 - Radar is an acronym for "radio detection andranging." A radar system usually operates in the ultra-high-frequency (UHF) ormicrowave partof the radio-frequency (RF) spectrum, and is used to detect theposition and/ormovement of objects. Radar can track storm systems, becauseprecipitation reflects electromagnetic fields at certainfrequencies. Radar can also render precise maps. Radar systemsarewidely usedin air-traffic control, aircraft navigation, and marine navigation.
High-power radar, using large dish antennas, has been used to measuredistances to themoon, other planets, asteroids, and artificial satellites. Fromunmannedspaceprobes, radar has been used to map Venus, whose surface is obscured atvisiblewavelengthsby a thick layer of clouds. Radar has been employed by NASA(the U.S. National Aeronautics and Space Administration) to make highly detailedtopographicalmaps of the earth's surface as well. Most radar systems determine position in two dimensions:azimuth (compassbearing) and radius (distance). The display is in polarcoordinates. Arotating antenna transmits RF pulses at defined intervals. The delaybetween atransmitted pulse and the echo, or return pulse, determines the radialposition of theplotted point(s) for each azimuth direction on the display. Thegreaterthe echodelay from a particular object in space, the farther from the displaycenterits pointappears. The maximum range of a UHF or microwave radar system dependsonthe heightof the antenna above average terrain, the topography of the surface in theregion, theatmospheric conditions in the region, and in some cases the level of radiobackgroundnoise. Radar is known to the general public for its use by lawenforcement indetermining the speeds of motor vehicles. This type of radar does notdisplay theexact position of an object, but determines its radial speed vector from the Doppler effect. A radardetector,which consists of a simple UHF/microwave broadband receiver, can be used ina car or truckto warn drivers of the presence of police radar. Radar detectors areillegal in somestates. The Weather Service uses so-called Doppler radartodetermine notonly the positions and extent of storm systems, but wind patterns andvelocitiesaloft. Doppler radar employs a combination of position-sensing andspeed-sensingradar, making it possible to ascertain the locations and intensity ofseverethunderstorms, hurricanes, and tornadoes. Radar has been used on the high-frequency (HF) radio bands,betweenapproximately 5 MHz and 20 MHz, in an attempt to obtain early warning intheevent of anuclear assault via ballistic missiles. The ionosphere refracts HFwaves, allowingmuch greater system range than is possible with radar at UHF or microwavefrequencies. During the 1970s and early 1980s, the signals from these systemsbecameinfamousbecause of the interference they caused. Radio amateurs coined thetermwoodpeckerto describe the sound of HF over-the-horizon radar pulses in communicationsreceivers.
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20 Oct 2008
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