Over-the-horizon radar, or OTH (sometimes also beyond the horizon, or BTH), is a design concept for radar systems to allow them to detect targets at very long ranges, typically up to thousands of kilometers. Several OTH radar systems were deployed starting in the 1950s and 60s as part of early warning radar systems, but these have generally been replaced by airborne early warning systems instead.
United States Air Force OTH-B
The US Air Force’s over-the-horizon-backscatter (OTH-B) air defense radar system is by several criteria the largest radar system in the world. Six one-million-watt OTH radars see far beyond the range of conventional microwave radars by bouncing their 5-28-MHz waves off the ionosphere, an ionized layer about 200 km above the earth. It was developed over 25 years at a cost of $1.5 billion to warn against Soviet bomber attacks when the planes were still thousands of miles from US air space.
Jindalee Operational Radar Network
The Jindalee Operational Radar Network (JORN) is an over-the-horizon radar network that can monitor air and sea movements across 37,000 km2. It has an official range of 3,000 km but depending upon certain atmospheric conditions has a range up to and including the Korean peninsula. It is used in the defence of Australia and can also monitor maritime operations, wave heights and wind directions.
US OTH Research
The Blossom Point facility is a 1,600 acre U.S. Army installation located near LaPlata, Maryland in southern Charles County. The Blossom Point Field Test Facility is an active facility under the U.S. Army Research Laboratory, Adelphi, Maryland.
Former Soviet Union OTH
Operational ballistic missile early warning was provided by 11 large HEN HOUSE detection and tracking radars deployed at six locations on the periphery of the USSR. Most of these radars are located outside the territory of Russia. These radars can distinguish the size of an attack, confirm the warning from the satellite and over-the-horizon radar systems, and provide target-tracking data in support of antiballistic missile (ABM) deployments.
