Over the Horizon radars are able detect targets even at a distance unlike most radars. This is because radio waves from low frequencies can travel beyond the horizon. These waves scatter around obstacles. This is the same method used to propagate medium-wave AM broadcasting lower than 1.6 MHz.
Australia, Russia, Britain and the United States use over-the horizon radars. These radars have the ability to detect targets hundreds of miles 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 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.
The ground wave above the horizon radar can detect targets at distances of hundreds of km. The ionosphere scatters the signal and reflects it back to the antenna. The radar can cover large areas without having to affect the earth's curvature. The range is much shorter than that provided by the sky wave over-the-horizon radar. Ground wave radars over the horizon can also detect large-area ocean dynamics parameters.
Skywave over the Horizon radar is a more advanced version of over-the-horizon radar. The radar uses ionosphere to reflect high-frequency signals. It has a small blind area at distances of about 100 kilometers. It is suitable for low-flying targets and stealth targets. By placing the radar on top of a mountain, you can increase 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 enables the radar to cover a large area, and is therefore more effective at sea-state remote sensing. Aside from being able to detect ships, it can also detect air targets at a distance of hundreds of kilometers.
Two radars are used in China to monitor the visual line of sight. Sunflower radar is one. This radar uses a single-hop radar that can detect targets at about four thousand km. 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 are stored in warm storage while others remain operational. In 1949, the USSR developed the first radar system that could detect the over-the-horizon. Numerous radar systems have since been developed. These include radars that are used for maritime surveillance, radars that are used for homeland security, and radars that are used to detect aircraft.