Over-the-horizon radar
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Over-the-horizon radar, or OTH (sometimes also beyond the horizon, or BTH), is a design concept for radar systems to overcome the problem that radio waves (a form of electromagnetic radiation) tend to travel in straight lines. This generally limits the range, due to the curvature of the Earth. For example, a radar mounted on top of a 10 metre mast has a range to the horizon of about 13,000 m, taking into account atmospheric refraction effects. If the target is above the surface this range will be increased accordingly, so a target 10 metres high can be detected by the same radar at 26 km. OTH radars use various techniques to see beyond the horizon, making them particularly useful in the early warning radar role.
The most common method of constructing an OTH radar is the use of ionospheric reflection. Only one range of frequencies regularly exhibits this behaviour: the high frequency (HF) or shortwave part of the spectrum from 3 – 30 MHz. The variability of the ionospheric propagation makes reliable target detection difficult. A practical system typically uses real-time monitoring of ionospheric charateristics by 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 a 1/2 degree beamwidth with a target at 120 km range will show the target as 1 km wide. Because of the long ranges at which OTH radars are used, the resolution is typically measured in tens of kilometres. This makes the backscatter system almost useless for target engagement, although this sort of accuracy is adequate for the early warning role. In order to achieve a beamwidth of 1/2 degree at HF an antenna array several kilometres long is required.
Since the ground and sea will also reflect these signals, 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, noting that moving targets will shift the returned signal's frequency. By simply filtering out all the signal close to the original broadcast frequency, the moving targets become visible. 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.
Much of the early research into OTH systems was carried out under the direction of Dr. William J. Thaler at the Naval Research Laboratory; The work was dubbed "Project Teepee" (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 away at Cape Canaveral, and nuclear explosions in Nevada at 1,700 miles. A greatly improved system, a testbed for an operational radar, was later built in 1961 as MADRE (Magnetic-Drum Radar Equipment) at Chesapeake Bay. As the names imply, both systems relied on the comparison of returned signals stored on magnetic drums, then the only high-speed storage systems available.
The first truly operational development was an Anglo-American experiment known as Cobra Mist. Built starting in the late 1960s, Cobra Mist used an enormous 10 MW broadcaster and could detect aircraft over the western USSR from its location in Suffolk. When the system started testing in 1972, however, an unexpected source of noise proved to render it unusable. They eventually abandoned the site in 1973, the source of the noise never having been identified.
The USAF's Rome Laboratory tried again with their AN/FPS-118 OTH-B. A prototype with a 1 MW broadcaster and a separate receiver was installed in Maine, offering coverage over a 60 degree arc between 900 to 3,300 km. 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 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, and a one-sector system facing south. With the ending of the cold war the Alaska and southern-facing sites were cancelled, the two so-far completed western sectors turned off, and the eastern ones used for countering drug trafficing.
The US Navy also created their own system, the AN/TPS-71 ROTHR (Relocatable Over-the-Horizon Radar), which covers a 64 degree wedge-shaped area at ranges between 500 to 1,600 nautical miles (925 to 3,000 km). ROTHR was originally intended to keep track of ship and aircraft movement over the Atlantic, and thus allow coordinated fleet movements well in advance of an engagement. A prototype ROTHR system was installed on the isolated Aleutian Island of Amchitka, Alaska, monitoring the eastern coast of Russia, in 1991 and used until 1993, but was later moved to Virginia where it also operates in the drug trafficing role covering Central America and the Caribbean. A second ROTHR was later set up in Texas, covering many of the same areas in the Atlantic, but also providing coverage over the Pacific as far south as Colombia.
The Soviets also operated an OTH system between 1976 and 1989. It was designated "Steel Yard" by NATO, but its loud and repetitive pulses in the middle of the shortwave radio bands led to it being known as the Russian Woodpecker by amateur radio (ham) operators. Upset by the interference, some amateur operators attempted to jam it by transmitting 'dots' from automatic morse keyers on the same frequency, although it is not clear if this had the desired effect. 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 more recent addition is the Jindalee over-the-horizon radar developed by the Australian Department of Defence in 1998 and completed in 2000. Jindalee is a multistatic radar (multiple-receiver) system using OTH-B, allowing it to have both long range as well as anti-stealth capabilities. Interestingly, Jindalee uses very little power, only 20 kW as compared to the US's OTH-B's 1 MW, yet offers far better range due to considerably improved electronics and signal processing.
An entirely different approach to over-the-horizon radar is to use creeping waves at much lower frequencies. Creeping waves are the scattering into the rear of an object due to diffraction, 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 itself, although processing the returned signal is quite 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, while a newer system has recently been used for coastal surveillance in Canada. Australia has also deployed a High Frequency Surface Wave RadarSenator Robert Hill, [Landmark Land Use Agreement For High Frequency Surface Radar], Ministerial Press Release number 33/2004 from the Australian Department of Defence, February 25, 2004.
References
External links
- [The Development of Over-the-Horizon Radar in Australia] - paper by D.H. Sinnott on the Australian Department of Defence website
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