Technology
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 (33 ft) mast has a range to the horizon of about 13 kilometres (8.1 mi), taking into account atmospheric refraction effects. If the target is above the surface this range will be increased accordingly, so a target 10 m (33 ft) high can be detected by the same radar at 26 km (16 mi). In general it is impractical to build radar systems with line-of-sight ranges beyond a few hundred kilometres. OTH radars use various techniques to see beyond the horizon, making them particularly useful in the early warning radar role.
A method of design for an OTH radar is the use of ionospheric reflection. 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 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 – 45 MHz. Given certain conditions in the atmosphere, radio signals in this frequency range will be reflected back towards the ground. The "correct" frequency to use 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 "backscatter" 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 Doppler effect, 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 itself.
The resolution of any radar depends on the width of the beam and the range to the target. For example a radar with a 1/2 degree beamwidth 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 1/2 degree at HF, an antenna array several kilometres long is required.
Read more about this topic: Over-the-horizon Radar
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