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 diffract around obstacles. This is the same propagation technique used in medium-wave AM broadcasting at 1.6 MHz.
Australia, Russia, Britain and the United States use over-the horizon radars. These radars can detect targets hundreds of kilometers away. These radars can also be used for monitoring ship movements across large areas. Unlike satellites, however, these radars are not able to continuously monitor an area.
There are two types: ground wave and skywave over the horizon radars. Sky wave radars can be used for surface surveillance and maritime surveillance. Ground wave radars over the horizon are used primarily for maritime surveillance. These two types are complementary, and their performance is dependent on their frequency and their range.
The ground wave from the horizon radar detects targets at hundreds of kilometers distance. The signal is reflected back to the receiving antenna by the ionosphere. This makes it possible for the radar to cover a large area without affecting the curvature of the earth. The range is a bit shorter than the sky wave over the horizon radar, however. 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. The radar uses the ionosphere as a reflector of high-frequency signal. It has a small blind area at distances of about 100 kilometers. This makes it suitable for detecting low-flying targets, and for detecting stealth targets. Mounting the radar on top can extend its range.
The skywave above the horizon radio serves as maritime surveillance. It can detect targets anywhere from 1000 to 4000 km away. The radar's signal reflects back to the receiving radio along 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. Apart from being capable of detecting ships, the radar can also detect targets in the air at distances up to hundreds and thousands of kilometers.
China has two radars that are used for the surveillance of the sea 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 LDJHC300 ground-wave over the horizon radar uses vertically polarized electromagnetic waves to detect targets.
The United States has several OTH radars. Some of them are kept in warm storage and others are operational. The first over the horizon radar system was developed by the USSR in 1949. Many radar systems were developed since then. These radars include radars used for maritime surveillance and radars used for homeland security.