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. Given certain conditions in the atmosphere, radio signals in this frequency range will be reflected back towards the ground. The "correct" frequency to use depends on the current conditions of the atmosphere, so systems using ionospheric reflection typically employ real-time monitoring of the reception of backscattered signals to continuously adjust the frequency of the transmitted signal.
After reflection off the atmosphere, a small amount of the signal will reflect off the ground back towards the sky, and a small proportion of that back towards the broadcaster. 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 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, 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. 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.
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 and 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 kilometers. 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 kilometers long is required.
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 system known as Cobra Mist. Built starting in the late 1960s, Cobra Mist used an enormous 10 MW transmitter 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 Soviets were also working on similar systems during this time, and started operation of their own experimental system in 1971. This was followed shortly thereafter by the first operational system, known in the west as Steel Yard, which started operation in 1976.
The USAF's Rome Laboratory had the first US success with their AN/FPS-118 OTH-B. A prototype with a 1 MW transmitter 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. 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 US border. Additionally, the range was extended to 3000 miles, crossing the equator. This was operated 40 hours a week at random times. Radar data was 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. But the influence from the Senators from Maine was not enough to save the operation and with the ending of the cold war 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 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 at Christmas Valley, Oregon and its receiver site near Tulelake, California. This work was completed by July of 2007 with the demolition and removal of the antenna arrays, leaving the buildings, fences and utility infrastructure at each site intact.
This was followed by their first operational system, known in the west as Steel Yard, which first broadcast in 1976. Built outside Gomel, near Chernobyl, it was aimed northward and covered the continental USA. 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. 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 Lower 48 as well as Alaska.
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 Radar.
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