside of its structure. The term ...
, that was to see over 3000 km. Podsolnukh (Sunflower) - Coast-horizon shortwave station short-range radar. Designed to detect surface and air targets at a distance of . Designed for use in coastal surface and air control systems within the economic zone. "Sunflower" allows operators to automatically and simultaneously detect, track and classify up to 300 offshore and 100 air objects beyond the radio horizon, and provide their coordinates to the targeting systems and armaments of ships and air defense systems. The radar passed state tests in 2008. Three stations are on duty, in the
Sea of Okhotsk The Sea of Okhotsk; Historically also known as , or as ; ) is a marginal sea of the western Pacific Ocean. It is located between Russia's Kamchatka Peninsula on the east, the Kuril Islands on the southeast, Japan's island of Hokkaido on the sou ...
, the
Sea of Japan The Sea of Japan is the marginal sea between the Japanese archipelago, Sakhalin, the Korean Peninsula, and the mainland of the Russian Far East. The Japanese archipelago separates the sea from the Pacific Ocean. Like the Mediterranean Sea, it ...
, and the
Caspian Sea The Caspian Sea is the world's largest inland body of water, described as the List of lakes by area, world's largest lake and usually referred to as a full-fledged sea. An endorheic basin, it lies between Europe and Asia: east of the Caucasus, ...
.


USA and UK


UK/US Cobra Mist

The first truly operational development was an Anglo-American system known as
Cobra Mist Cobra Mist was the codename for an Anglo-American experimental over-the-horizon radar station at Orford Ness, England. It was known technically as AN/FPS-95 and sometimes referred to as System 441a; a reference to the project as a whole. Cobra ...
, which began construction in the late 1960s. Cobra Mist used an enormous 10 MW transmitter and could detect aircraft over the western Soviet Union from its location in
Suffolk Suffolk ( ) is a ceremonial county in the East of England and East Anglia. It is bordered by Norfolk to the north, the North Sea to the east, Essex to the south, and Cambridgeshire to the west. Ipswich is the largest settlement and the county ...
. When system testing started in 1972, however, an unexpected source of noise rendered it largely unusable. The source of the noise was never identified and the site was abandoned in 1973. Other early UK/US systems from the same era include: * an installation at
RAF Akrotiri Royal Air Force Akrotiri, commonly abbreviated RAF Akrotiri (; ) is a large Royal Air Force (RAF) military airbase on the Mediterranean island of Cyprus. It is located in the Western Sovereign Base Area, one of two areas which comprise Akroti ...
on
Cyprus Cyprus (), officially the Republic of Cyprus, is an island country in the eastern Mediterranean Sea. Situated in West Asia, its cultural identity and geopolitical orientation are overwhelmingly Southeast European. Cyprus is the List of isl ...
and
Okinawa most commonly refers to: * Okinawa Prefecture, Japan's southernmost prefecture * Okinawa Island, the largest island of Okinawa Prefecture * Okinawa Islands, an island group including Okinawa itself * Okinawa (city), the second largest city in th ...
. Cobra Shoe was a reported Over The Horizon (Backscatter) (OTH-B) radar designed by
RCA Corporation RCA Corporation was a major American electronics company, which was founded in 1919 as the Radio Corporation of America. It was initially a patent trust owned by General Electric (GE), Westinghouse, AT&T Corporation and United Fruit Company ...
, designed to monitor ballistic missile tests in the interior of the Soviet Union, installed in the Western Sovereign Base Area (Akrotiri),
Cyprus Cyprus (), officially the Republic of Cyprus, is an island country in the eastern Mediterranean Sea. Situated in West Asia, its cultural identity and geopolitical orientation are overwhelmingly Southeast European. Cyprus is the List of isl ...
. Source is "U.S. declassified documents". Installed since around 1964; no details on when/whether it left service. * the Sugar Tree radar system.


U.S. Air Force

The
United States Air Force The United States Air Force (USAF) is the Air force, air service branch of the United States Department of Defense. It is one of the six United States Armed Forces and one of the eight uniformed services of the United States. Tracing its ori ...
Rome Laboratory Rome Laboratory (Rome Air Development Center until 1991) is a U.S. Air Force research laboratory for " command, control, and communications" research and development and is responsible for planning and executing the USAF science and technology pr ...
had the first complete success with their AN/FPS-118 OTH-B.AN/FPS-118 Over-The-Horizon-Backscatter (OTH-B) Radar
/ref> A prototype with a 1 MW transmitter and a separate receiver was installed in
Maine Maine ( ) is a U.S. state, state in the New England region of the United States, and the northeasternmost state in the Contiguous United States. It borders New Hampshire to the west, the Gulf of Maine to the southeast, and the Provinces and ...
, offering coverage over a 60-degree arc between . A permanent transmitting facility was then built at Moscow AFS, a receiving facility at Columbia Falls Air Force Station, and an operational center between them in
Bangor, Maine Bangor ( ) is a city in and the county seat of Penobscot County, Maine, United States. The city proper has a population of 31,753, making it the state's List of municipalities in Maine, third-most populous city, behind Portland, Maine, Portland ...
. The coverage could be extended with additional receivers, providing for complete coverage over a 180-degree arc (each 60 degree portion known as a "sector").
GE Aerospace General Electric Company, doing business as GE Aerospace, is an American aircraft engine supplier that is headquartered in Evendale, Ohio, outside Cincinnati. It is the legal successor to the original General Electric Company founded in 1892, w ...
was awarded the development contract, expanding the existing east coast system with two additional sectors, while building another three-sector system on the west coast, a two-sector system in
Alaska Alaska ( ) is a non-contiguous U.S. state on the northwest extremity of North America. Part of the Western United States region, it is one of the two non-contiguous U.S. states, alongside Hawaii. Alaska is also considered to be the north ...
, and a one-sector system facing south. In 1992, the Air Force contracted to extend the coverage 15 degrees clockwise on the southern of the three east coast sectors to be able to cover the southeast U.S. border. Additionally, the range was extended to , crossing the equator. This was operated 40 hours a week at random times. Radar data were fed to the U.S. Customs/Coast Guard C3I Center, Miami; Joint Task Force 4 Operations Center, Key West;
U.S. Southern Command The United States Southern Command (USSOUTHCOM), located in Doral in Greater Miami, Florida, is one of the eleven unified combatant commands in the United States Department of Defense. It is responsible for providing contingency planning, ope ...
Operations Center, Key West; and U.S. Southern Command Operations Center, Panama. Central Over-the-Horizon Backscatter Radar System While the four planned OTH-B systems would establish a surveillance zone around the east, west, and south perimeters of North America. The Central Radar System (CRS) was needed to complete the perimeter coverage of the southern approaches to North America. It was also needed to cover the near-shore ocean areas not covered by the East Coast and West Coast OTH-B systems. The CRS would consist of four sectors, each covering a 60-degree arc, for a total coverage arc of 240 degrees over the western, southwestern, southeastern, and eastern approaches to North America, including the Gulf of Mexico, the land area of Mexico, and the Pacific Ocean west and south of Mexico. The CRS would also cover near-shore areas along both the eastern and western coasts of North America that are not covered by the ECRS and WCRS because the OTH-B system functions only at a distance greater than 500 nmi from the receive antennas. Thus, CRS would complete coverage of those areas, overlapping the surveillance areas of the ECRS and WCRS. With the end of the Cold War, the influence of the two senators from Maine was not enough to save the operation and the Alaska and southern-facing sites were canceled, the two so-far completed western sectors and the eastern ones were turned off and placed in "warm storage," allowing them to be used again if needed. By 2002, the west coast facilities were downgraded to "cold storage" status, meaning that only minimal maintenance was performed by a caretaker. Research was begun into the feasibility of removing the facilities. After a period of public input and environmental studies, in July 2005 the U.S. Air Force Air Combat Command published a "Final Environmental Assessment for Equipment Removal at Over-the-Horizon Backscatter Radar - West Coast Facilities". A final decision was made to remove all radar equipment at the west coast sector's transmitter site at Christmas Valley Air Force Station outside
Christmas Valley, Oregon Christmas Valley is an unincorporated community in Lake County, Oregon, United States. The community was named after nearby Christmas Lake, usually dry, east of the present town and the site of the former Lake post office, which ran from 1906 u ...
and its receiver site near
Tulelake, California Tulelake ( ) is a city in northeastern Siskiyou County, California, United States. The town is named after nearby Tule Lake. Its population is 902 as of the 2020 census, down from 1,010 from the 2010 census. Tulelake peace officers are authori ...
. This work was completed by July 2007 with the demolition and removal of the antenna arrays, leaving the buildings, fences and utility infrastructure at each site intact. In 2018, development started on the high-frequency Tactical Multi-Mission Over the Horizon Radar (TACMOR), a technology prototype to expand air and maritime awareness over the Western Pacific. In 2022 construction of a TACMOR radar station in
Palau Palau, officially the Republic of Palau, is an island country in the Micronesia subregion of Oceania in the western Pacific Ocean. The Republic of Palau consists of approximately 340 islands and is the western part of the Caroline Islands ...
was agreed, expected to be operational in 2026.


U.S. Navy

The
United States Navy The United States Navy (USN) is the naval warfare, maritime military branch, service branch of the United States Department of Defense. It is the world's most powerful navy with the largest Displacement (ship), displacement, at 4.5 millio ...
created their own system, the AN/TPS-71 ROTHR (Relocatable Over-the-Horizon Radar), which covers a 64-degree wedge-shaped area at ranges from 500 to 1,600
nautical miles A nautical mile is a unit of length used in air, marine, and space navigation, and for the definition of territorial waters. Historically, it was defined as the meridian arc length corresponding to one minute ( of a degree) of latitude at the eq ...
(925 to 3,000 km). ROTHR was originally intended to monitor ship and aircraft movement over the Pacific, and thus allow coordinated fleet movements well in advance of an engagement. In 1991, a prototype ROTHR system was installed on the isolated Aleutian Island of
Amchitka Amchitka (; ;) is a volcanic, tectonically unstable and uninhabited island in the Rat Islands group of the Aleutian Islands in southwest Alaska. It is part of the Alaska Maritime National Wildlife Refuge. The island, with a land area of ro ...
, Alaska, monitoring the eastern coast of Russia. It remained in use until 1993, and the equipment was later removed into storage. The first production systems were installed in the test site in Virginia for acceptance testing, but were then transitioned to counter the
illegal drug trade The illegal drug trade, drug trafficking, or narcotrafficking is a global black market dedicated to the cultivation, manufacture, distribution and sale of drug prohibition, prohibited drugs. Most jurisdictions prohibitionism, prohibit trade, exce ...
, covering
Central America Central America is a subregion of North America. Its political boundaries are defined as bordering Mexico to the north, Colombia to the southeast, the Caribbean to the east, and the Pacific Ocean to the southwest. Central America is usually ...
and the
Caribbean The Caribbean ( , ; ; ; ) is a region in the middle of the Americas centered around the Caribbean Sea in the Atlantic Ocean, North Atlantic Ocean, mostly overlapping with the West Indies. Bordered by North America to the north, Central America ...
. The second production ROTHR was later set up in Texas, covering many of the same areas in the Caribbean, but also providing coverage over the Pacific as far south as
Colombia Colombia, officially the Republic of Colombia, is a country primarily located in South America with Insular region of Colombia, insular regions in North America. The Colombian mainland is bordered by the Caribbean Sea to the north, Venezuel ...
. It also operates in the anti-drug trafficking role. The third, and final, production system was installed in Puerto Rico, extending anti-drug surveillance past the equator, deep into South America.


Alternative Approaches to Over the Horizon Radar

Another common application of over-horizon radar uses surface waves, also known as groundwaves. Groundwaves provide the method of propagation for medium-wave AM broadcasting below 1.6 MHz and other transmissions at lower frequencies. Groundwave propagation gives a rapidly decaying signal at increasing distances over ground and many such broadcast stations have limited range. However, seawater, with its high conductivity, supports groundwaves to distances of or more. This type of radar, surface-wave OTH, is used for surveillance, and operates most commonly between 4 and 20 MHz. Lower frequencies enjoy better propagation but poorer radar reflection from small targets, so there is usually an optimum frequency that depends on the type of target. A different approach to over-the-horizon radar is to use
creeping wave According to the principle of diffraction, when a wave front passes an obstruction, it spreads out into the shadowed space. A creeping wave in electromagnetism or acoustics is the wave that is diffracted radially around the surface of a smooth bo ...
s or electromagnetic surface waves at much lower frequencies. Creeping waves are the scattering into the rear of an object due to
diffraction Diffraction is the deviation of waves from straight-line propagation without any change in their energy due to an obstacle or through an aperture. The diffracting object or aperture effectively becomes a secondary source of the Wave propagation ...
, which is the reason both ears can hear a sound on one side of the head, for instance, and was how early communication and broadcast radio was accomplished. In the radar role, the creeping waves in question are diffracting around the Earth, although processing the returned signal is difficult. Development of such systems became practical in the late 1980s due to the rapidly increasing processing power available. Such systems are known as OTH-SW, for ''Surface Wave''. The first OTH-SW system deployed appears to be a Soviet system positioned to watch traffic in the
Sea of Japan The Sea of Japan is the marginal sea between the Japanese archipelago, Sakhalin, the Korean Peninsula, and the mainland of the Russian Far East. The Japanese archipelago separates the sea from the Pacific Ocean. Like the Mediterranean Sea, it ...
. A newer system has recently been used for coastal surveillance in Canada, and is now offered for sales by Maerospace, Australia has also deployed a High Frequency Surface Wave Radar. Senator Robert Hill,
Landmark Land Use Agreement For High Frequency Surface Radar
'', Ministerial Press Release 33/2004 from the Australian Department of Defence, 25 February 2004


Notes


References

;Citations ;Bibliography *Peter Laurie
"An eye on the enemy over the horizon"
''New Scientist'', 7 November 1974, pp. 420–423. *Nathaniel Frissell and Lyndell Hockersmith
"Seeing Beyond: Over the Horizon Radar Systems and HF Propagation"
Virginia Tech, 2 December 2008


External links


Radar Systems on Shortwave
an extensive list of OTH and similar radar systems compiled by Wolf Hadel, August 2013
A Canadian Perspective on High-Frequency Over-the-Horizon Radar
{{Webarchive, url=https://web.archive.org/web/20150914200039/http://cradpdf.drdc-rddc.gc.ca/PDFS/unc81/p527279.pdf , date=2015-09-14  — paper by R. J. Riddolls, Defence R&D Canada, Ottawa
The Development of Over-the-Horizon Radar in Australia
nbsp;— paper by D.H. Sinnott on the Australian Department of Defence Web site
Google maps link
nbsp;— Russian "Steel-Yard" radar near
Chernobyl Chernobyl, officially called Chornobyl, is a partially abandoned city in Vyshhorod Raion, Kyiv Oblast, Ukraine. It is located within the Chernobyl Exclusion Zone, to the north of Kyiv and to the southwest of Gomel in neighbouring Belarus. ...
.
Final Environmental Impact Statement Part IIA - Proposed Central Radar System Over-the-Horizon Backscatter Radar Program
(1987)
Environmental Impact Statement - Proposed Sites - Central Radar System Over-the-Horizon Backscatter Radar Program
(August 1990) Radar Ground radars