What is an Over-the-Horizon Radar?

Over-the-Horizon (OTH) radars are radar systems that can detect targets very far away and beyond the range limit for ordinary radars. They operate in HF frequency range from 5 to 30 MHz and are suitable for detecting targets typically hundreds to thousands of kilometers away. These radars use powerful radio signals which are transmitted with the help of a large antenna or an  array of antennas. These signals get reflected by the ionosphere to reach the target. Over-the-Horizon (OTH) radars are usually used as early warning and threat detection systems. They are also used for navigation and surveillance in commercial and military ships.

OTH radars use two main techniques to detect long-range targets:

1. Shortwave Propagation: In shortwave propagation, radio signals in the HF range (3 to 30 MHz) are transmitted at a certain angle from an antenna or an array of antennas. These signals get refracted towards the ground by the ionosphere, thus allowing them to return to earth – beyond the horizon. Upon reaching the target, a portion of these signals get reflected back to the ionosphere and return to the source via the same path. 

For an efficient reflection process, the angle between the signal and the ionosphere, which is generally about 2–4° off the local horizon should be very precise. Transmitting a beam at this angle requires enormous antenna arrays and highly reflective ground along the path through which the signal is being sent. This is done by installing wire mesh mats extending up to 3 kilometers (1.9 mi) in front of the antenna. Real-time monitoring of atmospheric conditions, extreme low-noise amplifiers, and computers are used to efficiently and accurately locate and measure velocity of the target.

2. Surface Wave Propagation: In this technique, low frequency radio signals are transmitted and end up geting diffracted due to obstacles and follow the curving contour of the earth, traveling beyond the horizon. A backscatter of signals reflected by the target return to transmitter using the same path.

In this mode of propagation, radio signals have the longest range over sea. It requires a highly sensitive receiver system to detect the backscatter. Since these signals travel close to the surface, they are generally used for tracking ships, rather than aircrafts.