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Transcript of Over-the-horizon radar
is a type of radar system with the ability to detect targets at very long ranges, typically
up to 4000 km
. Several OTH radar systems were deployed starting in the 1950s and 1960s as part of early warning radar systems, but these have generally been replaced by airborne early warning systems instead. OTH radars have recently been becomes less-expensive ground based radars are once again being considered for roles such as maritime reconnaissance and drug enforcement.
Radio waves, a form of electromagnetic
radiation, tend to travel in straight lines.
This generally limits the detection range of
radar systems to objects on their horizon
due to the curvature of the Earth. For example, a radar mounted on top of a 10 m mast has a range to the horizon of about 13 km (8.1 mi), taking into account atmospheric refraction effects. In general it is impractical to build
radar systems with line-of-sight ranges
beyond a few hundred kilometres.
The most common type of OTH radar uses
. Given certain conditions in the atmosphere, radio signals broadcast up towards the ionosphere will be reflected back towards the ground. After reflection off the atmosphere, a small amount of the signal will reflect off the ground back towards the sky, and a small proportion of that will reflect back towards the broadcaster. Only one range of frequencies regularly exhibits this behaviour: the high frequency (HF) or shortwave part of the spectrum from 3 – 30 MHz. The
depends on the current conditions of the atmosphere
, so systems using ionospheric reflection typically employ real-time monitoring of the reception of backscattered signals to continuously adjust the frequency of the transmitted signal.
Given the losses at each reflection, this
signal is extremely small, which is one reason why OTH radars were not practical until the 1960s, when extremely low-noise amplifiers were first being designed. Since the signal reflected from the ground, or sea, will be very large compared to the signal reflected from a "target", some system needs to be used to distinguish the targets from the background noise. The easiest way to do this is to use the
, which uses frequency shift created by moving objects to measure their velocity. By filtering out all the backscatter signal close to the original transmitted frequency, moving targets become visible. This basic concept is used in almost all modern radars, but in the case of OTH systems it becomes considerably more complex due to similar effects introduced by movement of the ionosphere.
A second type of OTH radar uses much lower frequencies, ones that will diffract around the surface of the earth, and especially over the sea. Like the ionospheric high-frequency systems, the received signal from these groundwave systems is very low, and demands extremely sensitive electronics. Because these signals travel close to the surface and lower frequencies produce lower resolutions, low-frequency systems are generally used for tracking ships, rather than aircraft. However, the use of bistatic techniques and computer processing can produce higher resolutions, and has been used as of 1990s.
The resolution of any radar depends on the width of the beam and the range to the target. For example a radar with 0.5 degree beam width and a target at 120 km (75 mi) range will show the target as 1 km (0.62 mi) wide. Because of the long ranges at which OTH radars are used, the resolution is typically measured in tens of kilometres. This makes the backscatter system almost useless for target engagement, although this sort of accuracy is more than adequate for the early warning role. In order to achieve a beamwidth of 0.5 degree at HF, an antenna array several kilometres long is required.