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Introduction

(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 spectrum--namely, 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.