Radars in their simplest form, use radio signals to determine the location of moving targets. A transmitter sends a series of short bursts from an antenna toward a predetermined spot. Echoes from any objects in the signal’s path are detected by a receiver. The distance to an object is determined by the time difference between the pulse transmission and the echo’s arrival.
While this feature is quite useful, it had several drawbacks. Radio waves move in a straight line and require a clear line of sight between the antenna and the object being detected. Interference and dispersion caused by weather, atmospheric conditions, or other radio waves might also limit how far radar signals may travel and result in false-positive identifications.
At the very best, radar was only capable of detecting objects only to the line-of-sight horizon.
The ability to see things outside of the radar horizon was a problem and a scientific mystery that SRI International helped solve, resulting in the invention of over-the-horizon radars (OTHR).
Stanford University’s wide-aperture research facility, laid the groundwork for developing radar that can see over the horizon. During the late 1960s, SRI took over this advancement from Stanford and transformed radio waves’ use to detect distant things. In today’s world it’s radar ranges have been expanded to over thousands of miles away from the transmitter.
Now a days over-the-horizon radar (OTH) is very common among developed countries with systems located in the United States, China, Russia, UK, Australia, Canada, Germany, France, Brazil, etc.
Over-the-Horizon (OTH) radars are very powerful and are usually utilized as early warning and threat detection tools. Extremely useful for militaries in providing protection and intel gathering. They’re also used in commercial and military ships for navigation and surveillance.
What Is Over the Horizon Radar?
Over-the-horizon radar (OTH), sometimes referred to beyond the horizon (BTH), is a form of radar technology with the capacity to detect objects at long distances, often hundreds to thousands of kilometers, beyond the radar horizon.
Over-the-horizon radar operates in high-frequency(HF) range from 5 to 30 MHz.
Beginning in the 1950s and 1960s, several OTH radar systems were utilized as part of early warning radars, but these have mostly been supplanted by airborne early warning systems.
Most radars, which employ microwaves, generate radio waves in straight lines. Because of the shape of the Earth, radar systems’ detection ranges are restricted to things on their horizon (usually referred to as “line of sight” since the aircraft must be at least theoretically visible to a person at the location and elevation of the radar transmitter.
Siting an antenna on a high mountain might improve the range somewhat; however, line-of-sight ranges beyond a few hundred kilometers are generally impossible to achieve.
OTH radars employ various techniques to see well beyond the limit of traditional radar systems. The most frequent techniques are shortwave systems that refract their signals off the ionosphere for long-range detection, and surface wave systems, which use low frequency radio waves that diffract to follow the curvature of the Earth beyond the horizon.
The Ionosphere is a region in Earth’s upper atmosphere between 80 and 600 kilometers high where Extreme UltraViolet (EUV) and x-ray solar radiation ionize the atoms and molecules, producing a layer of electrons.
These systems can detect objects beyond a hundred kilometers away from modest, conventional radar stations. A chirp transmitter may be used to scan a number of high frequencies.
Skywave Systems
The most frequent type of OTH radar employs skywave or “skip” propagation, in which shortwave radio waves are refracted by an ionized layer in the atmosphere, the ionosphere.
When certain environmental conditions are present, radio signals sent into the sky at an angle will be refracted toward the ground and allowed to return to earth beyond the horizon, thanks to the ionosphere.
The remainder of this signal will be scattered off intended targets towards the sky, refracted through the ionosphere again, and returned to the receiving antenna along the same route. This effect is only seen in one range of frequencies on a regular basis: from 3 MHz to 30 MHz in the high-frequency (HF) or shortwave portion of the spectrum.
The best frequency to utilize is determined by the current atmosphere conditions and the solar cycle. For these reasons, skywave systems generally employ real-time monitoring of backscattered signal reception to constantly adjust the transmitted signal’s frequency.
Any radar’s resolution is determined by the breadth of the beam and the distance to the target. For example, a radar with a 1-degree beam width and a target at 120 kilometers (75 miles) will display it as 2 kilometers (1.2 miles) wide.
1.5 kilometers (0.93 mi) wide is needed to generate a 1-degree beam at the most frequent frequencies. Actual accuracy is even lower, with range resolution on the order of 20 to 40 kilometers (12–25 mi) and bearing accuracy of 2 to 4 kilometers (1.2–2.5 mi) being suggested. Even a two kilometer accuracy isn’t enough for weapons fire; it’s only good for early warning.
Another challenge is that the refraction process is highly sensitive to the angle between the signal and the ionosphere, which is generally confined to 2–4 degrees off the local horizon.
A beam at this angle necessitates the construction of enormous antenna arrays and highly reflecting ground along the signal’s route, usually accomplished by the use of wire mesh mats that extend 3 kilometers (1.9 miles) in front of the antenna. OTH systems are thus costly to develop and impossible to move due to their high cost and immobility.
Given the refraction losses, this “backscatter” signal is extremely faint, which is one reason why OTH radars were not practical until the 1960s, when extremely low-noise amplifiers were first invented.
Since the signal refracted from the ground, or sea, will be much stronger than that refracted from a “target,” some method must be employed to distinguish targets from background noise.
The Doppler effect, which makes use of frequency shift produced by moving objects to measure their speed, is the simplest approach to do so. Moving targets become apparent by filtering out all backscatter signals near to the original transmitted frequency. Even the smallest amount of movement may be observed with this technique; speeds as slow as 1.5 knots (2.8 km/h) are possible to pick up.
Modern radars almost all employ this basic principle, but OTH systems make it considerably more difficult owing to the same ionospheric effects that cause movement. To measure the movement of the ionosphere and update the main radar in real time, most systems utilized a second transmitter broadcasting directly up at the ionosphere.
OTH systems required the use of computers, which is another reason they were not truly useful until the 1960s, when solid-state high-performance systems were developed.
Ground Wave Systems
A different sort of OTH radar uses much lower longwave frequencies. Radio waves at these frequencies can bend around objects and follow the curving contour of the earth, traveling beyond the horizon.
By the same route, the echo’s were reflected back to the transmitter location. These ground waves have the greatest range over water. The signal received from these ground wave systems is very faint, and sophisticated electronics are required.
Because low-frequency radio waves propagate near the surface and lower resolutions are required, low-frequency systems are generally employed to track ships rather than aircraft. However, bistatic methods and computer processing can produce higher resolutions, which have been utilized since the 1990s.