Over-the-horizon

Over-the-horizon radar, or OTH (sometimes called beyond the horizon, or BTH), 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systems were deployed starting in the 1950s and 1960s as part of early warning radar systems, but these have generally been replaced by airborne early warning systems instead. OTH radars have recently been making a comeback, as the need for accurate long-range tracking becomes less important with 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.

The super high frequency radio waves used by most radars, called microwaves, travel in straight lines. This generally limits the detection range of radar systems to objects on their horizon (generally referred to as "line of sight" since the aircraft must be at least theoretically visible to a person at the location and elevation of the radar transmitter) due to the curvature of the Earth. 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), taking into account atmospheric refraction effects. If the target is above the surface, this range will be increased accordingly, so a target 10 m (33 ft) high can be detected by the same radar at 26 km (16 mi). 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 reflect their signals off the ionosphere for very long-range detection, and surface wave systems, which use high 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 microwave frequencies using a chirp transmitter.

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