The History of Radar | Radar History: Isle of Wight Radar During The Second World War | Radar: The Basic Principle
Radar Technology: Main Components | Radar Technology: Side Lobe Suppression | Radar Technology: Airborne Collision Avoidance
Radar Technology: Antennas | Radar Technology: Antenna Beam Shapes | Radar Technology: Monopulse Antennas | Radar Technology: Phased Array Antennas | Radar Technology: Continuous Wave Radar | Theoretical Basics: The Radar Equation
Theoretical Basics: Ambiguous Measurements | Theoretical Basics: Signals and Range Resolution
Theoretical Basics: Ambiguity And The Influence of PRFs | Theoretical Basics: Signal Processing | Civilian Radars: Police Radar | Civilian Radars: Automotive Radar | Civilian Radars: Primary and Secondary Radar
Civilian Radars: Synthetic Aperture Radar (SAR) | Military Applications: Overview | Military Radars: Over The Horizon (OTH) Radar
How a Bat's Sensor Works | Low Probability of Intercept (LPI) Radar | Electronic Combat: Overview | Electronic Combat in Wildlife
Radar Countermeasures: Range Gate Pull-Off | Radar Countermeasures: Inverse Gain Jamming | Advanced Electronic Countermeasures

It is commonly assumed that nobody can look around corners. However, some radars can. The 'corner' to be looked around is the horizon, and more often than not there are interesting surface objects which are below the horizon but still at ranges where surveillance coverage is desired. The beams emitted by 'ordinary' radars operating at microwave frequencies follow the rules of ray optics - their signals propagate along a straight line and that's about it. These radars can be overcome by aircraft approaching at extremely low altitudes (on the order of 10m) - a tactics employed by military aircraft and drug transporters alike.

Over the Horizon radars close the low altitude gap and provide advance warning which directly translates into reaction time. There are two different mechanisms which can be exploited: HF Skywave and HF Ground Wave. The HF or 'High Frequency' range is the part of the electromagnetic spectrum between 3MHz and 30MHz.

HF Ground Wave Propagation: Diffraction

The calculation of propagation paths via ray optics is only valid if the wavelength is much smaller than the dimensions of any other objects which happen to be around. The case of sending an HF signal of some 100m wavelength along the boundary between air and a surface cannot be handled with the methods of ray optics. Rather, it is to be dealt with using the laws of diffraction between media of different dielectric properties. Diffraction means that the beam of electromagnetic energy is bent into the direction of the material which is more 'dense', or has the bigger epsilon value. The net effect is that an HF signal is hugging the ground, or creeping along the boundary. This is the HF ground wave.

Of course, diffraction occurs with light as well. A famous example is the sunset, preferably observed from a bar at the shores of a South Pacific island. When the sun appears to be touching the ocean's surface then it actually is already below the horizon. The density of the atmosphere significantly decreases with height and at this time of day, the sunlight enters the atmosphere from a grazing angle. Thus the propagation path gets bent towards the ground and even human eyes get a chance to look around the corner. Diffraction is also responsible for the dark orange or red colour that the sun takes on in this situation: the effect of diffraction is more pronounced at longer wavelengths, and the red/orange parts of the visible spectrum represent the longer wavelengths. At sunset, sunlight still contains the yellow, green and blue colours (that is, the shorter wavelengths) too but these get diffracted to a lesser degree. Thus, only the red/orange parts get bent down and eventually hit the ground whereas the other colours only 'touch' the atmosphere and leave it through the back door, so to speak. To sum it up, the atmosphere works like a huge prism which splits sunlight into its components and sends them into different directions.

HF ground wave has been employed for communications since the first days of radio, but research into radar applications began no earlier than in the late 1980s.

The HF ground wave can yield up to 200km coverage over sea, with no gap in the elevation coverage. Application areas are surveillance of littoral waters or detection of sea-skimming missiles. Some prototypes exist, but operational types are not yet known.

HF Skywave Propagation: Using a Mirror

Imagine that a mirror was suspended from the ceiling of a room, and in parallel to the ground. Then, by looking from an angle upward into the mirror, it would be possible to see things even if there was an obstacle in the direct line of sight. In the case of radars operating in the HF frequency range, such a mirror is available. It is the ionosphere, one of several layers of Earth's atmosphere which is located some 200km above ground level.

The atmosphere is exposed to radiation which emanates from the sun, and some of this radiation is powerful enough to separate electrons from the gas atoms - that is, to ionise the gases and create a plasma layer. Within a plasma, electrons are free to move around. High electron mobility is precisely what makes up the reflecting properties of a mirror. The ionosphere doesn't make up an ideal mirror because the height of the reflecting layers varies with the time of day and depends on other factors too. Furthermore, there is no abrupt boundary separating the ionosphere from the lower atmosphere layers. In other words, the ionosphere is a mirror which yields a somewhat blurred image and keeps floating up and down. The reflecting properties of the ionosphere are limited to the frequency range between some 4MHz and around 28MHz, depending on the time of day.

Since the 1920s, radio communication services have been exploiting this effect for extremely long-range communication links. Skywave propagation can yield as much as 3000km coverage with a single 'hop,' or 6000km if the beam is bounced off the ionosphere twice, being reflected from the surface of an ocean in-between. As it makes use of the ionospheric mirror, this type of OTH radar is also called OTH Backscatter, or OTH-B radar.

Needless to say, an echo received from 6000km away is incredibly weak. Therefore it takes complicated signals, highly-sophisticated signal processing schemes and very sensitive receivers in order to achieve anything useful. Due to the low frequency and the large wavelengths associated with them, OTH-B radars require extremely big antenna arrays which stretch out for some two to three kilometres. OTH-B radars are nowadays operational in the USA (ROTHR) and Australia (Jindalee, JORN), for purposes like such as the detection of drug traffic over the Gulf of Mexico, long range surveillance, oceanography, or monitoring hurricanes.

History: Overview | Isle of Wight Radar During WWII
Technology: Basic Principle | Main Components | Signal Processing | Antennae | Side Lobe Suppression | Phased Array Antennae | Antenna Beam Shapes | Monopulse Antennae | Continuous Wave Radar
Theoretical Basics: The Radar Equation | Ambiguous Measurements | Signals and Range Resolution | Ambiguity and PRFs
Civilian Applications: Police Radar | Automotive Radar | Primary and Secondary Radar | Airborne Collision Avoidance | Synthetic Aperture Radar
Military Applications: Overview | Over The Horizon | Low Probability of Intercept | How a Bat's Sensor Works
Electronic Combat: Overview | Electronic Combat in Wildlife | Range Gate Pull-Off | Inverse Gain Jamming | Advanced ECM | How Stealth Works | Stealth Aircraft