Over the Horizon radars (OTHRs), unlike most radars, can detect targets even from a faraway distance. This is because radio waves from low frequencies can travel beyond the horizon. These waves diffract around obstacles. This is the same method used to propagate medium-wave AM broadcasting lower than 1.6 MHz.
Over the horizon radars are used by the United States, Australia, Russia, and Britain. These radars can locate targets up to a thousand kilometers away. These systems can also monitor ship movements across large areas. These radars, however, are not able monitor an area continuously like satellites.
There are two types, ground wave radars and skywave radars, of over-the-horizon radars. Ground wave radars that are over the horizontal are usually used for maritime surveillance. Skywave radars, on the other hand, are used for surveillance of the surface. Both are complementary and performance is dependent both on their frequency as well as their range.
Ground waves over the horizon radar are capable of detecting targets from hundreds of kilometers away. The ionosphere reflects the signal back to the antenna. The radar can cover large areas without having to affect the earth's curvature. However, the range is somewhat shorter than that of the skywave over 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 over the horizon radar is an advanced form of over the horizon radar. The radar uses ionosphere to reflect high-frequency signals. It has a narrow blind zone, which is located at a distance between a few hundred and a few hundred km. This makes the radar suitable for detection of low-flying targets or stealth targets. Mounting the radar on top can extend its range.
The skywave radio above the horizon is used for maritime surveillance. It can detect targets as far away as 1000 to 4000 kilometers. The radar signal is reflected back at the receiving antenna on the same path. This allows the radar's signal to be reflected back at the receiving antenna along the same path, making it more efficient for remote sensing of sea-states. This radar is capable of detecting ships and air targets over distances of hundreds of km.
Two radars are used in China to monitor the visual line of sight. Sunflower radar, one of the two radars used to monitor the sea visual line of sight in China, uses a single hop radar to detect targets up to four thousand kilometers away. The other is the LD-JHC300 ground wave over the horizon radar, which uses vertically polarized high-frequency electromagnetic waves to detect targets.
The United States has many OTH radars. Some of them are kept in warm storage and others are operational. In 1949, the USSR invented the first system for over-the-horizon radar. Since then, many radar systems have been developed. There are radars for maritime surveillance, homeland security and aircraft detection.