Radar is an acronym for "radio detection and ranging." A radar system usually operates in the ultra-high-frequency (UHF) or microwave part of the radio-frequency (RF) spectrum, and is used to detect the position and/or movement of objects. Radar can track storm systems, because precipitation reflects electromagnetic fields at certain frequencies. Radar can also render precise maps. Radar systems are widely used in air-traffic control, aircraft navigation, and marine navigation.
High-power radar, using large dish antennas, has been used to measure distances to the moon, other planets, asteroids, and artificial satellites. From unmanned spaceprobes, radar has been used to map Venus, whose surface is obscured at visible wave lengths by a thick layer of clouds. Radar has been employed by NASA(the U.S. National Aeronautics and Space Administration) to make highly detailed topographical maps of the earth's surface as well.
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Most radar systems determine position in two dimensions:azimuth (compass bearing) and radius (distance). The display is in polar coordinates. A rotating antenna transmits RF pulses at defined intervals. The delay between a transmitted pulse and the echo, or return pulse, determines the radial position of the plotted point(s) for each azimuth direction on the display. The greater the echo delay from a particular object in space, the farther from the display center its point appears. The maximum range of a UHF or microwave radar system depends on the height of the antenna above average terrain, the topography of the surface in the region, the atmospheric conditions in the region, and in some cases the level of radio background noise.
Radar is known to the general public for its use by law enforcement in determining the speeds of motor vehicles. This type of radar does not display the exact position of an object, but determines its radial speed vector from the Doppler effect. A radar detector,which consists of a simple UHF/microwave broadband receiver, can be used in a car or truck to warn drivers of the presence of police radar. Radar detectors are illegal in some states.
The Weather Service uses so-called Doppler radar to determine not only the positions and extent of storm systems, but wind patterns and velocities aloft. Doppler radar employs a combination of position-sensing and speed-sensing radar, making it possible to ascertain the locations and intensity of severe thunderstorms, hurricanes, and tornadoes.
Radar has been used on the high-frequency (HF) radio bands,between approximately 5 MHz and 20 MHz, in an attempt to obtain early warning in the event of a nuclear assault via ballistic missiles. The ionosphere refracts HF waves, allowing much greater system range than is possible with radar at UHF or microwave frequencies. During the 1970s and early 1980s, the signals from these systems became infamous because of the interference they caused. Radio amateurs coined the term woodpecker to describe the sound of HF over-the-horizon radar pulses in communications receivers.