Home / Technology / Electronics & EW / Countries integrating Over-The-Horizon (OTH) radars into air defence networks to detect and track stealth Aircrafts, Aircraft carriers and hypersonic missiles

Countries integrating Over-The-Horizon (OTH) radars into air defence networks to detect and track stealth Aircrafts, Aircraft carriers and hypersonic missiles

The signals of Conventional microwave radars propagate in a straight line and cannot detect objects beyond their line of sight.  The maximum range of these radars is limited by the radio horizon – slightly far away than the optical horizon. OTH radars use very long wave lengths with special properties of propagation.


Over The Horizon Radar (OTHR) utilises the Ionosphere to reflect the radiated signal  to detect and track targets beyond the horizon. There are two types of OTH radars:


Skywave OTHR

The first is the Skywave OTHR that utilises the refractive properties of the ionosphere to refract or bend transmitted HF electromagnetic waves back to Earth. When these refracted HF waves hit a radar reflective (metal) surface of sufficient size — either airborne or maritime — some of the energy is reflected back along the transmission path to the OTHR receiver. Sophisticated computer systems then process the received energy to discern objects within the radar’s footprint.   Skywave OTHR that are able to  detect aircraft and ships at very long ranges (between 500 km and 3000 km – ignoring any double bounces).

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OTHR systems operate on the Doppler principle, where an object can be detected if its motion toward or away from the radar is different from the movement of its surroundings. OTHRs are typically made up of very large fixed transmitter and receiver antennas (called ‘arrays’). The location and orientation of these arrays determines the lateral limits or arc of radar’s coverage. The extent of OTHR coverage in range within this arc is variable and principally dependent on the state of the ionosphere.

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OTHRs do not continually ‘sweep’ an area like conventional radars but rather ‘dwell’ by focusing the radar’s energy on a particular area – referred to as a ‘tile’ – within the total area of coverage. The transmitted HF energy can be electronically steered to illuminate other ‘tiles’ within the OTHR’s coverage as required to satisfy operational tasking or in response to intelligence cuing.


These radars are typically capable of estimating the 2-dimensional coordinates of targets; namely, latitude, longitude, speed and heading. They can perform simultaneous tracking of separate targets.  Such radars offer extremely long detection ranges (from 700 to 4000 km) but also very low resolution (from some hundreds of meters up to 20 km).


Scientists have proved that aircraft could be detected at a range of up to 3,000km on a single-hop propagation and ballistic missile launches can be detected at a range of up to 6,000km on a two-hop propagation. Such radars can detect stealth aircrafts and ships from extremely long detection ranges from 700 to 4000 and also being employed for border protection, disaster relief and search and rescue operations.


Under certain atmospheric conditions, only specific radio frequencies will get reflected back towards the ground. The “correct” frequency to use depends on the current conditions of the atmosphere. So systems using ionospheric reflection need real-time monitoring of the reception of backscattered signals to continuously adjust the frequency of the transmitted signal.


High frequency surface wave radar (HFSWR) systems

Other type of OTH radars are Surface wave radar systems, in particular high frequency surface wave radar (HFSWR) systems,  that operate from coastal installations, so that the radar energy can couple into the salt water.  High Frequency Surface Wave Radar (HFSWR) takes advantage of the diffraction of electromagnetic waves over the conducting ocean surface. The transmitted signal follows the curved ocean surface, and a system can detect aircraft, and ships, beyond the visible horizon,  at ranges out to roughly 300 km. HFSWR exploits a phenomenon known as a Norton wave propagation whereby a vertically polarised electromagnetic signal propagates efficiently as a surface wave along a conducting surface.

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The successful detection of a target by a surface wave radar system traditionally involves compromises between a number of factors, including propagation losses, target radar cross-section, ambient noise, man-made interference, and signal-related clutter. In detecting a target at roughly 150 kilometers using HFSWR large error tolerances are experienced in both range (.+.1 to 2 km) and azimuth (.+.1. degree.) due to limited band width availability and physical antenna size constraints.


OTHR and HFSWR radars in Air Defense netoworks

OTHR and HFSWR radars have become important element of air  defence networks of many countries including China, Australia, Iran, US and Russia.  They  have the capability to defeat the stealth of aircrafts like the Northrop Grumman B-2 Spirit, F-35 or F-22 by detecting and tracking them from hundreds of kilometers.


Some of the OTH radars are JORN of Australia, ROTH of Ratheon USA, NOSTRA-DAMUS of ONERA, France and STEEL YARD of NIIDAR, Russia. All of them operate from approximately 5-30 MHz. The most powerful radar is Steel yard of Russia which transmits 1500 KW. OTH Radars being low frequency band radars possess anti-stealth capabilities, offering considerable capabilities of detecting targets such as stealth planes.


Stealth techniques such as shaping have been designed with the aim to reflect most of the radar energy away from an expected radar antenna and not back to it. However, techniques such as shaping and coating with Radar-Absorbent Materials (RAM) is most effective in microwave frequencies mainly in the X and Ku bands, and is less effective at Longer wavelengths such as VHF or High frequency (HF) radars such as OTHR. When the wavelength of the incident electromagnetic (EM) wave is comparable to the physical dimension of the object, it results in enhancement of RCS and in large amplitude oscillations in the RCS. This is due to the resonance effect between the direct reflection from the target and scattered waves which “creep” around it.



In response to US plans to pull out of the INF Treaty and the active development of its hypersonic technology, Russia is building a network of radar stations best suited to detect missile launches from afar. Russia is seeking to install Container-type over-the-horizon (OTH) radar stations along its borders. The move will substantially boost Moscow’s capabilities to monitor airspace and detect missile launches, particularly in case of hypersonic projectiles.


In Dec 2019, the first Konteiner radar, situated in the semi-autonomous Russian republic of Mordovia, officially began operations, six years after Russia finished building that system. From there, it provides early warning and general monitoring coverage of Russia’s western flank, reportedly including most of Europe and portions of the Middle East.

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Russia says it will set up a new, long-range over-the-horizon radar system to help “control” the Arctic by providing additional early warning and monitoring capability with regards to various potential threats, including aircraft, cruise missiles, and hypersonic weapons. The Russian Ministry of Defense revealed that the next 29B6 Konteiner radar would be installed in the Arctic on Dec. 2, 2019, according to state media outlet TASS. Russia has been carrying out rapid Arctic militarization by building New airbases, icebreakers, ground forces, missiles and and carrying out military exercises there.

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Container OTH can detect Hypersonic Missiles

The Container (29B6) 2-coordinate OTH radar was created by the Scientific and Research Institute for Long-Distance Radio Communication (NIIDAR). The equipment can be stored in transport containers and delivered anywhere, reducing the need for on-site construction work.


Konteiner is an extremely large, bistatic system consisting of separate high-frequency transmitter and receiver arrays. The complete transmitter array, which has 36 masts, is just over 1,440 feet wide, while the 144 masts that make up the receiver are spread across an area 4,265 feet wide. The transmitter and receiver sites in Mordovia are around 186 miles from each other.


The system provides fixed coverage in one particular direction across a 180-degree arc. It can reportedly detect and track objects right up to the edge of space and between 1,240 and 1,864 miles away, depending on the size and type of the target, as well as weather and other atmospheric conditions. The 29B6 radar bounces its signals off Earth’s ionosphere, an upper region of the atmosphere, to detect targets at such extreme ranges. It does also mean that it is blinded to threats outside of its field of view, though.


The 29B6 OTH radar was reported as deployed for trials near the town of Kovylkino in Mordovia in December 2013. This is where the receiving antennas and main processing equipment were located. The transmitting antennas were located outside Gorodets in Nizhny Oblast. This was the first OTH radar capable of monitoring airspace up to a range of over 3,000km. Its coverage included airspace over Poland, Germany, and the Baltics all the way to Turkey, Syria, and Israel. Unlike the Duga, the Container’s signal is reflected by the ionosphere only once, allowing for more accurate target detection. This allows it to track even small planes over Western Europe. The station can detect a mass launch towards Russia of cruise missiles, deployments of aircraft and hypersonic cruise missiles, when they are ready.


By 2018, the Container system was significantly improved. In early December, the Ministry of Defense announced that upgraded OTH radar is now on trial duty in Russia’s Mordovia region. In six months, it is expected to be fully operational and enter regular service. Basically, the same location was used for the deployment of a new OTH radar station that is far superior to the old Container (29B6). The new station has a larger detection zone. Now it is capable of discovering aerodynamic targets in a 240-degre arc, compared to 180 degrees for the old system. The computing systems and tracking stability have been improved significantly.


The new Container radar is capable of detecting and tracking over 5,000 air targets simultaneously. It can also track a small plane right on the runway, or detect launch and track the warhead of short or intermediate range ballistic missiles, which have smaller radar cross-sections than their intercontinental cousins.


There is one more important thing. When hypersonic aircraft and short- and intermediate-range missile warheads fly at high altitudes, an ionization signature forms around them in the form of a compression wave in front and an ionization wake behind. This makes them easier to detect by OTH radar already trained to analyze the state of the ionosphere.


In the nearest future, a number of Container-type OTH radar stations are expected to be deployed in the Russian Far East, Central Siberia, and in Kaliningrad in the Baltic region. The Russian military believes that 10 to 12 Container-type OTH radar stations will be enough to cover the entire airspace outside of the borders of Russia.


The OTH coverage will complement traditional radar stations of the upgraded Voronezh type, which serve as part of Russia’s ICBM launch early warning system.


Russian researchers had to develop new equipment and processing algorithms to compensate for the interference caused by Sun’s radiation in the ionosphere. It takes sophisticated mathematical algorithms to isolate relevant targets and, and more so to determine their velocity and direction based on the Doppler shift.


The combat-worthy 5N32 Duga was ready to enter service in 1971. The space component of the ICBM launch early-warning system is pretty good at detecting launches from US soil. But it is not capable of getting the coordinates for targets. They managed to detect American space rockets launched at Cape Canaveral. Following multiple upgrades, the Duga was able to provide consistent detection of Space Shuttle launches and Titan ballistic missiles from Cape Kennedy – 7,000-9,000km away. Nevertheless, all attempts to detect Minuteman missile launches from Vandenberg Air Force Base via the polar section of the ionosphere were in vain.


Despite the success, the Soviet OTH program had to give way to the space-based ballistic missile launch early warning technology. The space component of the ICBM launch early-warning system is pretty good at detecting launches from US soil. But it is not capable of getting the coordinates for targets. In the early 1990s, over-the-horizon radar technology became relevant again.


Chinese OTH radars

China is reported to have developed its first OTH-B radar back in 1967; Since the 1980s two further installations have possibly been added to the inventory, with at least one system looking out into the China Sea area reportedly to target (US Navy) aircraft carriers. Backscatter systems function at the upper end of the High Frequency (HF) band, typically between 12 and 28 MHz. OTH-B radars are bistatic systems, this is where the transmitter and receiver use different antennas at widely separated locations to achieve detection results.


China’s OTH-B is said to use Frequency Modulated Continuous Wave (FMCW) transmissions to enable Doppler measurements, the suppression of static objects and the display of moving targets. In 2008 Asian military sources told Richard Fisher that China had placed a new long range OvertheHorizon (OTH) radar station in Hainan Island. Then at the February 2009 IDEX show in Abu Dhabi a Russian source confirmed to Fisher the sale to China of the 300km range PodsolnukhE surfacewave OTH radar.


China may have deployed as many as three Over-the-Horizon (OTH) sky-wave radar systems by 2005. China aspires to use to target aircraft carriers. These systems could be used in an early warning capacity. China also may have deployed at least one surface-wave OTHR. OTH radars could also detect stealth aircraft and locate inter-continental ballistic missiles and other types of missiles fired by other countries. The radar could allegedly confirm the target of an enemy within a minute after launching and could issue an early warning three minutes later.


China has reportedly set up a high-tech radar system in Inner Mongolia with a detection range of up to 3,000 kilometers, a move to spy on South Korean and Japanese military maneuvers, according to Chinese media. The installation comes amid a spat with South Korea on the deployment of a Terminal High-Altitude Area Defense (THAAD) battery from the US. THAAD is a missile defense system Beijing and Russia fear could be a tool to be used to spy its military activities.  Its first OTH is set up in the Hubei-Henan-Anhui triangle. All two radars are used to monitor the entire western Pacific if used together with spy satellites.


Recently it has been revealed that China has developed a new ship-based OTH radar system. Liu Yongtan, an academician of the Chinese Academy of Sciences (CAS) and the Chinese Academy of Engineering (CAE) from the Harbin Institute of Technology, is credited to have upgraded the China’s radar technology and developing an advanced compact size radar for the PLA Navy’s carrier fleet to “maintain constant surveillance over an area the size of India,” the Post reported. The floating radar “will increase our navy’s information gathering capabilities in critical areas” including the South China Sea, Indian Ocean and Pacific Ocean, he was quoted as saying in the Post.

China is not the only nation developing the technology. A major US defence contractor, Raytheon, was granted a patent in 2016 for a similar system. The Raytheon design involves a transmitting vessel and several receiving ships with antennae mounted on the deck. Radio waves are directed into the sky by the transmitter to be collected by the receiving ships, which then pass on the signals to aircraft carriers via satellite or airborne relays.


China deploying Anti-Stealth OTH Radar in the South China Sea

In 2015 Victor Robert Lee of The Diplomat reported, “Fiery Cross Reef, Subi Reef and Mischief Reef are China’s largest military installations in the Spratlys, but they are still under construction and do not exhibit the more sophisticated defensive capabilities now present at China’s smaller bases on four other reefs in the Spratlys: Cuarteron, Gaven, Hughes, and Johnson South.”


“These facilities are being equipped with state-of-the-art sensor towers, weapons tracking and firing platforms and tracking/firing guidance radars, as well as an array of electronic sensors and satellite communications infrastructure. For example, a satellite image taken August 23 shows that Cuarteron has a new antenna farm that Rogers considers reminiscent of Australia’s Jindalee over-the-horizon radar network, which has a range of up to 3,000 kilometers.” “China appears to be building an anti-stealth radar system on an artificial island in the middle of the South China Sea, where a military-grade system would be useful in detecting stealth aircraft in the contentious and contested area,” Kyle Mizokami reports in Popular Mechanics.


China’s Anti-ship ballistic missile system can target US aircraft carriers through OTH radar and satellites

A constellation of satellites and at least one over-the-horizon radar give its Anti-Ship Ballistic Missile (ASBM) system the capability to work out the position of U.S. aircraft carriers at sea, according to assessments published by researchers at the National Institute of Advanced Studies in Bangalore. Land-based ballistic missiles, carrying manoeuvrable warheads with conventional munitions, could then, if needed, target the aircraft carriers at a distance of about 2,000 km.


Although the land-based ballistic missiles can target aircraft carriers using just the Yaogon constellation, the number of targeting opportunities become fewer if cloud cover obscures the view of satellites with optical sensors, observed Prof. Chandrashekar. China’s constellation of Yaogan military satellites includes those for electronic intelligence (ELINT) gathering that detect radio signals and other electronic emissions from an aircraft carrier and its associated warships. China currently has three clusters of ELINT satellites that provide global surveillance. By incorporating an over-the-horizon radar that can continually track aircraft carriers up to a distance of about 3,000 km, the Chinese gain the flexibility to launch the ballistic missiles whenever they choose, he pointed out.



Raytheon Canada Limited (RCL),  will design, build and install two over-the-horizon radar sites in Canada’s polar region to determine what effects, if any, the Aurora Borealis has on target detection along the Canadian north. These two contracts, totaling $30 million, will enable Defence Research and Development Canada to conduct a feasibility study of using sky-wave Over-The-Horizon Radar technology, in the arctic, to determine the effect of the Aurora Borealis on target detection beyond line-of-site.  “Raytheon built and operates a similar radar system in the U.S. which has been key to defending America’s borders,” said David Appel, director for mission systems at Raytheon IIS. “A full over-the-horizon radar will monitor the arctic, as those waters have become more accessible to shipping traffic.”


Ghadir, Iran’s over-the-horizon radar

Ghadir, is an Iranian over the horizon radar Ghadir is a 360°, 3D-radar, with a ceiling of 300 km, and a maximum range of 1,100 km. Unlike other OTHR’s, Ghadir doesn’t use FMCW modulation. Instead, it uses a shaped pulsed system which makes the edges of the signal hard to define. Because of this, the bandwidth of this signal can vary greatly, ranging from around 60 kHz to splattering over 1MHz, depending on the power of the received signal for the user

A senior Iranian Army general  spoke about plans of  Islamic Republic plans to unveil a variety of over-the-horizon radar systems covering a distance of 3,000 kilometers. Brigadier General Farzad Esmaili, the commander of Iran’s Khatam al-Anbiya Air Defense Base, said that the radars can help the Khatam al-Anbiya Air Defense Base “detect and monitor aircraft flying beyond [Iran’s] borders.”

Australia’s Jindalee Operational Radar Network

The Jindalee Operational Radar Network (JORN) is an over-the-horizon radar (OTHR) network that can monitor air and sea movements across 37,000 km2. It has a normal operating range of 1,000 km to 3,000 km. It is used in the defence of Australia, and can also monitor maritime operations, wave heights and wind directions.

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The JORN defence system is a network of three remote over-the-horizon radars in Queensland, Western Australia and the Northern Territory. These radars are dispersed across Australia — at Longreach in Queensland, Laverton in Western Australia and Alice Springs in the Northern Territory — to provide surveillance coverage of Australia’s northern approaches.  It provides wide-area surveillance to support the Australian Defence Force’s air and maritime operations, border surveillance, disaster relief, and search and rescue operations.


The JORN radars have an operating range of 1000–3000km, as measured from the radar array. Of note, the Alice Springs and Longreach radars cover an arc of 90 degrees each, whereas the Laverton OTHR coverage area extends through 180 degrees. JORN does not operate on a 24 hour basis except during military contingencies. Defence’s peacetime use of JORN focuses on those objects that the system has been designed to detect, thus ensuring efficient use of resources.

Operation and uses

JORN’s main ground stations comprise a control centre, known as the JORN Coordination Centre (JCC), at RAAF Base Edinburgh in South Australia and three transmission stations: Radar 1 near Longreach, Queensland, Radar 2 near Laverton, Western Australia and Radar 3 near Alice Springs, Northern Territory. The JORN network is operated by No. 1 Radar Surveillance Unit RAAF (1RSU). Data from the JORN sites is fed to the JORN Coordination Centre at RAAF Base Edinburgh where it is passed on to other agencies and military units.


Officially the system allows the Australian Defence Force to observe air and sea activity north of Australia to distances up to 4000 km. This encompasses all of Java, Irian Jaya, Papua New Guinea and the Solomon Islands, and may include Singapore. However, in 1997, the prototype was able to detect missile launches by China over 5,500 kilometres (3,400 mi) away. The “backscatter” signal is extremely small due to reflection losses. The very long wave length used by such low frequency radar make it very difficult to pick out the relatively small target presented by an aircraft against the very large target presented by the earth. It takes a huge amount of data processing to pick large targets out of earth clutter.


For an aircraft or maritime vessel to be detected, it must possess a radar reflective (metal) surface of sufficient size so that sufficient HF radar energy is reflected back along the transmission path to the JORN receiver. “JORN is expected to detect air objects equivalent in size to a BAe Hawk-127 aircraft or larger and maritime objects equivalent in size and construction to an Armidale-class patrol boat or larger,” according to Australian Air force.


Australia’s JORN Phase 6 upgrade

The JORN project (JP2025) has had 5 phases, and has cost approximately A$1.8 billion. The ANAO Audit report of June 1996 estimated an overall project cost for Phase 3 of $1.1 billion. Phase 5 costs have been estimated at $70 million. Phase 6 costs expect to be $1.2 billion.


JORN achieved Final Operational Capability in 2014 under Project JP 2025 Phase 5. The enhancements to JORN under that phase provided greater integration and connectivity between the JORN radar sites, its control systems and wider Defence command and surveillance networks.


In March 2018 it was announced that BAE Systems Australia will undertake the $1.2 billion upgrade to Australia’s Jindalee Operational Radar Network which will take 10 years to complete.  The upgrade to the over-the-horizon radar (OTHR) network is designed to ‘open’ the system’s architecture enabling the insertion of next generation technologies and extend the operational life of JORN to beyond 2042. This will allow for an improved scan rate to boost radar coverage and detection capabilities which Ms Payne says are vital to keep JORN at the cutting edge. “It will allow us to execute more surveillance tasks at once and see smaller targets,” she said on Monday.

Cryogenic Sapphire Oscillator – ‘The Sapphire Clock’

Scientists at the Institute for Photonics and Advanced Sensing have successfully built and marketed the so-called sapphire clock. Based on a lab-grown, 1,200-carat sapphire, it ticks 10 billion times per second to produce hyper-accurate measurements. The sapphire clock offers a 1000-fold improvement in timing precision, which delivers an improved ability for Australian Defence to identify threats to Australia using the Jindalee Over-The-Horizon Radar Network. The performance of the radar is critically dependent on the purity and quality of the signals that are broadcast: if one can broadcast noiseless signals then will be possible to detect smaller objects that are further away and which are moving slower. This additional surveillance power is critically important in a defence context by providing additional insight.


When applied to the JORN radar application it delivers a signal that is more than 1000 times purer than its existing approach. It is important to note that this improvement can still be delivered despite the existing JORN solution making use of the best commercial devices that money can buy. To answer this call for better radar signals the Sapphire Clock team commenced working closely with the High-frequency Radar team at DST Group who are responsible for the research behind the JORN project.


In addition to the remarkable Sapphire Clock, the team has developed two additional technologies that directly result in purer signals for JORN, and which thus assist Australian defence to be better able to observe threats to Australia. The team has developed ultra-low noise synthesis technology that can take the clock signals and generate the frequencies that are needed by the radar: it can do this while preserving the signal purity of the clock. The team has also developed signal dissemination technology, which enables them to deliver the pure signals to the numerous locations necessary to broadcast the JORN signals effectively. The range of technologies developed by the team provides the revolutionary leap in the performance in this outstanding Australian invention.

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