A device or system that transmits radio waves and receives and analyzes their reflections in order to determine the location and speed of objects, such as aircraft.
The word “radar” is an acronym for “radio detection and ranging,” where ranging refers to finding the distance to a target. Radar works in a fashion similar to that supposed by the early Greeks for the operation of the eye. The Greeks imagined that rays shot out from a person’s eye, and that people saw objects as their personal rays struck those objects and somehow returned information. The concept was one of being able to reach out and touch and feel objects from a distance.
Radar reaches out by sending out a beam of radio waves oscillating electric and magnetic fields. When a radio wave passes a given point, the electric field strength at that point goes up and down in much the same way that the water level at a point on the ocean goes up and down as a water wave passes. The distance between adjacent crests in a radio wave is the wavelength, and the number of waves that pass a given point during one second is the frequency. The frequency multiplied by the wavelength gives the speed of the waves. The speed of radio waves is very nearly the speed of light, 3 × 105 kilometers per second. Light itself is an electromagnetic wave, but it has a much higher frequency than radio waves. At the speed of light, it takes only 2.5 seconds for radio waves to travel to the Moon and back.
A radar set usually consists of a transmitter, a transmitting antenna, a receiving antenna, a receiver, a computer, and a display. Normally, the same antenna is used both to transmit and to receive. The transmitter causes a current to flow back and forth in the antenna, causing radio waves of the same frequency as the current to travel outward from the antenna. When radio waves strike objects, the waves are reflected and absorbed, depending upon the waves’ frequency and the properties of the objects. Metals, for example, are particularly reflective. When waves are reflected, a small fraction of the reflected energy may return to the radar antenna as an echo. The receiver amplifies this echo, and then the computer extracts information from the amplified echo and prepares this information to be displayed.
A common type of radar, with a revolving antenna, sends out a short burst of waves and listens for an echo. The direction in which the antenna was pointing when it received the echo gives the target’s direction, and the time delay between sending and receiving gives the target’s distance. If the elapsed time is the time between sending the burst and receiving the echo, then the target’s range is one-half the elapsed time multiplied by the speed of radar waves, or about 3 × 105 kilometers per second.
When the same antenna is used both to transmit and to receive, there must be some way to keep the stronger transmitted signal from completely swamping the weaker return echos. In the pulsed operation just described, this is done by timing. The transmission burst lasts about one microsecond, then the radar listens. The wait time during which the radar set listens for echos before sending out another pulse is keyed to the faintest echo that can be reliably detected. If targets up to 150 kilometers (90 miles) away can be detected, and radar waves can travel this distance and back in one millisecond, the pattern of pulse transmission and listening can be repeated about every millisecond.
When a radar wave is reflected from a moving target, the frequency of the wave changes in a fashion described as the Doppler effect. The target’s speed can be determined from this change in frequency. If two targets are at the same distance and have the same radar reflective properties, a brighter echo indicates a larger object. Because radar reflectivity depends upon the shape and composition of the target, a better method to determine size is to send out a series of very short pulses, each lasting only a nanosecond or two. A large target may reflect two or more of these pulses, and the maximum distance between the echos yields the approximate size of the target.
A simple wire antenna will send radio waves outward in all directions; however, a carefully spaced group of several antennas can concentrate most of the radio waves into a beam. Such antenna groups must be several times the size of the wavelength they broadcast, and they work reasonably well from 3 million to 300 million hertz (cycles per second), or 100-meter to 1-meter wavelengths. The largest radar system in the world is the U.S. Air Force’s over-the-horizon backscatter (OTH-B) air defense radar system, built to detect a Soviet bomber attack from thousands of kilometers away but also used to study ocean currents and waves. Each of the six transmitting antennas are 1.1 kilometers long, and the receiving antennas are 1.5 kilometers long. They operate between 5 and 28 megahertz, from 60- to 1.1-meter wavelengths. These wavelengths bounce off the ionosphere, about 200 kilometers above the ground, and reflect back down to the earth’s surface.
The need for finer resolution and more portable radar sets eventually led to the development of radar wavelengths only centimeters long. Such short wavelengths can be formed into a searchlight-like beam by reflecting them from a parabolic metal dish. Because the paths of these wavelengths are not bent by the ionosphere, they must have a straight line of sight to the target. However, they will pass through the ionosphere and can be used to track objects in space. Regardless of the type of antenna used, radar beams spread wider as they travel outward from the antenna. The amount of spreading is smaller for shorter wavelengths and for bigger antennas. That is, the narrowest beams are formed, and the finest details can be seen, with radars using the shortest wavelengths and the largest antennas.
The properties of the atmosphere also affect the choice of operating frequency. Atmospheric attenuation is negligible for frequencies up to 1 gigahertz (1 billion hertz). Above 3 gigahertz (1-centimeter wavelength), however, radar absorption by raindrops becomes significant, so weather radars operate at these frequencies. Above 12 gigahertz, clouds begin to absorb the radar waves.
The development of radar was such a natural outgrowth of experiments with radio transmission that it was independently invented and developed by several countries during the 1930’s. Probably more than any other device, radar dictated the course of World War II. Even before the war, Great Britain had begun installation of chain home (CH) radar stations along its coasts, with radar antennas on towers up to 110 meters high. Germany began massive bomber attacks on Britain in August, 1940. Chain home radars were so effective at giving warning and allowing the badly outnumbered Royal Air Force (RAF) fighters to position themselves for maximum effect, that by November of that year, daytime bomber attacks had stopped. The CH radar system determined the direction and elevation of an approaching aircraft by comparing the intensity of signals received at different antennas in the chain. When night attacks began the following year, CH radars were used to guide friendly fighters toward enemy bombers until the fighters got close enough to pick up the bombers on the short-range (5-kilometer) radar the fighters now carried. This technique was so successful that night attacks were also stopped.
Radar was also put to other uses. In order to aid radar operators to distinguish between friendly and enemy aircraft, identification, friend or foe (IFF) beacons were developed and used by the Allies. These were small radar receiver/transmitters that broadcast a coded radar signal that identified a craft as friendly when they detected a probing radar wave. Another device, a radar altimeter, is simply a small radar set that sends pulses toward the ground and determines the height from the time it takes for the echos to return. The atomic bomb dropped on Hiroshima in 1945 carried four radar altimeters and was fused to explode when any two measured the height as less than 600 meters (2,000 feet).
Had German submarines been able to cut off the flow of supplies and personnel from the United States and Canada to Great Britain and Europe, the Allied invasion of Europe would have been impossible. At first, the German submarines were very successful in sinking Allied ships, but then the Allies began to hunt the submarines with radar. As submarine losses mounted, the Germans equipped their submarines with radar detectors, and the warning they gained allowed the submarines to be safely hidden underwater by the time attack aircraft arrived.
The British then made one of the most important technological advances of the war, the microwave-cavity magnetron, a device for generating high-power radio waves of 10 centimeters or less. Shorter wavelengths meant radar antennas could be smaller, a great advantage in an aircraft, and smaller targets, such as submarine periscopes, could be detected. The German radar detectors could not pick up the short wavelength the Allies were now using, and the tide turned against them. In 1942, the Germans sank 8,245,000 tons of Allied shipping while losing 85 submarines. In 1944, they sank only 1,422,000 tons, but lost 241 submarines.
The familiar weather radar displays distances and directions to radar targets in a maplike image. A moving target such as a storm can be tracked by following its image on the radar screen as its position changes with time. Air traffic controllers use an extension of this method to guide aircraft in the vicinity of busy airports. A sophisticated version of this type of radar is used by the E-3 Sentry, or Airborne Warning and Control System (AWACS) aircraft, a modified Boeing 707 carrying a 9-meter (30-foot) radar dome. When aloft, AWACS can detect low-flying targets more than 375 kilometers (250 miles) away. Special equipment subtracts out the ground clutter that would swamp ordinary radars, thereby allowing AWACS controllers to monitor all the air traffic in the area and to direct friendly aircraft. AWACS assisted in thirty-eight of the forty air-to-air shoot-downs of the 1991 Persian Gulf War.
The efforts of civilian air traffic controllers have contributed to making air travel far safer than automobile travel. Airport surveillance radar (ASR) is a medium-range system that detects and tracks aircraft within about 50 miles of the radar installation. Controllers use this radar as they direct aircraft landings, takeoffs, and flight patterns. Air route surveillance radar (ARSR) tracks aircraft en route between airports. The ARSR-4 uses a wavelength of about 21 centimeters and has a range of about 400 kilometers. It broadcasts a series of pulses that interrogates the radar beacon or transponder carried by all large aircraft. The transponder broadcasts a reply from which the aircraft’s identity, range, and direction can be determined. An air traffic controller follows the aircraft’s progress and delivers instructions. When the aircraft leaves one controller’s sector, it is progressively handed off to controllers in the sectors through which it flies until reaching the destination airport.
Radar sets can be designed to track a target automatically. During the Korean War, the U.S. Army used radar to track mortar shells. A shell follows a parabolic trajectory, and if the radar can follow it for more than one-half of its trajectory, its launch point can be deduced, and artillery fire can be directed against the mortar. The radar dish used could slew, or pivot, quickly in any direction, and a mask partially blocked the center of the radar beam. When the radar locked onto a target, the target was positioned in the center of the beam, where the return echo would be relatively weak because of the mask. If the target drifted from the beam’s center, the echo strengthened, and the radar set used this information to move the antenna and keep the target centered. Although a similar scheme can be used to track aircraft, schemes that maximize the echo are more common. In any case, a relatively narrow beam must be used for tracking.
Although mechanical systems can neither move quickly enough to track rockets and nearby fast-moving aircraft nor track multiple targets, phased-array radars can. These arrays consist of hundreds or even thousands of small antenna pods mounted in a regular array on a reflecting surface. Each pod is like a four-leaf clover, with each leaf replaced by a pencil-length rod pointing back toward the reflector at an approximate 45-degree angle. The term “phase” refers to position in the wave cycle. When all of the antennas are in phase, they begin broadcasting the beginning of a wave at the same time, and the radar beam is strongest straight ahead. If, instead, neighboring rows of antennas begin to broadcast at progressively later times, the radar beam will be tilted off to one side. When the radar receives a target echo, a computer can calculate where the target should be a fraction of a second later and direct the beam at that point. It takes only millionths of a second to switch the beam between targets so that a phased array can track one hundred or more targets virtually simultaneously.
The U.S. Air Force maintains Pave Paws radars at Cape Cod, Massachusetts; Beale, California; and Clear Air Force Station, Alaska. “Pave” is an Air Force program name, and “Paws” is an acronym of phased-array warning system. Each Pave Paws site has twin antennas consisting of 1,792 radiating elements mounted on massive reflecting faces measuring 31 meters across. The primary assignment for these installations is to detect and track intercontinental ballistic missiles or missiles launched from submarines at the United States. The Pave Paws radar beams extend 5,500 kilometers into space and are also used to track satellites.
The heart of the U.S. Navy’s Aegis combat system is a 4-megawatt phased-array radar mounted on a special ship that is also equipped with missiles and a Phalanx close-in weapons system (CIWS) for destroying attacking aircraft and missiles.
The crew of an aircraft carrying a radar detector will know whether the craft is being observed. Once alerted, the crew might eject strips of aluminum foil, called chaff. Clouds of chaff appear as new targets on the radar screen and confuse the radar operator. The U.S. Eighth Air Force dropped more than 10 million pounds (4.5 million kilograms) of aluminum foil during World War II. Specially equipped aircraft, such as Ferrets, and later, Wild Weasels, determine the location and frequencies of fire-control radars and jam them by broadcasting radar noise.
Modern radar countermeasures include recording the fire-control radar signals and then beaming them back at the ground installation, thus making false targets appear at various distances and directions. When the aircraft are close enough, pilots can fire high-speed antiradiation missiles (HARMs) that home in on the fire-control radar. This presents the fire-control radar operator with an impossible choice: In order to shoot down the attacking aircraft, the operator must turn on the fire-control radar. However, if the radar is on for more than a few seconds, a HARM can lock in on its beam. In the initial stage of the Persian Gulf War, F-4G Phantom Wild Weasels flew 2,596 sorties and used this technique to devastate the formidable Iraqi air defenses.
Perhaps the best radar countermeasure is to make an aircraft invisible to radar. The radar echos from an aircraft’s rounded fuselage fan out over a broad range of directions, including back toward the originating antenna. Stealth aircraft are made with many flat surfaces that are tilted to deflect the reflected radar beam away from the originating antenna. In order to reduce the radar echo when it is observed from behind, a “W” shape is used for the wing’s trailing edge. Right-angled corners such as those between the tail and fuselage of a normal aircraft are eliminated, because they can return strong radar echos. It is such right angles that make highway signs coated with corner reflector crystals appear to light up when lit by a car’s headlights. Carbon fiber materials and coatings that absorb radar waves are used extensively. The F-117A Nighthawk can get 90 percent closer to ground-based radar than a normal aircraft before it can be detected. During the opening minutes of the Persian Gulf War, eight Nighthawks followed a wave of Tomahawk cruise missiles and arrived at Bagdad undetected by ground radar. Their presence was announced only by bombs falling on their targets. The massive B-2 stealth bomber first saw combat in Yugoslavia during March, 1999. It carries eight times the bomb load of the F-117.
Baxter, James Phinney III. Scientists Against Time. Cambridge, Mass.: MIT Press, 1968. A popular book about the important inventions of World War II. Brookner, Eli. “Phased-Array Radars.” Scientific American 252, no. 2 (February, 1985). A good, basic description of how phased-array radars work. Jensen, Homer, et al. “Side-Looking Airborne Radar.” Scientific American 237, no. 2 (October, 1977). A slightly technical description of how terrain-mapping radar works. Page, Robert Morris. The Origin of Radar. Garden City, N.Y.: Anchor Books, 1962. Written for the general public by the Director of Research at the U.S. Naval Laboratory, a scientist who helped develop radar.
Air traffic control
World War II
A radar receiver locates objects and measures the distance to them by sending out short bursts of radio waves and measuring how long it takes for an echo of the bounced-back wave to return.