Over The Horizon Radar

In this lecture, we are going to learn about the Over The Horizon Radar, types of OTH radar, and the application of OTH radar in detail. So let’s start with the introduction of Over The Horizon Radar.

Over The Horizon Radar

  • Over-the-horizon radar or OTH (sometimes also beyond the horizon, or BTH) is a designed concept for radar systems to overcome the problem that radio waves ( a form of electromagnetic radiation) tend to travel in straight lines. This generally limits the range, due to the curvature of the earth.
  • For example, a radar mounted on top of a 10-meter must-have range to the horizon of about 13,000m, taking into account the atmospheric refraction effect. If the target is above the surface this range will be increased accordingly, so a target 10 meters high can be detected by the same radar at 26 km.
  • OTH radars use various techniques to see beyond the horizon, making them particularly useful in the early warning radar role.
  • The most common method of constructing an OTH radar is the use of ionospheric reflection. Only one range of frequencies regularly exhibits this behavior: the high frequency (HF) or shortwave part of the spectrum from 3-30 MHz. The variability of the ionospheric propagation makes reliable target detection difficult. A practical system typically uses real-time monitoring of ionospheric characteristics by the reception of backscattered signals to continuously adjust the frequency of the transmitted signal.
Over The Horizon Radar

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The resolution of any radar depends on the width of the beam and the range of the target. 
  • For example, a radar with a 1/2 degree beamwidth with a target at 120 km range will show the target as 1 km wide. Because of the long ranges at which OTH radars are used, the resolution is typically measured in tens of kilometers. This makes the backscatter system almost useless for target engagement, although this sort of accuracy is adequate for the early warning role. In order to achieve a beamwidth of 1/2 degree at HF an antenna array several kilometers long is required.
  • Since the ground and sea will also reflect these signals, 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, noting that moving targets will shift the returned signal’s frequency. By simply filtering out all the signal close to the original broadcast frequency. the 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 the movement of the ionosphere.

Types of OTH radar

  • Over The Horizon Radar (OTH Radar) is used for surveillance over wide areas, well beyond the range of normal ‘line of sight’ radar systems. HF Radar operates normally at frequencies between approximately 5 MHz and 35 MHz.

Two alternative propagation modes:

  1. Skywave OTH Radar, and
  2. Surface Wave OTH Radar may be used, depending on the required range.

1. Sky Wave OTH Radar Uses Ionospheric Propagation:

  • Skywave OTH Radar depends entirely on the ionosphere for successful operation. A suitable frequency is selected and then the Sky wave OTH Radar bounces radio waves from the ionosphere, receiving tiny signals back from reflecting surfaces such as the sea, islands, ships, and aircraft which may be hundreds or even thousands of kilometers away.

2. Surface Wave OTH Radar Follows The Sea-Air Interface:

  • Surface Wave Radar is much less expensive and it covers a smaller range than Sky wave OTH Radar. The efficiency of surface wave radio propagation is lower than sky wave propagation, so greater power is required for smaller ranges. However, Surface Wave Radar can operate independently of the ionosphere, thus eliminating one significant aspect of variations and complexity. Surface Wave OTH Radar typically is proposed for ranges up to 250 or 300 km, although, with sufficient power, some applications suggest results can be obtained at 500 km.
  • OTH Radar systems (Skywave or Surface Wave) can measure the range, azimuth, signal amplitude, and Doppler characteristics of observed objects. Significant differences in capital cost are directly related to the desired accuracies of these parameters, together with overall system reliability.

Advantages of an OTH Radar System

  • The advantages of an OTH (Over-The-Horizon) radar system include:
1.Long-range detection:OTH radar systems can detect targets at ranges of up to several thousand kilometers, which is much farther than other types of radar.
2.Wide coverage:OTH radar systems have a wide coverage area, which allows them to monitor large areas and detect targets beyond the horizon.
3.Independence from weather conditions:OTH radar systems can operate in all weather conditions, including rain, fog, and snow.
4.Difficult to jam:OTH radar systems use a high-frequency band, making them difficult to jam compared to other radar systems.
5.Cost-effective:OTH radar systems are generally less expensive than other long-range radar systems, such as satellite-based systems.
6.Flexibility:OTH radar systems can be easily relocated and reconfigured to adapt to changing operational requirements.
  • Overall, the advantages of an OTH radar system make it a valuable tool for military and civilian applications such as surveillance of air and maritime traffic, border control, and early warning systems for missile attacks.

Limitations of an OTH Radar System

  • The limitations of an OTH (Over-The-Horizon) radar system include:
1.Lower resolution and accuracy:OTH radar systems typically have lower resolution and accuracy compared to other types of radar, such as line-of-sight radar. This is due to the complexity of the ionosphere, which can cause errors in the target location.
2.Susceptibility to ionospheric disturbances:OTH radar systems are affected by ionospheric disturbances, which can cause signal degradation and errors in the target location. These disturbances can be caused by solar activity or other atmospheric phenomena and can make the radar system less reliable.
3.Vulnerability to clutter:OTH radar systems can be affected by clutter, which is the reflection of radio waves from the ground or other objects that are not of interest. This can make it difficult to distinguish between real targets and clutter.
4.High power consumption:OTH radar systems require a lot of power to operate, which can make them less portable and more difficult to deploy in remote locations.
5.Environmental impact:OTH radar systems can have an environmental impact due to the high-power radio waves they emit, which can interfere with other radio communications and potentially harm wildlife.

Application for OTH Radar

Applications for OTH Radar systems are as follows :

  1. Sea-state monitoring and the wide-area surveillance of Exclusive Economic Zones.
  2. Checking for smuggling, piracy, illegal immigrants, and illegal fishing fleets.
  3. Major offshore assets such as oil and gas platforms also can be observed by OTH Radar systems.
  4. Sea-state monitoring can produce data on wave height and direction, assisting in weather forecasting and in the study of ocean dynamics.
  • Australia has achieved a position of world leadership in OTH Radar, through the development of the Jindalee OTH Sky wave Radar System at Alice Springs in the center of Australia, by the Australian Defense Science and Technology Organization (DSTO).
  • This success has encouraged the Australian government to expand the Jindalee system to include additional radar sites, in Queen’s land and Western Australia. Telstra, the Australian telephone company, is now undertaking this work. The expanded system is known as JORN, the Jindalee OTH Radar Network.

Frequently Asked Questions on OTH radar system

Answer: An Over-The-Horizon (OTH) radar system is a type of radar that can detect targets over long distances by reflecting its signals off the ionosphere, a layer of the Earth’s atmosphere.

Answer: An OTH radar system works by transmitting radio waves at a specific frequency towards the sky, where they are reflected off the ionosphere and redirected back to Earth. By analyzing the reflected signal, the radar system can detect and locate objects beyond the horizon.

Answer: OTH radar systems have the advantage of being able to detect targets over very long distances, typically up to several thousand kilometers. They can also operate independently of weather conditions and are difficult to jam.

Answer: The main limitations of OTH radar systems are that they have lower resolution and accuracy compared to other types of radar, and they can be affected by ionospheric disturbances, which can cause signal degradation and errors in the target location.


OTH radar systems are used for various military and civilian applications, including surveillance of air and maritime traffic, border control, and early warning systems for missile attacks.

Hello friends, my name is Trupal Bhavsar, I am the Writer and Founder of this blog. I am Electronics Engineer(2014 pass out), Currently working as Junior Telecom Officer(B.S.N.L.) also I do Project Development, PCB designing and Teaching of Electronics Subjects.

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