Unlike most radars, Over the Horizon radars (OTHRs) are able to detect targets even at a long distance. Radio waves at low frequencies can travel far 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 have the ability to detect targets hundreds of miles away. These systems can also be used to monitor ship movements over a large area. However, these radars cannot continuously monitor an area like satellites.
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. They are complementary and their performance is dependent upon their frequency and range.
The ground wave over the horizon radar is able to detect targets at a distance of hundreds of kilometers. The signal is reflected back to the receiving antenna by the ionosphere. 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. Ground wave over the Horizon radars are capable of detecting targets from a distance. They can also detect large ocean dynamics parameters.
The sky wave radar above the horizon is a sophisticated form of the over-the horizon radar. The radar uses the high-frequency signals of the ionosphere to reflect them. It has a small blind zone at a distance of a few hundred kilometers. This makes it useful for detecting low flying targets and for detecting stealthy targets. By placing the radar on top of a mountain, you can increase its range.
The sky wave above the horizon radio is used to maritime surveillance. It can detect targets up to 1000 kilometers away. The radar's signals are reflected back to their receiving antenna along the same route. This allows the radar cover a larger area, which makes it more effective for sea-state remote sensing. It can detect aircraft targets up to hundreds of kilometers away, in addition to ships.
China has two radars that are used for the surveillance of the sea visual line of sight. Sunflower radar is one. It uses a single stop radar to detect targets from a distance of around four thousand kilometers. The LD-JHC300 groundwave over the horizon radar uses vertically polarized high frequency electromagnetic waves to identify targets.
The United States owns several OTH radars. Some of them are in warm storage, while others are operational. The USSR was the one to develop the first over the horizon radar systems in 1949. Since then, many radar systems have 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.