Over-the-horizon radar
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), considering atmospheric refraction effects.
The most common type of OTH radar, OTH-B (backscatter),[3] 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.
[1] 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.
[1] 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.
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.
For this role, the delay line was not usable, and the magnetic drum, recently introduced, provided a convenient and easily controlled variable-delay system.
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.
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.
[10] Canada has been investigating the use of High Frequency Surface Wave Radar (HFSWR) for surveillance of the 200 nautical mile Exclusive Economic Zone (EEZ) 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.
[13] The following is a quote from the October 2002 Operational Evaluation (OPEVAL) performed by Canadian Department of National Defence:[14] "HFSWR is a beneficial addition to the Recognized Maritime Picture (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."
[17] In June 2019, MAEROSPACE was granted a global license to design,, manufacture, and internationally market the Canadian HFSWR System and its derivatives.
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 3,000 kilometres (1,900 mi), in a 360-degree arc.
A demonstration site[23] 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.
After successful trials of existing system, India is expected to develop a large OTH radar based on same design.
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 Novaya Zemlya.
The Soviet Union 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.
[33] Other early UK/US systems from the same era include: The United States Air Force Rome Laboratory had the first complete success with their AN/FPS-118 OTH-B.
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.
The Central Radar System (CRS) was needed to complete the perimeter coverage of the southern approaches to North America.
[38] 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.
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.
[41] In 2018, development started on the high-frequency Tactical Multi-Mission Over the Horizon Radar (TACMOR), a technology prototype to expand air and maritime awareness over the Western Pacific.
[43][44] The United States Navy created their own system, the AN/TPS-71 ROTHR (Relocatable Over-the-Horizon Radar), which covers a 64-degree wedge-shaped area at ranges from 500 to 1,600 nautical miles (925 to 3,000 km).
In 1991, a prototype ROTHR system was installed on the isolated Aleutian Island of Amchitka, Alaska, monitoring the eastern coast of Russia.
The third, and final, production system was installed in Puerto Rico, extending anti-drug surveillance past the equator, deep into South America.
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.
A newer system has recently been used for coastal surveillance in Canada, and is now offered for sales by Maerospace,[45] Australia has also deployed a High Frequency Surface Wave Radar.
