Over the Horizon radars or OTHRs can detect targets from a very long distance. This is unlike any other radars. Radio waves at low frequencies can travel far beyond the horizon. These waves can scatter around obstacles. This propagation method is used for medium-wave AM broadcasting below 1.66 MHz.
The United States, Australia and Russia use over-the-horizon radars. These radars can detect targets hundreds to thousands of kilometers away. These radars can also monitor ship movements in large areas. Unlike satellites, however, these radars are not able to continuously monitor an area.
There are two types, ground wave radars and skywave radars, of over-the-horizon radars. Skywave radars and ground wave radars are both used for maritime surveillance. These two types are complementary, and their performance is dependent on their frequency and their range.
The ground wave above the horizon radar can detect targets at distances of hundreds of km. The ionosphere mirrors the signal back at the receiving antenna. This allows the radar to cover large areas without affecting the earth's curvature. However, this range is not as long as the skywave above the horizon radar. Aside from being able to detect targets at a long distance, ground wave over the horizon radars can also detect large area ocean dynamics parameters.
The sky wave radar above the horizon is a sophisticated form of the over-the horizon radar. This radar uses the ionosphere for high-frequency signals to reflect. It has a small blind zone at a distance of a few hundred kilometers. This makes it ideal for detecting low-flying targets as well as stealth targets. The radar's range can be extended by placing it on top of a mountain.
The skywave above the horizon radio serves as maritime surveillance. It can detect targets anywhere from 1000 to 4000 km away. The radar signal is reflected back at the receiving antenna on the same path. This enables the radar to cover a large area, and is therefore more effective at sea-state remote sensing. This radar is capable of detecting ships and air targets over distances of hundreds of km.
China has two radars used to monitor the sea visual line. Sunflower radar is one. It uses a single stop radar to detect targets from a distance of around four thousand kilometers. The LD-JHC300 groundwave over the horizon radar uses vertically polarized high frequency electromagnetic waves to identify targets.
The United States has many OTH radars. Some of them remain in warm storage, but others are operational. The USSR created the first over-the horizon radar system in 1949. Since then, numerous radar systems has been developed. These radars include radars used for maritime surveillance and radars used for homeland security.