Radar is an object-detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio waves or microwaves which bounce off any object in their path. The object reflects some of the energy back towards the radar dish or antenna. This reflected energy is then detected and analyzed by a computer which provides information on the object’s location and speed.
Radar has been used for military purposes since World War II and is also used in law enforcement (to detect speeding motorists), air traffic control, meteorology (tracking rain and snowstorms), oceanography (measuring wave height), earthquake detection, and industrial process control. Radar was originally called RDF (radio detecting finder).
How Radar Works
Radar works by sending out pulses of radio waves or microwaves which reflect off any object in their path back to the radar receiver. The time it takes for the pulse to return can be used to determine how far away an object is. By analyzing the Doppler effect of the returning signal, it is also possible to determine an object’s speed and direction.
The strength of the returned signal can be used to determine the size and shape of an object. Larger objects reflect more energy back than smaller ones so they show up better on radar images. For example, aircraft are easily detected by radar because they are large metal objects with few places for the energy to escape from. However, small objects such as birds or insects are much harder to detect because they reflect very little energy back to the receiver. Even large objects can be difficult to detect if they are not made of metal or if they have a smooth surface that reflects energy away from the receiver instead of back towards it. This is why stealth aircraft are designed to be difficult for radar to detect.
The Basics Of Pulse-Doppler Radar
Pulse-Doppler radar is a type of moving target indicator (MTI)radar that uses Doppler shift processingto distinguish targets from clutterreturns originating from stationary background scenery such as land massesand buildings(“stationary clutter”). Since stationary clutter generally does not move along with respect totarget motion relativetothe transmitter/receiverlineof sight(LOS), its Doppler shift relative totargetreturns will bedifferentfromthatof targets; i.,e.,will exhibita “differential”Dopplershift.”Clutter” mayalso includeechoesfromprecipitation(rainor snow)and atmospheric turbulence (“chaff”). Unlikeinpulsedenvelope MTI systemswhich only utilize one pulse per PRTintervalfor both transmittingand receiving(i.,e.,are “noncoherent”),pulse-Dopplersystemstransmit separate transmitand receivepulses;i.,ethey employtransmittercoherence(seeCoherentradarsignalprocessing).Since each transmitted pulse illuminates alarge volumeof space encompassingmany potentialtargetsas well as stationaryclutter,”range gating”is employed sothatthe receivedsignal issubjecteddetailed Dopplershift analysisonly over alimited volumeof spaceanotherwise knownasthe “gate”.Also unlikepulsedenvelopeMTIsystemswhich utilizea constantPRFoverallscanningtime (“dwell time”),most modernpulse-Dopplersystemsemploy whatisknownasa”staggeredPRF”;i.,esome sortoffixedratioor fixedincrementbetweenthetwo PRFsusedbytransmitterandreceiver,.This techniquehelpsto mitigate groundclutterreturnsand other forms offalse alarmsobtainingwith constantPRFsystemswhen scanningnear themaximumunambiguousvelocity.(For moreinformationaboutpracticalconsiderationsoftransmittingandreceivingseparate pulsesin pusle-doppersystemssee:http://www2a .cdc .gov/cdcup /library /spl /tcrdata/TCRpdf6l 1l3 .pdf.)