U.S. Navy Relocatable Over-the-Horizon Radar station How a OTH radar works: A powerful signal from a large transmitting (left) reaches a target beyond the horizon by reflecting off the , and the echo signal from the target (right) returns to the receiving antenna by the same route.
Over-the-horizon radar, or OTH (sometimes called beyond the horizon, or BTH), is a type of system with the ability to detect targets at very long ranges, typically hundreds to thousands of kilometres, beyond the , which is the distance limit for ordinary . Several OTH radar systems were deployed starting in the 1950s and 1960s as part of systems, but these have generally been replaced by systems. OTH radars have recently been making a comeback, as the need for accurate long-range tracking becomes less important with the ending of the , and less-expensive ground-based radars are once again being considered for roles such as maritime reconnaissance and drug enforcement.
The super high frequency used by most radars, called , travel in straight lines. This generally limits the detection range of radar systems to objects on their (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 10 m (33 ft) mast has a range to the horizon of about 13 kilometres (8.1 mi), taking into account atmospheric refraction effects. If the target is above the surface, this range will be increased accordingly, so a target 10 m (33 ft) high can be detected by the same radar at 26 km (16 mi). 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 reflect their signals off the for very long-range detection, and systems, which use high frequency radio waves that, due to , 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 microwave frequencies using a .
The most common type of OTH radar uses or "skip" propagation, in which radio waves are reflected off an layer in the atmosphere, the . Given certain conditions in the atmosphere, radio signals transmitted at an angle into the sky will be reflected towards the ground by the , allowing them to return to earth beyond the horizon. A small amount of this signal will be scattered off desired targets back towards the sky, reflect off the ionosphere again, and return to the receiving antenna by the same path. Only one range of frequencies regularly exhibits this behaviour: the or part of the from 3–30 MHz. The best frequency to use depends on the current conditions of the atmosphere and the . 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 120 km (75 mi) range will show the target as 2 km (1.2 mi) wide. To produce a 1 degree beam at the most common frequencies, an antenna 1.5 kilometres (0.93 mi) wide is required. Due to the physics of the reflection process, actual accuracy is even lower, with range resolution on the order of 20 to 40 kilometres (12–25 mi) and bearing accuracy of 2 to 4 kilometres (1.2–2.5 mi) being suggested. Even a 2 km accuracy is useful only for early warning, not for weapons fire.
Another problem is that the reflection 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 3 kilometres (1.9 mi) in front of the antenna. OTH systems are thus very expensive to build, and essentially immobile.
Given the losses at each reflection, 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 reflected from the ground, or sea, will be very large compared to the signal reflected 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 , 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 1.5 knots (2.8 km/h).
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 , another reason OTH systems did not become truly practical until the 1960s, with the introduction of high-performance systems.
A second type of OTH radar uses much lower frequencies, in the 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 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.
Engineers in the 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. at the . 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 600 miles (970 km) away at , and nuclear explosions in at 1,700 miles (2,700 km). A greatly improved system, a testbed for an operational radar, was built in 1961 as MADRE (Magnetic-Drum Radar Equipment) at . It detected aircraft as far as 3,000 kilometres (1,900 mi) using as little as 50 kW of broadcast energy.
As the names imply, both of the NRL systems relied on the comparison of returned signals stored on . In an attempt to remove from radar displays, many late-war and post-war radar systems added an 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 of the radar, or PRF. For long-range use, the PRF was very long to start, and deliberately changed in order 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 in the early 1960s. This consisted of several antennas positioned to be four wavelengths apart, allowing the system to use phase-shift to steer the direction of sensitivity and adjust it to cover Singapore, Calcutta and the UK. This system consumed 25 miles (40 km) of electrical cable in the antenna array.
The first truly operational development was an Anglo-American system known as , 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 . When system testing started in 1972, however, an unexpected source of noise rendered it largely unusable. They abandoned the site in 1973, the source of the noise never having been identified.
Other early UK/US systems from the same era include:
Obsolete US Air Force OTH-B (AN/FPS-118) radar
The had the first complete success with their OTH-B. A prototype with a 1 MW transmitter and a separate receiver was installed in , offering coverage over a 60 degree arc between 900 and 3,300 km. A permanent transmitting facility was then built at , a receiving facility at , and an operational center between them in . 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").
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 , 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 3,000 miles (4,800 km), 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 Operations Center, Key West; and U.S. Southern Command Operations Center, Panama.
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 outside and its receiver site near . 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.
The created their own system, the ROTHR (Relocatable Over-the-Horizon Radar), which covers a 64 degree wedge-shaped area at ranges from 500 to 1,600 (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. A prototype ROTHR system was installed on the isolated Aleutian Island of , Alaska, monitoring the eastern coast of Russia, in 1991 and used until 1993. 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 , covering and the . 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 . 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.
Beginning as early as the 1950s, the Soviets had also studied OTH systems. Their first experimental model appears to be the Veyer (Hand Fan), which was built in 1949. The next serious Soviet project was , built outside on the coast near . 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 .
This was followed by their first operational system , known in the west as , which first broadcast in 1976. Built outside Gomel, near , 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 (ham) operators. The Soviets 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 in Siberia, also covering the continental United States and Alaska.
In early 2014, the Russians announced a new system, called , that was to see over 3000 km.
(Sunflower) - Coast-horizon shortwave station short-range radar. Designed to detect surface and air targets at a distance of 450 km. Designed for use in coastal systems control surface and air situation within the 200-mile economic zone. "Sunflower" allows operators to automatically beyond the radio horizon simultaneously detect, track and classify up to 300 offshore and 100 air objects, determine their coordinates and provide them targeting complexes and systems of armament of ships and air defense systems. Radar has passed state tests in 2008. Currently on duty are three stations - in the Sea of Okhotsk, the Sea of Japan, and the Caspian Sea.
A more recent addition is the developed by the Australian in 1998 and completed in 2000. It is operated by of the . Jindalee is a (multiple-receiver) system using OTH-B, allowing it to have both long range as well as anti- capabilities. It has an official range of 3,000 kilometres (1,900 mi), but in 1997 the prototype was able to detect missile launches by over 5,500 kilometres (3,400 mi) 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.
The 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 B2 Spirit 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 2,000 kilometers, in a 360 degree arc. The frequency range used is from 6 to 30 MHz.
Launched officially in 2009, the French developed a new over-the-horizon radar (High Frequency Surface Wave Radar – HFSWR) capable of monitoring maritime traffic up to 200 nautical miles 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.
A number of OTH-B and OTH-SW radars are reportedly in operation in China. Few details are known of these systems. However, transmission from these radars causes much interference to other international licensed users.
One set of Chinese OTH-B radars is found on Google map for the and .
Iran is working on an OTH radar called , with a reported range of 3,000 kilometres. It is currently operational.
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 100 kilometres (62 mi) 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 or at much lower frequencies. Creeping waves are the scattering into the rear of an object due to , 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 . A newer system has recently been used for coastal surveillance in Canada, and is now offered for sales by as Coast Watcher. Australia has also deployed a High Frequency Surface Wave Radar.