Over-the-horizon radar
Over-the-horizon radar, sometimes called beyond the horizon radar, is a type of radar system with the ability to detect targets at very long ranges, typically hundreds to thousands of kilometres, beyond the radar horizon, 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 systems, but airborne early warning 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, 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 waves used by most radars, in the form of microwaves, propagate in straight lines. This generally limits the detection range of radar systems to objects on their horizon 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 for very long-range detection, and surface wave systems, which use low-frequency radio waves that, due to diffraction, 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.
Skywave systems
The most common type of OTH radar, OTH-B, uses skywave or "skip" propagation, in which shortwave radio waves are refracted off an ionized layer in the atmosphere, the ionosphere, 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 or shortwave part of the spectrum from 3–30 MHz. The best frequency to use depends on the conditions of the atmosphere and the sunspot cycle. 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, 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 computers, 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, uses much lower frequencies, in the longwave 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 waves 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
can degrade the ability of OTH to detect targets. Such clutter can be caused by atmospheric phenomena such as disturbances in the ionosphere caused by geomagnetic storms or other space weather events. This phenomenon is especially apparent near the geomagnetic poles, where the action of the solar wind on the earth’s magnetosphere produces convection patterns in the ionospheric plasma.History
Engineers in the Soviet Union 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 work was dubbed "Project Teepee". Their first experimental system, MUSIC, became operational in 1955 and was able to detect rocket launches away at Cape Canaveral, and nuclear explosions in Nevada at. A greatly improved system, a testbed for an operational radar, was built in 1961 as MADRE at Chesapeake Bay. It detected aircraft as far as 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 magnetic drums. In an attempt to remove clutter from radar displays, many late-war and post-war radar systems added an acoustic delay line 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 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 in the early 1960s. This consisted of several antennas positioned four wavelengths apart, allowing the system to use phase-shift beamforming 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 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. Jindalee is a multistatic radar system using OTH-B, allowing it to have both long range as well as anti-stealth capabilities. It has an official range of, but in 1997 the prototype was able to detect missile launches by China 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 for surveillance of the 200 nautical mile exclusive economic zone 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 performed by Canadian Department of National Defence: "HFSWR is a beneficial addition to the Recognized Maritime Picture. 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 and Europe. 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, 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 announced that Canada would purchase JORN radar technology from Australia, for deployment over the Arctic.