Over-the-horizon_radar

# Over-the-horizon radar **Over-the-horizon radar** (OTH), also called beyond-the-horizon radar, is a class of [[Radar|radar]] built to find and track targets hundreds to thousands of kilometres away, well past the point at which the curvature of the Earth blocks an ordinary line-of-sight set (see [[Radar_horizon]]). It reaches that range by declining the straight path an ordinary [[Microwave|microwave]] beam takes: a [[Skywave|skywave]] system sends high-frequency energy up to refract off the [[Ionosphere|ionosphere]] and return to Earth far beyond the horizon, while a ground-wave system keeps a lower-frequency wave hugging a conducting surface, usually the sea, well past where a raised beam would climb into empty sky. A three.js microsim elsewhere on this page renders the skywave geometry directly, turning the ionospheric bounce that extends a set's reach into the skip zone of unreachable ground that opens up close to the radar. Developed from the 1950s as a way to watch for bombers, and later for missile launches, at ranges no line-of-sight radar could reach, OTH radar fell out of the front rank of early-warning technology once airborne and satellite sensors took over that mission during the [[Cold_War|Cold War]]. It has since found a second career: a skywave or ground-wave installation is far cheaper to build and run than a fleet of aircraft or a constellation of satellites, and several countries now operate or are developing systems for maritime surveillance, border security and counter-narcotics work as well as for air defence. ## Technology An over-the-horizon radar buys its enormous range by giving up an assumption every other radar design makes: that a target sits within [[Line-of-sight_propagation|line of sight]] of the antenna. [[Radio_wave|Radio waves]] near the top of the high-frequency band, roughly 3 to 30 megahertz, make the trade possible, because they are the highest frequencies still able to interact strongly enough with the ionosphere, a region of the upper [[Atmosphere_of_Earth|atmosphere]], or with a conducting sea surface to be steered around the planet's curve rather than escaping into space or dying out within sight of the coast. ### Skywave systems A skywave set transmits obliquely upward, aiming a pulse at a layer of the ionosphere ionised by solar ultraviolet and X-ray radiation. Below a critical frequency set by how densely that layer is ionised, the layer refracts the wave enough to turn it back toward the ground; above that frequency the wave passes through and is lost to space. The highest frequency that still returns along a given path is the maximum usable frequency, and an operator retunes below it, often several times a day, as the ionosphere's ionisation rises and falls with the sun's elevation. One bounce, or hop, typically carries a signal a few thousand kilometres before it reaches the ground again, and a further reflection off the sea or land surface can send it up for another hop beyond that, which is how a single installation watches a strip of the Earth thousands of kilometres from its antennas. Between the outer edge of any ground wave near the transmitter and the inner edge of the first skywave return lies a skip zone the radar cannot see at all; the three.js companion sketch renders exactly this geometry, opening and closing the skip zone as the modelled frequency and reflecting height change. ### Ground wave systems A ground-wave set instead keeps its energy close to the surface, using a lower part of the high-frequency band and a vertically polarised wave that a conducting surface guides along its own curvature rather than lets escape upward. [[Ground_wave|Ground-wave]] propagation works best over salt water, whose high conductivity keeps the loss per kilometre low, and far worse over dry land or ice, so a ground-wave radar is chiefly a maritime tool. Its reach is far shorter than a skywave hop, typically some hundreds of kilometres rather than thousands, but it does not depend on the ionosphere's mood at all: coverage is continuous from the coast outward, with no skip zone and no maximum usable frequency to chase through the day. ### Limitations What a skywave path buys in range it spends in precision and cleanliness. The same layer that bends a beam back to Earth is itself moving, rippling and occasionally turbulent, so an echo returns carrying a spread of Doppler shifts contributed by the medium as well as by the target; separating a slow-moving ship or a manoeuvring aircraft from that [[Clutter_(radar)|clutter]], and from the similar clutter left by meteor trails, is a large part of an over-the-horizon radar's signal processing rather than an afterthought to it. A skywave return can also arrive by more than one hop or ionospheric layer at once, a form of [[Multipath_propagation|multipath]] that smears a single target's echo across several closely spaced delays rather than the single clean spike a line-of-sight radar records. A target's true position is likewise uncertain until the ionosphere's current height and tilt are known well enough to convert a measured [[Doppler_effect|Doppler]] shift and a skywave delay into an actual range and bearing, an inversion problem with no fixed answer because the medium doing the bending has none either. Because a single hop can span thousands of kilometres, an over-the-horizon radar also runs a far lower [[Pulse-repetition_frequency|pulse-repetition frequency]] than a microwave set, since the unambiguous range fixed by the interval between pulses must itself stretch across the whole skywave path. Operating in the high-frequency band also means operating in one of the noisiest parts of the radio spectrum. Distant thunderstorms, the galaxy itself and human sources such as power lines and industrial equipment all raise the noise floor a high-frequency receiver works against, and that external noise, not the receiver's own [[Johnson–Nyquist_noise|thermal noise]], usually sets the limit on how weak an echo can still be detected: near 30 megahertz, a receiving antenna in or near a city can see a noise temperature on the order of a million kelvin from man-made sources alone, far above anything the receiver itself contributes.[^ellhfnoise] Integrating a return coherently for a long interval recovers some of that lost margin, concentrating a target's energy into a narrow Doppler line while the noise stays spread across the band and improving the effective [[Signal-to-noise_ratio|signal-to-noise ratio]], at the cost of the minutes, rather than milliseconds, a skywave radar needs to form a single detection. The same long wavelength that lets a skywave beam bend around the Earth also works against [[Stealth_technology|stealth]] shaping optimised for microwave radar, since airframe features shaped to scatter a centimetre-scale wave away from the receiver are only a small fraction of a high-frequency wavelength across and scatter far less selectively at these frequencies. ## History The idea that a radio wave could be bounced deliberately off the ionosphere to find a target beyond the horizon followed close on the heels of ionospheric research itself: engineers studying how the layer reflected signals for long-distance communication recognised that the same reflection could carry a radar pulse out and an echo back, and military laboratories pursued the idea through the 1950s and 1960s as a way to watch for bomber and, later, ballistic-missile launches thousands of kilometres beyond any line-of-sight set.[^cn-early] By the 1970s a Western and an Eastern bloc of over-the-horizon radars stood as part of the wider early-warning network of the Cold War. One Soviet-era system's broadband pulse became well known outside military circles, since it was strong enough to be heard by [[Shortwave_radio|shortwave]] listeners worldwide and earned an informal nickname, "the Woodpecker," in the amateur-radio press of the day.[^cn-soviet] The end of the Cold War removed the original justification for many of these systems just as satellites and airborne radar were taking over the missile- and bomber-warning mission in any case, and several large installations, including the joint UK-US Cobra Mist set described under Systems below, were shut down or placed in storage during the 1990s. Over-the-horizon radar did not disappear with them: Australia's Jindalee programme, begun as ionospheric research in the 1950s and 1970s, grew into an operational network in the following decades and became the model most later systems have followed, while the United States kept a maritime-surveillance capability alive for [[United_States_Air_Force|Air Force]] and Navy missions also described below. The lower cost of a ground-based skywave or ground-wave installation, next to a fleet of patrol aircraft or a constellation of satellites, is what has kept the technology in service for maritime surveillance, border protection and counter-narcotics work long after its original Cold War mission receded. ## Systems ### Australia The Jindalee Operational Radar Network (JORN) is the longest-running and most thoroughly documented skywave system in service. It grew out of ionospheric research conducted from the 1950s, and a first experimental array, Jindalee Stage A, went up near Alice Springs in the mid-1970s; the demonstrator was extended through the 1980s before the Commonwealth committed to a full three-radar network in 1991.[^jorn] Two further installations, at Longreach in Queensland and Laverton in Western Australia, joined the Alice Springs radar when the network reached full operational capability in 2003, giving continuous high-frequency coverage of Australia's northern and western approaches out to 1,000–3,000 kilometres, sufficient to track an aircraft the size of a light strike jet or a vessel the size of a coastal patrol boat.[^jorn] The network is operated by the Royal Australian Air Force from a coordination centre at RAAF Base Edinburgh and has been progressively upgraded since, most recently under a mid-life redesign begun in 2018. ### Brazil Brazil has studied high-frequency skywave coverage of its Amazonian and South Atlantic approaches as part of its wider airspace- and border-surveillance effort, though a dedicated operational over-the-horizon radar comparable to Australia's has not been as widely documented in open sources.[^cn-intl] ### Canada Canada has taken part in North American over-the-horizon radar research alongside the United States, evaluating the technology's usefulness for approaches over the Arctic and the North Atlantic, without fielding a dedicated system of its own.[^cn-intl] ### China Open defence-industry reporting describes China as operating both skywave and surface-wave over-the-horizon radars along its coast, intended to extend maritime domain awareness across the Western Pacific and the South China Sea well beyond the reach of shipborne or coastal microwave radar.[^cn-intl] ### France The French national aerospace research agency, ONERA, has developed and continued to invest in an experimental over-the-horizon radar named Nostradamus, built around a circular antenna array rather than the linear arrays more common elsewhere, which the agency has described as capable of monitoring air movement at ranges beyond 3,000 kilometres.[^nostradamus] ### India India has been reported to be developing an indigenous over-the-horizon radar capability for early warning and maritime surveillance, though published technical detail remains limited.[^cn-intl] ### Iran Iran has announced radar programmes it describes as capable of over-the-horizon detection, without the kind of independent technical verification available for the longer-established Australian, British, Russian and United States systems.[^cn-intl] ### Soviet Union / Ukraine / Russia Soviet military research produced at least one large skywave early-warning radar from the 1970s as part of the wider Cold War missile-warning network described in History above; the system's characteristic wideband pulse interference was heard on shortwave receivers around the world for years and is well documented in the amateur-radio literature of the period, even though the facility's official designation and precise operating history are not independently confirmed here.[^cn-soviet] Russia is reported to continue development of over-the-horizon systems for the same early-warning mission today.[^cn-soviet] ### USA and UK #### UK/US Cobra Mist Cobra Mist, given the military designation AN/FPS-95, was a joint British-American skywave radar built by RCA for the [[United_States_Air_Force|US Air Force]] at Orfordness on the Suffolk coast of England. Construction began in 1967 and finished in 1971, and the radar, operating across 6 to 40 megahertz from a fan-shaped [[Antenna_array|antenna array]] of eighteen strings each some 550 metres long, became operational in January 1973 with the mission of watching Soviet aircraft and missile activity over Eastern Europe.[^cobramist] It never worked as intended: the receiver picked up a persistent, unexplained noise well above what its designers had predicted, degrading performance so badly that the project was shut down after only about six months, in June 1973, despite several investigations into the noise's source.[^cobramist] #### U.S. Air Force The Air Force fielded a production successor, the AN/FPS-118 Over-the-Horizon Backscatter radar, in the 1970s and 1980s, with an eastern sector run from Bangor Air National Guard Base in Maine and a western sector run from Mountain Home Air Force Base in Idaho, each pairing a separate transmit and receive site and together covering approaches to North America out to some 1,800 nautical miles.[^othb] Built for exactly the Cold War air-defence mission that was fading as the system matured, the eastern sector stopped operating in October 1997 and the western sector was placed in caretaker status around the same time, and both have since been kept for eventual sale or transfer rather than returned to service.[^othb] #### U.S. Navy The Navy's AN/TPS-71 Relocatable Over-the-Horizon Radar (ROTHR) has fared differently, having found a lasting peacetime role. Operated by the Fleet Surveillance Support Command from Chesapeake, Virginia, with a second transmit-receive site at Corpus Christi, Texas, ROTHR uses the 5-to-28-megahertz band and a receiving array some 2.6 kilometres long that forms its bearing estimate through digital [[Beamforming|beamforming]] to resolve direction to about half a degree, covering a wedge some 500–1,600 nautical miles deep, the Virginia system alone watching more than 2.2 million square miles of the Caribbean.[^rothr] Built originally for tactical warning of air and surface threats, ROTHR has been used for years in full-time support of United States counter-narcotics surveillance, tracking small aircraft and vessels that line-of-sight coastal radar cannot see far enough out to catch.[^rothr] ## Alternative approaches to over the horizon radar Bending a wave around the Earth's curvature is not the only way to see past the horizon; another is to raise the sensor instead of the wave. An aircraft or an aerostat carrying an ordinary microwave radar climbs high enough that its own line of sight reaches hundreds of kilometres, trading the ionosphere's unpredictability and the ground wave's short reach for the cost and limited endurance of keeping a platform aloft, which is the substitution that let airborne early-warning aircraft take over much of the Cold War-era skywave and ground-wave mission. A satellite goes further still, watching an entire ocean at once from an orbit no propagation trick is needed to reach, at the price of revisit time and a much larger programme to build and launch. A second alternative keeps the ionospheric or ground-wave path but removes the expensive high-power transmitter from the equation. [[Bistatic_radar|Bistatic]] and [[Passive_radar|passive]] configurations instead receive the skywave or ground-wave energy that someone else's shortwave broadcast, communications link or dedicated illuminator is already sending out, extracting range and bearing from an echo of a signal the receiving site never had to generate itself. The saving is real: a receive-only site is cheaper to build, harder to detect and immune to [[Radar_jamming_and_deception|jamming]] aimed at its own transmitter, though it depends entirely on a suitable illuminator being on the air at the time and frequency a target happens to be visible. ## Microsims A three.js companion elsewhere on this page renders the skywave geometry described in Technology above, tracing a wave up to the ionosphere and back down to show the skip zone open and close as the modelled frequency and reflection height change. Neither this article nor that companion carries a two-dimensional p5.js sketch of its own; the pulse-timing and Doppler ideas an over-the-horizon radar depends on are instead shown by the sketches carried by neighbouring articles. *Try:* in the [[Radar]] sketch, watch how a pulse's round-trip delay converts directly to range on the A-scope, and how dragging the pulse-repetition frequency moves the unambiguous-range ring; an over-the-horizon set faces the same ambiguity, just stretched over a path that climbs to the ionosphere and back rather than a straight line to the target. *Try:* in the [[Doppler_effect]] sketch, change the source speed and watch the wavefronts bunch ahead of it and spread out behind; a moving patch of ionosphere does the same bunching and spreading to a skywave echo, which is the spread-Doppler clutter described in Limitations above. *Try:* in the [[Sonar]] sketch, drag the assumed sound speed away from its true value and watch every measured range slide off the faint true-range ticks; an over-the-horizon radar has the same problem when the assumed ionospheric reflection height is wrong, sliding every measured range and bearing away from the target's real position. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Over-the-horizon_radar) : [Wikitube](https://en.wikitube.io/wiki/Over-the-horizon_radar) Skeleton mirrored at revision 1375101024. Prose, emphasis and the microsims are Wikitube's own. ## Notes The propagation physics in Technology above — ionospheric refraction, the maximum usable frequency, ground-wave attenuation over sea water — is standard radio-propagation material and carries no footnote of its own, per the Wikitube style guide's §6.1. Page numbers in References below are PDF pages of the open textbook edition cited there. ## References [^ellhfnoise]: Ellingson, S. *Radio Systems Engineering, Revised First Edition*. 2023, pp. 104-107 (PDF pages). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering . CC BY-NC. [^cn-early]: Citation needed: a primary 1950s ionospheric-research or military technical report describing the first deliberate use of an oblique ionospheric echo for ranging would confirm the laboratory, country and date this account summarises. [^cn-soviet]: Citation needed: a primary Soviet or Russian military technical source, or a peer-reviewed history, giving the Cold War-era skywave radar's official designation, site and in-service dates, and confirming Russia's current systems, would settle the details only informally sourced here. [^jorn]: Australian Department of Defence, Defence Science and Technology Group. "Jindalee Operational Radar Network." https://www.dst.defence.gov.au/innovation/jindalee-operational-radar-network . Also: Peterson-Schriever Space Force Base, "Jindalee Operational Radar Network," U.S. Space Force fact sheet: https://www.petersonschriever.spaceforce.mil/About-Us/Fact-Sheets/Display/Article/1059651/jindalee-operational-radar-network/ . [^cn-intl]: Citation needed: primary defence-ministry technical releases for Brazil, Canada, China, India and Iran would confirm or correct the programmes and capabilities summarised for each in this section; open secondary reporting is the basis for these sentences and is not independently verified here. [^nostradamus]: ONERA (French national aerospace research agency). "Nostradamus peut surveiller les mouvements aériens à plus de 3 000 km de distance." https://onera.fr/fr/pepites/nostradamus-peut-surveiller-les-mouvements-aeriens-a-plus-de-3000-km-de-distance . [^cobramist]: Griffiths, H. "The History Column: Cobra Mist." *IEEE Aerospace and Electronic Systems Magazine*, 2023. https://doi.org/10.1109/MAES.2023.3247972 . [^othb]: United States Air Force, Air Combat Command. "Over the Horizon Backscatter Radar: East and West." Fact sheet. https://www.acc.af.mil/About-Us/Fact-Sheets/Display/Article/199120/over-the-horizon-backscatter-radar-east-and-west/ . [^rothr]: Federation of American Scientists. "AN/TPS-71 ROTHR (Relocatable Over-the-Horizon Radar)." https://nuke.fas.org/guide/usa/airdef/an-tps-71.htm . ## External links This article's only interactive companion is the three.js microsim named in the Microsims section above; the current build carries no p5.js sketch about over-the-horizon radar specifically, so there is no separate live-sketch or editor-fork link to add here.