RADAR

(from radio detecting and ranging), electromagnetic device used to detect and locate objects at distances and under conditions of lighting or obscuration that would render the unaided eye useless. It also provides a means for measuring precisely the distance, or range, to an object and the speed at which the object is moving toward or away from the observing unit. Radar systems operate by transmitting electromagnetic waves, most commonly of microwave frequency, toward an object and receiving the waves reflected from it. The properties of the received radio waves, or echoes, are amplified and analyzed by a signal processor. The processed signals are then converted into a form usable by a human operator or by a device (e.g., antiaircraft gun) controlled by the radar unit. Information about the target object (e.g., distance, direction, or altitude) is typically displayed on the screen of a cathode-ray tube, which may provide a maplike image of the area scanned by the radar beam, as, for example, in the case of the Plan Position Indicator (PPI). There are several types of radar. Each variety involves different kinds of signals from the radar transmitter and makes use of different properties of the received echo. By far the most widely used type of radar is pulse radar. It is so called because the transmitter is keyed to send out short, very intense bursts or pulses of electromagnetic energy, with a relatively long interval between pulses. The receiver picks up echoes from the closest objects soon after the transmission of a pulse, from objects at intermediate range later on, and from the most distant objects near the end of the interpulse interval. When sufficient time has elapsed to permit the reception of echoes from the most distant objects of interest, the transmitter sends another short pulse and the cycle repeats. The delay between the transmission of a pulse and the reception of an echo arises from the fact that the radar waves used travel at the exceedingly high (but finite) speed of lightnamely, 300,000 km (186,000 miles) per second. In units convenient in radar applications, this speed equals 300 m (nearly 1,000 feet) per microsecond. Because the electromagnetic energy from the radar transmitter has to travel the distance from the radar set to the target twiceonce out and once back as an echoeach microsecond of delay between the transmitting of the pulse and the receiving of an echo corresponds to 150 m (roughly 500 feet) of distance between the radar unit and the target. Extremely short intervals must be timed to achieve absolute precision in range. If an error in range of only 4.6 m (15 feet) can be tolerated, time intervals must be measured with an accuracy of 1/30 of a microsecond. Electronic timing and display techniques permit such measurements to be made with considerable ease and reliability. A second general type of radar is continuous-wave (CW) radar. In this technique radar signals are transmitted continuously, rather than in short bursts. Because the resultant continuous echo cannot be associated with a specific part of the transmitted wave, it is not possible to derive range information from simple continuous-wave radar. This technique, however, can be used to determine the speed of the target by measuring Doppler shifti.e., a change in observed frequency produced by motion. A signal transmitted at a particular frequency is coupled to the antenna through a duplexer (a device that permits the use of a single antenna for both transmission and reception) and is radiated into space. When the transmitted signal is interrupted by a radially moving target, the reflected signal will be altered in frequency. Although simple continuous-wave radar cannot measure distance, a more sophisticated variation known as frequency-modulated radar is able to do so. This technique involves tagging each part of the transmitted radio signal, rendering it recognizable upon reception. The signal is tagged by altering the frequency continuously. When an echo is received, its frequency differs from that of the signal leaving the transmitter at that time. If the rate of frequency change is known, the difference in frequency can be interpreted as the distance from the target. One other significant form of radar is laser radar, or lidar, in which very narrow signal beams of laser light are transmitted instead of radio radiation. Lidar operates at exceedingly high frequenciesroughly 100,000 times higher than radio frequencies. Most radio detection systems generate signals with frequencies ranging from several megahertz to 40 gigahertz. The development of radar can be traced to the experimental work of the German physicist Heinrich Hertz. During the late 1880s Hertz proved the existence of radio waves and demonstrated that they behave much like light waves (e.g., they can be reflected by objects, just as light is reflected by a mirror). Christian Hlsmeyer, a German engineer, was one of the first to apply Hertz's findings. He developed a simple radio echo device for use in navigation and obtained a patent for it in 1904. His primitive radarlike system, however, failed to attract interest because of its severe technical limitations. The possibility of using the radio reflection phenomenon for detection purposes was further explored after the Italian engineer Guglielmo Marconi elaborated its principles in 1922. Soon afterwards, the United States Naval Research Laboratory tested his proposal, employing continuous-wave radiation to detect a ship passing between a radio transmitter and receiver. The operating principle of pulse ranging was developed in 1925 by two American physicists, Gregory Breit and Merle A. Tuve, while engaged in ionospheric research. They succeeded in measuring the height of the Earth's ionosphere by bouncing radio pulses off the ionized layer of air and determining the amount of time taken by the echoes to return. During the 1930s several countries, including Great Britain, France, the United States, Germany, and Japan, initiated research on radar systems capable of detecting aircraft and surface vessels at long range and under conditions of poor visibility. Before the outset of World War II, Britain had constructed a network of radar stations designed to provide early warning against approaching enemy aircraft. By late 1939 Germany had begun production of similar ground-based aircraft warning units called Freya. Within a few years the British developed an aircraft-intercept radar set small enough to be installed on fighters, and the United States introduced radar equipment that could be used to direct gunfire. Moreover, cooperative efforts by British and American researchers over the duration of the war resulted in the development of a reliable high-power microwave radar system particularly suited for automatic fire control and long-distance aircraft interception. The main outlines of radar-system design were reasonably well defined at the close of World War II. Since the late 1940s radar development has included improvements of components and circuitry, with an increasing use of solid-state electronic devices from transistors to very-large-scale integrated (VLSI) circuits. The introduction of new scanning methods and the adoption of high-speed digital computers for signal processing have also contributed significantly to the efficiency and reliability of radar equipment. These and other technological advances have given rise to a wide variety of new radar applications. In military uses, remarkable attainments in transmitters of ever higher power and in receivers of greater sensitivity have made possible networks of extremely long-range radars for early warning of intercontinental ballistic missiles. In the late 20th century the United States and Canada jointly operated a radar network known as Space Detection and Tracking System (SPADATS) for identifying and monitoring artificial satellites launched into Earth orbit. Other modern-day military applications include the use of radar for missile guidance and for surveillance (e.g., mapping radar carried by reconnaissance planes). Radar has found numerous and varied civilian applications as well. It has become an important navigational aid for commercial airplanes and marine vessels. Virtually all major airports have surveillance and precision-approach radar systems, which enable air-traffic controllers to monitor and direct the movements of approaching and departing aircraft so as to prevent collisions. With these systems, controllers also are able to help guide pilots to safe landings when visibility is poor. More and more ships, including small fishing and pleasure craft, are equipped with simple radar units suitable for coastline navigation. In many ports large radar surveillance sets have been installed ashore overlooking the harbour and approach waters in order to assist shipping. The radar operator observing ship movements in the confined waters advises pilots of harbour traffic conditions from moment to moment via radiotelephone. Radar also serves as a valuable tool in astronomical studies. Radar techniques not only permit more accurate measurement of distances than optical methods do but also make possible the study of planetary and satellite surface features. So far, astronomers have employed radar to map the surfaces of the Moon, Mars, and Venus in considerable detail. (See also radio and radar astronomy.) Another field of science that has benefited from radar is meteorology. Ground-based and airborne radars are used to aid weather forecasters in making short-range predictions. Such equipment can locate and track approaching storms for several hundred kilometres because strong radar echoes are reflected from cloud droplets, ice crystals, raindrops, and hailstones. Other kinds of meteorological observations, such as those of atmospheric aerosols, dust, and molecules, are commonly conducted with laser radar. Continued miniaturization of circuitry and auxiliary equipment has enabled the designing of smaller portable radar units. The handheld continuous-wave radar gun employed by the police for detecting speeding vehicles is a notable example. An even smaller, lightweight unit is a laser-radar sensory device developed for use in canes for the blind. (from radio detecting and ranging), electromagnetic sensor used for detecting, locating, tracking, and identifying objects of various kinds at considerable distances. It operates by transmitting electromagnetic energy toward objects, commonly referred to as targets, and observing the echoes returned from them. The targets may be aircraft, ships, spacecraft, automotive vehicles, and astronomical bodies, or even birds, insects, and raindrops. Radar can not only determine the presence, location, and velocity of such objects but can sometimes obtain their size and shape as well. What distinguishes radar from optical and infrared sensing devices is its ability to detect faraway objects under all weather conditions and to determine their range with precision. Radar is an active sensing device in that it has its own source of illumination (a transmitter) for locating targets. In certain respects, it resembles active sonar, which is used chiefly for detecting submarines and other objects underwater; however, the acoustic waves of sonar propagate differently from electromagnetic waves and have different properties. Radar typically operates in the microwave region of the electromagnetic spectrumnamely, at frequencies extending from about 400 megahertz (MHz) to 40 gigahertz (GHz). It has, however, been used at lower frequencies for long-range applications (frequencies as low as several megahertz, which is the HF, or short-wave, band) and at optical and infrared frequencies (those of laser radar, or lidar). The circuit components and other hardware of radar systems vary with the frequency used, and systems range in size from those small enough to fit in the palm of the hand to those so enormous as to take up several football fields. These differences notwithstanding, the basic principles of operation of all radar systems remain the same. Radar underwent rapid development during the 1930s and '40s to meet the needs of the military. It is still widely employed by the armed forces, and many advances in radar technology have in fact been subsidized by the military. At the same time, radar has found an increasing number of important civilian applications, notably air traffic control, remote sensing of the environment, aircraft and ship navigation, speed measurement for industrial applications and for law enforcement, space surveillance, and planetary observation. Additional reading Complete basic information on all forms of radar is presented in Merrill I. Skolnik, Introduction to Radar Systems, 2nd ed. (1980); and Merrill I. Skolnik (ed.), Radar Handbook, 2nd ed. (1990), a comprehensive reference work written for radar engineers, covering the various aspects of radar technology. S.S. Swords, Technical History of the Beginnings of Radar (1986), surveys the developments prior to World War II, focusing on early European achievements. Henry E. Guerlac, Radar in World War II, 2 vol. (1987), is an excellent, authoritative history written by a professional historian with a background in science. Merrill I. Skolnik (ed.), Radar Applications (1988), is a collection of technical papers describing the various applications of radar, such as air traffic control, military air defense, aircraft and spacecraft surveillance, and remote sensing of the environment. David K. Barton, Modern Radar System Analysis (1988), provides a fine technical presentation of many aspects of radar design. Merrill I. Skolnik Major applications of radar Areas of application Over the years, radar has found many and varied uses for both civilian and military purposes. A sampling of some of the more significant applications is given here. Military Radar originally was developed to meet the needs of the military, and it continues to have significant application for military purposes. It is used to detect aircraft, missiles, artillery and mortar projectiles, ships, land vehicles, and satellites. In addition, radar controls, guides, and fuzes weapons; allows one class of target to be distinguished from another; aids in the navigation of aircraft and ships; performs reconnaissance; and determines the damage caused by weapons to targets. The importance of radar in modern warfare is borne out by the many measures designed to negate its effectiveness (in addition to direct attack, which is an option for any military target of value). Attempts to degrade military radar capability include electronic warfare (jamming, deception, chaff, decoys, and interception of radar signals), antiradiation missiles that home on radar transmissions, reduced radar cross-section targets to make detection more difficult (stealth), and high-power microwave energy transmissions to degrade or burn out sensitive receivers. A major objective of military radar development is to insure that a radar system can continue to perform its mission in spite of the various measures that attempt to degrade it. Types of radar Radar systems may be categorized according to the function they performe.g., aircraft surveillance, surface (ground or sea) surveillance, space surveillance, tracking, weapon control, missile guidance, instrumentation, remote sensing of the environment, intruder detection, or underground probing. They also may be classified, as in the listing below, on the basis of the particular radar technique they employ. It is difficult to give in only a few words precise and readily understandable descriptions of the many types of radar available. The following survey is necessarily brief and qualitative. Additional information about each radar type can be found in the books listed in the Bibliography. Simple pulse radar This is by far the most widely used technique and constitutes what might be termed conventional radar. (For a discussion of its fundamentals, see above Fundamentals of radar: Pulse radar.) All but the last two techniques outlined below employ a pulse waveform; however, they have additional features that give an enhanced performance as compared to simple pulse radar.