Over-the-horizon Radar - Alternate OTH Approaches

Alternate OTH Approaches

Another common application of over-horizon radar uses surface waves, also known as groundwaves. Groundwaves provide the method of propagation for medium-wave AM broadcasting below 1.6 MHz and other transmissions at lower frequencies. Groundwave propagation gives a rapidly decaying signal at increasing distances over ground and many such broadcast stations have limited range. However seawater, with its high conductivity, supports groundwaves to and from distances of 100 km or more. This type of radar, surface-wave OTH, is used for surveillance and operates most commonly between 4 and 20 MHz. Lower frequencies enjoy better propagation but poorer radar reflection from small targets, so there is usually an optimum frequency that depends on the type of target being detected.

An entirely different approach to over-the-horizon radar is to use creeping waves or electromagnetic surface waves at much lower frequencies. Creeping waves are the scattering into the rear of an object due to diffraction, which is the reason both ears can hear a sound on one side of the head, for instance, and was how early communication and broadcast radio was accomplished. In the radar role, the creeping waves in question are diffracting around the Earth itself, although processing the returned signal is quite difficult. Development of such systems became practical in the late 1980s due to the rapidly increasing processing power available. Such systems are known as OTH-SW, for Surface Wave.

The first OTH-SW system deployed appears to be a Soviet system positioned to watch traffic in the Sea of Japan, while a newer system has recently been used for coastal surveillance in Canada. Australia has also deployed a High Frequency Surface Wave Radar.