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.
The United States, Australia and Russia use over-the-horizon radars. These radars can locate targets up to a thousand 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 of radars are complementary. Their performance is dependent on how frequent they are used and 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. This allows radar to cover a wide area without affecting earth's curvature. However, the range is somewhat shorter than that of the skywave over the horizon radar. Ground wave over horizon radars can detect ocean dynamics parameters in large areas, and are capable of detecting targets at long distances.
The sky wave above the radar is a new form of radar. The radar uses ionosphere to reflect high-frequency signals. 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. You can extend the radar's range by placing it on top a mountain.
The skywave over the horizon radio can be used to monitor maritime targets. It can detect targets at distances of up to 1000 km. 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. It can detect aircraft targets up to hundreds of kilometers away, in addition to ships.
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 LDJHC300 groundwave above the horizon radar uses vertically polarized low-frequency electromagnetic wave to detect targets.
The United States is home to several OTH-radars. Some of these radars are still in warm storage, others are still operational. The USSR created the first over-the horizon radar system in 1949. Many radar systems were developed since then. These radar systems include radars designed for maritime surveillance, radars intended for homeland security, as well as radars capable of detecting aircraft.