And the Ion will lie down with the Amp

High Frequency Over-The-Horizon radars are a niche capability, which are seeing increasing investment from several nations around the world.

Like other NATO nations in the Arctic region, Canada is facing a renewed threat from a strategically assertive Russia. The two countries are separated by the Arctic Ocean and the US state of Alaska. Nonetheless, any air and ballistic missile attacks targeting Canada and the US mainland would traverse through the skies above Canada. It is therefore imperative to detect any such attacks as early as possible. The Canadian government is therefore acquiring a High Frequency (HF: 3 MHz to 30 MHz) over-the-horizon backscatter (OTH-B) radar. The radar will detect potential air and maritime threats thousands of kilometres from the country’s northern regions. In July 2025, Ottawa announced it had chosen two sites for the radar. The transmitting antenna will be located near Bexley Township, north of Toronto in southern Canada, while Clearview Township, 94 km to the southeast of Bexley will host the receiving antenna. The radar is expected to be operational from 2029, according to the Canadian government.

Many readers will be familiar with HF radio signal use for communications, but perhaps less so regarding HF signal employment for radar. High frequency signals exploit the ionosphere to provide intercontinental ranges for communications. The ionosphere is an atmospheric layer between 48-965 km above sea level. This atmospheric layer is prone to ionisation caused by the sun’s interaction with it. The sun produces ultraviolet and X-Ray radiation, and as this radiation collides with the ionosphere, atoms in the layer become stripped of their electrons which in turn become free electrons. These free electrons effectively stop HF signals leaving the atmosphere, reflecting them back to Earth. If the radio signals are aimed at an angle towards the ionosphere, they will bounce off this layer at a reciprocal angle. In effect, the ionosphere is used as a naturally occurring satellite dish. By bouncing the signal off the ionosphere, it avoids the curvature of the Earth which would otherwise obstruct a radio transmission from one point to another on the surface.

As an example, an HF signal aimed at a 45° angle to a point in the ionosphere 200 km away will be refracted to Earth at an angle of 90° and travel a further 200 km back to the surface. Basic trigonometry reveals that the distance between the radio making the transmission, and the receiver is 283 km. The Earth, however, is not flat and the planet’s curvature must be considered. Another calculation to measure arc length reveals that the range between the two radios is in fact 313 km. The shallower the angle at which the signal is aimed at the ionosphere, the greater the range. Conversely, the steeper the angle at which the signal is aimed, the shorter the range will be.

The ability of radio waves to bounce off the ionosphere makes HF signals attractive for use in radar. As with HF radio, radar signals are bounced off the ionosphere to avoid the Earth’s curvature. This process allows HF radar signals to potentially detect targets beyond the horizon. This attribute also makes HF ideal for early warning. The interest in using HF radar is not so much to determine target range, but in determining if a potential target is in the area where the radar signal meets the surface after refraction.

Waves not pulses

Many radars use pulses of RF energy in their signals to determine the range of a target. A pulse of RF is transmitted by the antenna into the ether. The pulse travels at the speed of light (299,792,458 m/s) and hits a target. This collision causes the pulse to be reflected to the radar’s antenna as an echo. By measuring the time the process takes, and halving it, the radar determines the range between its antenna and the target.

Instead of using RF pulses, HF OTH radars use continuous wave transmissions. To summarise, the radar will send out a constant stream of RF energy at a particular frequency. This stream will perform the same ionospheric refraction process discussed above, exploiting a process called Doppler shift. Doppler shift ascertains if a target of interest is in the radar’s field-of-view. The Doppler effect is the principle by which the frequency of a wave appears to increase or decrease in frequency relative to the observer. Radio waves have peaks and troughs; the more peaks and troughs a radio waves performs per second, the higher the signal’s frequency will be and vice versa.

An oft-quoted example explaining the Doppler effect is what happens to the sound of an emergency vehicle siren when it is approaching or driving away from an observer. As a police car approaches the frequency, and hence the tone, of the signal seems to increase. Conversely, the frequency seems to decrease as the police car drives away. In reality the frequency of the siren is unchanged, but not for the observer. The peaks and troughs take progressively less time to reach the observer as the car approaches, causing a rise in frequency, and progressively more time to reach them as the car drives away. With radar, the frequency of the echo will increase relative to the frequency of the original transmission, if the target is approaching the radar’s antenna. The echo will decrease in frequency if the target is moving away. This difference in frequency is known as Doppler shift.

If no targets are moving in the HF radar’s field-of-view, the frequency of the transmitted signal and the echo will be the same. The movement of the sea can cause Doppler shift but can be predicted with reasonable accuracy based on tidal patterns and meteorology. Doppler shift thresholds can also be programmed into the radar so that the sea’s natural motion is ignored to avoid false alarms. If an HF radar transmits a signal on a frequency of 6 MHz and detects an echo of 6 MHz, no moving targets are in the radar’s field-of-view. If the radar receives an echo with a frequency of 6.000,000.36025 MHz, this will indicate a moving target has been detected. The frequency difference between the outgoing signal and the echo, in this case 0.36052 Hz can be multiplied by the speed of light and divided by the outgoing 6 MHz signal. This calculation gives a velocity indication of 18 m/s). Has the radar detected a warship underway? It is impossible to say for certain, but this could be confirmed with the dispatch of a maritime patrol to provide a confirmation. An echo with a frequency of 6.000,006.18428 MHz may indicate that the radar has detected a combat aircraft moving at 308 m/s.

Bistatic and backscatter

HF radars work to detect the faint echoes refracted by a target. The further a radio signal must go, the less energy it has when it reaches its destination, in this case, the target. When a radar signal hits a target, the echoes can zoom off in all manner of directions. The task of an HF radar’s receiving antenna is to detect at least some of the very low powered echoes scattered back towards it. Therefore, an HF radar’s receiving antenna must be particularly sensitive.

As an example, let us assume a radar transmits a signal with a strength of 100 mW. Radio engineering uses decibels (dB) as a measure of signal strength, with 100 mW corresponding to 20 dB. The radar’s transmitting antenna can produce 30 dB of gain. Gain is a measurement of how much of a radar signal’s power the antenna can focus in a specific direction. The higher the gain, the more signal power hits the target. The combat aircraft travelling at 308 m/s has a radar cross section 1 m² and is at 313 km range. The strength of the signal returned as an echo by the jet will be infinitesimally small compared to the strength of the outgoing 50 dB signal when combining the power with the gain. The echo will have a strength of -88.44 dB, which is a loss of -138.44 from the strength of the outgoing signal.

Weak signals make bi-static architectures essential for HF OTH radars. Most radars transmit and receive their outgoing signals and incoming echoes using the same antenna. The radar transmits a pulse from its antenna and then listens for an echo. The antenna will not transmit while listening to hear the relatively weak signal reflected as an echo. HF OTH radars are always transmitting because they produce a continuous wave which cannot be switched off. A second, receiving antenna is thus needed to detect an echo and must be sited some distance from the transmitting antenna. If positioned too close, and all it would hear is the signal from the transmitting antenna, and the echo would be simply inaudible. Position the receiving antenna far enough away, and the weak signal will be comparatively easier to detect.

Spotting the radars

The use of this bistatic architecture can make HF OTH radars relatively easy for Open-Source Intelligence (OSINT) analysts to locate. These radars often have characteristic antenna arrays pointing in the direction of a particular area of interest for the nation operating the system. The OTH radars will also be recognisable as they typically have two arrays (transmit and receive) positioned some distance apart. A handful of countries operate such radars, but this number may increase in the future.

One of the most famous is Australia’s Jindalee Operational Radar Network, better known as JORN. JORN was developed by BAE Systems and is operated by the Royal Australian Air Force (RAAF). Three radars comprise the JORN architecture: one is in Queensland in northeastern Australia, while a second is in Western Australia with a third in the Northern Territories in the centre of the country. Collectively, these radars watch a swathe of the country’s northwestern air and maritime approaches covering an area of 37,000 km². Targets can be detected at ranges of between 1,000 km and 3000 km, according to open sources. Given the size of Australia’s locale, and the Asia-Pacific region in general, the country needs adequate warning time of incoming threats. The People’s Republic of China (PRC) has emerged as the pre-eminent strategic rival for the US and regional allies such as Australia in this region. Any threats to Australian interests are thus likely to come from the northwest; the area under surveillance by JORN.

Confidential sources have shared with the author that the PRC is building a network of HF OTH radars along the country’s East China and South China Sea coasts. These radars are being sited to provide overlapping coverage of the air and maritime approaches to the PRC at comparable ranges to those offered by JORN. The PRC’s anti-access/area-denial (A2AD) posture places a premium on deterring the US and her allies moving forces to within striking range of targets in the country, and in the PRC’s strategic locale. For the PRC’s A2AD posture to be effective it is imperative that potential threats such as US Navy Aircraft Carrier Strike Groups (CSGs) are detected as early as possible. HF OTH radars will help significantly in this regard. A CSG could be detected through numerous Doppler shift returns showing vessels moving. Aircraft flying at various speeds in a relatively small area around the ships may also be determined. Satellite imagery could confirm the presence of a CSG. The vessels could then be engaged with kinetic effects like People’s Liberation Army Rocket Force DF-21 (NATO reporting name: CSS-5) medium-range ballistic missiles (MRBMs), dubbed ‘carrier killers’. Open sources say this missile has a range of circa 1,700 km.

 

OSINT analysis notes that the Islamic Republic of Iran has deployed several HF OTH radars around the country to detect air and maritime targets at significant range. At least one Ghadir HF OTH system, located 140 km northwest of the city of Tabriz in northwestern Iran, is thought to have been destroyed by the Israeli Air Force (IAF) during the short war between Israel and Iran in June 2025. The radar’s destruction illustrates that such installations would be lucrative targets early in any conflict. Ghadir radars are fixed installations with characteristic designs which could be easily identifiable from satellite imagery.

Canada’s planned AOTHR (Arctic Over-the-Horizon Radar) acquisition, and Australia’s JORN system, show HF OTH radars are appealing to allied nations. Not every country needs such surveillance but the ability to detect air and maritime targets at range across vast distances can be useful. These radars are vital for nations needing to monitor large swathes of water or airspace for potential threats. HF OTH radars do not replace satellite surveillance, but their unique attributes provide useful additional early warning capabilities. Much as Western strategic rivals like Iran, the PRC and Russia may widen and deepen their HF OTH radar coverage to enhance regional A2AD postures, allied nations may follow their example.

Dr Thomas Withington