Over the Horizon radars can detect targets at a great distance, unlike other radars. Radio waves from low frequencies are capable of traveling beyond the horizon. These waves scatter around obstacles. This is the same method of propagation used in medium-wave AM broadcasting below 1.6 MHz.
Over the horizon radars are used by the United States, Australia, Russia, and Britain. These radars are capable of detecting targets from hundreds to thousands kilometers away. These radars are also useful in monitoring ship movements over large areas. These radars, however are not capable of continuously monitoring an area, as satellites are.
There are two types. Ground wave over the horizon radars are generally used for maritime surveillance, while sky wave radars are used for surface surveillance. 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 reflect the signal back to its receiving antenna. This allows the radar coverage to be large without affecting any curvature of earth. 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 above the radar is a new form of radar. The radar uses the ionosphere as a reflector of high-frequency signal. 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. You can extend the radar's range by placing it on top a mountain.
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's signals are reflected back to their receiving antenna along the same route. This allows radar to cover more area and improve its effectiveness at remote sensing in sea states. This radar is capable of detecting ships and air targets over distances of hundreds of km.
China has two radars for surveillance of the maritime visual line of vision. One of them is the Sunflower radar, which uses a single-hop radar to detect targets at a distance of about four thousand kilometers. The LDJHC300 groundwave above the horizon radar uses vertically polarized low-frequency electromagnetic wave to detect targets.
The United States owns several OTH radars. Some of them remain in warm storage, but others are operational. The first over the horizon radar system was developed by the USSR in 1949. Since then, many radar systems have been developed. These radar systems include radars designed for maritime surveillance, radars intended for homeland security, as well as radars capable of detecting aircraft.