Early-warning radar

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PAVE PAWS Early-warning radar PAVE PAWS Radar Clear AFS Alaska.jpg
PAVE PAWS Early-warning radar

An early-warning radar is any radar system used primarily for the long-range detection of its targets, i.e., allowing defences to be alerted as early as possible before the intruder reaches its target, giving the air defences the maximum time in which to operate. This contrasts with systems used primarily for tracking or gun laying, which tend to offer shorter ranges but offer much higher accuracy.

Radar object detection system based on radio waves

Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. A radar system consists of a transmitter producing electromagnetic waves in the radio or microwaves domain, a transmitting antenna, a receiving antenna and a receiver and processor to determine properties of the object(s). Radio waves from the transmitter reflect off the object and return to the receiver, giving information about the object's location and speed.


EW radars tend to share a number of design features that improve their performance in the role. For instance, EW radar typically operates at lower frequencies, and thus longer wavelengths, than other types. This greatly reduces their interaction with rain and snow in the air, and therefore improves their performance in the long-range role where their coverage area will often include precipitation. This also has the side-effect of lowering their optical resolution, but this is not important in this role. Likewise, EW radars often use much lower pulse repetition frequency to maximize their range, at the cost of signal strength, and offset this with long pulse widths, which increases the signal at the cost of lowering range resolution.

Optical resolution describes the ability of an imaging system to resolve detail in the object that is being imaged.

The pulse repetition frequency (PRF) is the number of pulses of a repeating signal in a specific time unit, normally measured in pulses per second. The term is used within a number of technical disciplines, notably radar.

The canonical EW radar is the British Chain Home system, which entered full-time service in 1938. It used a very low pulse repetition of 25 pps and very powerful transmissions (for the era) reaching 1 MW in late-war upgrades. The German Freya and US CXAM (Navy) and SCR-270 (Army) were similar. Post-war models moved to the microwave range in ever-increasingly powerful models that reached the 50 MW range by the 1960s. Since then, improvements in receiver electronics has greatly reduced the amount of signal needed to produce an accurate image, and modern examples the transmitted power is much less; the AN/FPS-117 offers 250 nautical miles (460 km; 290 mi) range from 25 kW. EW radars are also highly susceptible to radar jamming and often include advanced frequency hopping systems to reduce this problem.

Chain Home

Chain Home, or CH for short, was the codename for the ring of coastal Early Warning radar stations built by the Royal Air Force (RAF) before and during the Second World War to detect and track aircraft. Initially known as RDF1, and given the official name Air Ministry Experimental Station Type 1 in 1940, the radar units themselves were also known as Chain Home for most of their life. Chain Home was the first early warning radar network in the world, and the first military radar system to reach operational status. Its effect on the outcome of the war made it one of the most powerful weapons of what is today known as the "Wizard War".

Freya radar early-warning radar

Freya was an early warning radar deployed by Germany during World War II; it was named after the Norse Goddess Freyja. During the war, over a thousand stations were built. A naval version operating on a slightly different wavelength was also developed as the Seetakt.

CXAM radar radar system deployed on United States Navy ships

The CXAM radar system was the first production radar system deployed on United States Navy ships, operating in the mid-high VHF frequency band of 200 MHz. It followed several earlier prototype systems, such as the NRL radar installed in April 1937 on the destroyer Leary; its successor, the XAF, installed in December 1938 on the battleship New York; and the first RCA-designed system, the CXZ, installed in December 1938 or January 1939 on the battleship Texas. Based on testing in January 1939, where the XAF was more reliable, the US Navy ordered RCA to build six XAF-based units for deployment and then shortly thereafter ordered 14 more.


The first early-warning radars were the British Chain Home, the German Freya, the US CXAM (Navy) and SCR-270 (Army), and the Soviet Union RUS-2  [ ru ]. By modern standards these were quite short range, typically about 100 to 150 miles (160 to 240 km). This "short" distance is a side effect of radio propagation at the long wavelengths being used at the time, which were generally limited to line-of-sight. Although techniques for long-range propagation were known and widely used for shortwave radio, the ability to process the complex return signal was simply not possible at the time.


The SCR-270 was one of the first operational early-warning radars. It was the U.S. Army's primary long-distance radar throughout World War II and was deployed around the world. Its also known as the Pearl Harbor Radar, since it was an SCR-270 set that detected the incoming raid about 45 minutes before the December 7, 1941 attack on Pearl Harbor commenced.

Radio propagation behavior of radio waves as they travel, or are propagated, from one point to another, or into various parts of the atmosphere

Radio propagation is the behavior of radio waves as they travel, or are propagated, from one point to another, or into various parts of the atmosphere. As a form of electromagnetic radiation, like light waves, radio waves are affected by the phenomena of reflection, refraction, diffraction, absorption, polarization, and scattering. Understanding the effects of varying conditions on radio propagation has many practical applications, from choosing frequencies for international shortwave broadcasters, to designing reliable mobile telephone systems, to radio navigation, to operation of radar systems.

Wavelength spatial period of the wave—the distance over which the waves shape repeats, and thus the inverse of the spatial frequency

In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is thus the inverse of the spatial frequency. Wavelength is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings and is a characteristic of both traveling waves and standing waves, as well as other spatial wave patterns. Wavelength is commonly designated by the Greek letter lambda (λ). The term wavelength is also sometimes applied to modulated waves, and to the sinusoidal envelopes of modulated waves or waves formed by interference of several sinusoids.

Cold War

To counter the threat of Soviet bombers flying over the Arctic, the U.S. and Canada developed the DEW Line. Other examples have since been built with even better performance. An alternative early warning design was the McGill Fence, which provided "line breaking" indication across the middle of Canada, with no provision to identify the target's exact location or direction of travel. Starting in the 1950s, a number of over-the-horizon radars were developed that greatly extended detection ranges, generally by bouncing the signal off the ionosphere.

Distant Early Warning Line former system of radar stations in the far northern Arctic region of Canada

The Distant Early Warning Line, also known as the DEW Line or Early Warning Line, was a system of radar stations in the far northern Arctic region of Canada, with additional stations along the North Coast and Aleutian Islands of Alaska, in addition to the Faroe Islands, Greenland, and Iceland. It was set up to detect incoming Soviet bombers during the Cold War, and provide early warning of any sea-and-land invasion.

Canada Country in North America

Canada is a country in the northern part of North America. Its ten provinces and three territories extend from the Atlantic to the Pacific and northward into the Arctic Ocean, covering 9.98 million square kilometres, making it the world's second-largest country by total area. Canada's southern border with the United States is the world's longest bi-national land border. Its capital is Ottawa, and its three largest metropolitan areas are Toronto, Montreal, and Vancouver. As a whole, Canada is sparsely populated, the majority of its land area being dominated by forest and tundra. Consequently, its population is highly urbanized, with over 80 percent of its inhabitants concentrated in large and medium-sized cities, many near the southern border. Canada's climate varies widely across its vast area, ranging from arctic weather in the north, to hot summers in the southern regions, with four distinct seasons.

Over-the-horizon radar type of radar

Over-the-horizon radar, or OTH, is a type of radar system with the ability to detect targets at very long ranges, typically hundreds to thousands of kilometres, beyond the radar horizon, which is the distance limit for ordinary radar. Several OTH radar systems were deployed starting in the 1950s and 1960s as part of early warning radar systems, but these have generally been replaced by airborne early warning systems. OTH radars have recently been making a comeback, as the need for accurate long-range tracking becomes less important with the ending of the Cold War, and less-expensive ground-based radars are once again being considered for roles such as maritime reconnaissance and drug enforcement.

Modern day

Today the early warning role has been supplanted to a large degree by airborne early warning platforms. By placing the radar on an aircraft, the line-of-sight to the horizon is greatly extended. This allows the radar to use high-frequency signals, offering high resolution, while still offering long range. A major exception to this rule are radars intended to warn of ballistic missile attacks, like BMEWS, as the high-altitude exo-atmospheric trajectory of these weapons allows them to be seen at great ranges even from ground-based radars.

Airborne early warning and control Airborne system of surveillance radar plus command and control functions

An airborne early warning and control (AEW&C) system is an airborne radar picket system designed to detect aircraft, ships and vehicles at long ranges and perform command and control of the battlespace in an air engagement by directing fighter and attack aircraft strikes. AEW&C units are also used to carry out surveillance, including over ground targets and frequently perform C2BM functions similar to an Air Traffic Controller given military command over other forces. When used at altitude, the radar on the aircraft allows the operators to detect and track targets and distinguish between friendly and hostile aircraft much farther away than a similar ground-based radar. Like a ground-based radar, it can be detected by opposing forces, but because of its mobility, it is much less vulnerable to counter-attack.

Ballistic Missile Early Warning System

The RCA 474L Ballistic Missile Early Warning System was a United States Air Force Cold War early warning radar, computer, and communications system, for ballistic missile detection. The network of 12 radars, which was constructed beginning in 1958 and became operational in 1961, was for detecting "a mass ballistic missile attack launched on northern approaches [for] 15 to 25 minutes' warning time" also provided Project Space Track satellite data.

Early systems

1950s through 70s

AWACS plane E-3aawa.jpg
AWACS plane

Operational systems

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