Over the Horizon radars are able detect targets even at a distance unlike most radars. Because radio waves of low frequencies travel beyond the horizon, this is possible. These waves scatter around obstacles. This is the same method used to propagate medium-wave AM broadcasting lower than 1.6 MHz.
Australia, Russia and Britain use radars above the horizon. 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. Unlike satellites, however, these radars are not able to continuously monitor an area.
Over the horizon radars can be either ground wave or skywave. Sky wave radars can be used for surface surveillance and maritime surveillance. Ground wave radars over the horizon are used primarily for maritime surveillance. Both are complementary and performance is dependent both on their frequency as well as their range.
Ground wave radar can detect targets up to hundreds of kilometers away. The ionosphere reflects the signal back to the antenna. This makes it possible for the radar to cover a large area without affecting the curvature of the earth. However, the range is somewhat shorter than that of the skywave over 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 skywave radar over the horizon is an advanced form over the-horizon radar. The radar uses the ionosphere as a reflector of high-frequency signal. It has a relatively small blind zone located at about a 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 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. The radar can not only detect ships but also air targets from hundreds of kilometers away.
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 LDJHC300 groundwave above the horizon radar uses vertically polarized low-frequency electromagnetic wave to detect targets.
The United States has many 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. Many radar systems were developed since then. These include radars that are used for maritime surveillance, radars that are used for homeland security, and radars that are used to detect aircraft.