Radar communication 2
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    Radar communication 2 Radar communication 2 Document Transcript

    • RADAR COMMUNICATIONABSTRACTThis long range radar antenna (approximately 40m (130ft) indiameter) rotates on a track to observe activities near the horizon.RADAR is an acronym for Radio Detection And Ranging orRadio Angle Detection And Ranging. It is a system used todetect, range (determine the distance of), and map objects such asaircraft and rain. Strong radio waves are transmitted, and a receiverlistens for any echoes. By analysing the reflected signal, the reflectorcan be located, and sometimes identified. Although the amount ofsignal returned is tiny, radio signals can easily be detected andamplified.Radar radio waves can be easily generated at any desired strength, detected at even tinypowers, and then amplified many times. Thus radar is suited to detecting objects at very
    • large ranges where other reflections, like sound or visible light, would be too weak todetect.INTRODUCTION In 1887, the German physicist Heinrich Hertz began experimenting with radiowaves in his laboratory. He found that radio waves could be transmitted through differenttypes of materials, and were reflected by others. The existence of electromagnetic waveswas predicted earlier by James Clerk Maxwell, but it was Hertz who first succeeded ingenerating and detecting radio waves experimentally. The scientists named CHRISTIAN HUELSMEYER , NIKOLA TESLA,A.H.TAYLOR , L.C.YOUNG and ROBERT WATSON – WATT researchedand developed the fundamentals of RADAR technology in 1900’s. PRINCIPLESReflection
    • DescriptionElectromagnetic waves reflect (scatter) from any large changein the dielectric or diamagnetic constants. This means that asolid object in air or vacuum, or other significant change inatomic density between object and whats surrounding it, willusually scatter radar (radio) waves. This is particularly true ofelectrically conductive materials such as metal and carbonfiber, making radar particularly well suited to the detection ofaircraft and ships. Radar absorbing material, containingresistive and sometimes magnetic substances, is used onmilitary vehicles to reduce radar reflection. This is the radioequivalent of painting something a dark color.Radar waves scatter in a variety of ways depending on the size (wavelength) of the radiowave and the shape of the target. If the wavelength is much shorter than the targets size,the wave will bounce off in a way similar to the way light bounces from a mirror. If thewavelength is much longer than the size of the target, the target is polarized, like a dipoleantenna. This is described by Rayleigh Scattering (like the blue sky). When the twolength scales are comparable, there may be resonances. Early radars used very longwavelengths that were larger than the targets and received a vague signal, whereas somemodern systems use shorter wavelengths (a few centimeters or shorter) that can imageobjects as small as a loaf of bread or smaller.Radio waves always reflect from curves and corners, in a way similar to glint from arounded piece of glass. The most reflective targets for short wavelengths have 90° anglesbetween the reflective surfaces. A surface consisting of three flat surfaces meeting at asingle corner, like the corner on a block, will always reflect directly back at the source.These so-called corner cubes are commonly used as radar reflectors to make otherwisedifficult-to-detect objects easier to detect, and are often found on boats in order toimprove their detection in a rescue situation and reduce collisions. For generally the samereasons objects attempting to avoid detection will angle their surfaces in a way toeliminate inside corners and avoid surfaces and edges perpendicular to likely detection
    • directions, which leads to "odd" looking stealth aircraft. These precautions do notcompletely eliminate reflection because of diffraction, especially at longer wavelengths.Electromagnetic waves do not travel well underwater; thus for underwater applications,sonar, based on sound waves, has to be used instead of radar.PolarizationPolarization is the direction that the wave vibrates. Radars use horizontal, vertical, andcircular polarization to detect different types of reflections. For example, circularpolarization is used to minimize the interference caused by rain. Linear polarizationreturns usually indicate metal surfaces, and help a search radar ignore rain. Randompolarization returns usually indicate a fractal surface like rock or dirt, and are used bynavigational radars. Brightness can indicate reflectivity as in this 1960 weather radar image. The radars frequency, polarization, and receiver determine what it can observe.Distance measurementTransit timeThe easiest way to measure the range of an object is to broadcast a short pulse of radiosignal, and then time how long it takes for the reflection to return. The distance is one-half the product of round trip time (because the signal has to travel to the target and thenback to the receiver) and the speed of the signal. Where c is the speed oflight in a vacuum, and τ is the round trip time. For RADAR the speed of signal is thespeed of light, making the round trip times very short for terrestrial ranging. For this
    • reason accurate distance measurement was difficult until the introduction of highperformance electronics, with older systems being accurate to perhaps a few percent.The receiver cannot detect the return while the signal is being sent out – there is no wayto tell if the signal it hears is the original or the return. This means that a radar has adistinct minimum range, which is the length of the pulse divided by the speed of light,divided by two. In order to detect closer targets you have to use a shorter pulse length.A similar effect imposes a specific maximum range as well. If the return from the targetcomes in when the next pulse is being sent out, once again the receiver cannot tell thedifference. In order to maximize range, one wants to use longer times between pulses, theinter-pulse time.Frequency modulationAnother form of distance measuring radar is based on frequency modulation. Frequencycomparison between two signals is considerably more accurate, even with olderelectronics, than timing the signal. By changing the frequency of the returned signal andcomparing that with the original, the difference can be easily measured.This technique can be used in radar systems, and is often found in aircraft radaraltimeters. In these systems a "carrier" radar signal is frequency modulated in apredictable way, typically varying up and down with a sine wave or sawtooth pattern ataudio frequencies. The signal is then sent out from one antenna and received on another,typically located on the bottom of the aircraft, and the signal can be continuouslycompared.Since the signal frequency is changing, by the time the signal returns to the aircraft thebroadcast has shifted to some other frequency. The amount of that shift is greater overlonger times, so greater frequency differences mean a longer distance, the exact amountbeing the "ramp speed" selected by the electronics. The amount of shift is thereforedirectly related to the distance travelled, and can be displayed on an instrument. Thissignal processing is similar to that used in speed detecting doppler radar. See the articleon continuous wave radar for more information.
    • Speed measurementSpeed is the change in distance to an object with respect to time. Thus the existing systemfor measuring distance, combined with a little memory to see where the target last was, isenough to measure speed. At one time the memory consisted of a user making grease-pencil marks on the radar screen, and then calculating the speed using a slide rule.However there is another effect that can be used to make much more accurate speedmeasurements, and do so almost instantly (no memory required), known as the Dopplereffect. Practically every modern radar uses this principle in the pulse-doppler radarsystem. It is also possible to make a radar without any pulsing, known as a continuous-wave radar (CW radar), by sending out a very pure signal of a known frequency. Returnsignals from targets are shifted away from this base frequency via the Doppler effectenabling the calculation of the speed of the object relative to the radar.POSITION MEASUREMENTRadio signals broadcast from a single antenna will spread out in all directions, andlikewise a single antenna will receive signals equally from all directions. This leaves theradar with the problem of deciding where the target object is located.Early systemsEarly systems tended to use omni-directional broadcast antennas, with directionalreceiver antennas which were pointed in various directions. For instance the firstsystem to be deployed, Chain Home, used two straight antennas at right angles forreception, each on a different display.One serious limitation with this type of solution is that the broadcast is sent out in alldirections, so the amount of energy in the direction being examined is subject to theinverse-square law. To get a reasonable amount of power on the "target", the broadcastshould also be steered. More modern systems used a steerable parabolic "dish" to create atight broadcast beam, typically using the same dish as the receiver. Such systems oftencombined two radar frequencies in the same antenna in order to allow automatic steering,or radar lock.
    • Not all radar antennas must rotate to scan the sky.RADAR EQUATIONThe amount of power Pr returning to the receiving antenna is given by the radar equation:WherePt = transmitter power,Gt = gain of transmitting antenna,Ar = effective aperture (area) of receiving antenna,σ = Radar Cross Section, or scattering coefficient of target,Rt = distance from transmitter to target,Rr = distance from target to receiver.In the common case where the transmitter and receiver are at the same location, Rt = Rrand the term Rt² Rr² can be replaced by R4, where R is the range. This yields:This shows that the received power declines as the fourth power of the range, whichmeans that the reflected power from distant targets is very, very small.Other mathematical developments in radar signal processing include time-frequencyanalysis (Weyl Heisenberg or wavelet), as well as the chirplet transform which makes useof the fact that radar returns from moving targets typically "chirp" (change theirfrequency as a function of time, as does the sound of a bird or bat).
    • FREQUENCY BANDSThe traditional band names originated as code-names during World War II and are still inmilitary and aviation use throughout the world in the 21st century. They have beenadopted in the United States by the IEEE, and internationally by the ITU. Most countrieshave additional regulations to control which parts of each band are available for civilianor military use.Other users of the radio spectrum, such as the broadcasting and electroniccountermeasures (ECM) industries, have replaced the traditional military designationswith their own systems.Radar Frequency BandsBand Frequency Wavelength NotesName Range RangeHF 3-30 MHz 10-100 m coastal radar systems;high frequency P for previous, applied retrospectively to early radarP < 300 MHz 1 m+ systemsVHF 50330MHz 0.9-6 m very long range,groundpenetrating;veryhighfrequency 300-1000 Very long range (e.g. ballistic early warning), groundUHF 0.3-1 m MHz penetrating; ultra high frequency long range air traffic control and surveillance; L forL 1-2 GHz 15-30 cm long terminal air traffic control, long range weather, marineS 2-4 GHz 7.5-15 cm radar; S for short a compromise (hence C) between X and S bands;C 4-8 GHz 3.75-7.5 cm weatherX 8-12 GHz 2.5-3.75 cm missile guidance, marine radar, weather; in the USA the narrow range 10.525GHz ±25MHz is used for airport
    • radar. high-resolution mapping, satellite altimetry; frequencyKu 12-18 GHz 1.67-2.5 cm just under K band (hence u) Mapping, short range, airport surveillance; frequency just above K band (hence a) Photo radar, used toKa 27-40 GHz 0.75-1.11 cm trigger cameras which take pictures of license plates of cars running red lights, operates at 34.300 ± 0.100 GHz.mm 40-300 GHz 1 - 7.5mm millimetre band, subdivided as belowV 40-75 GHz 4.0 - 7.5 mm used as a visual sensor for experimental autonomousW 75-110 GHz 2.7 - 4.0 mm vehicles, high-resolution meterological observationSPECIFIC RADAR SYSTEMSActive Electronically Scanned Array (AESA)An Active Electronically Scanned Array (AESA) is a revolutionary type of radarwhose transmitter and receiver functions are composed of numerous smalltransmit/receive (T/R) modules that each scan a small fixed area, negating the need for amoving antenna. AESA radars feature short to instantaneous (millisecond) scanning ratesand have desirable low-probability of intercept characteristicsContinuous-wave radarContinuous-wave radar system is a radar system where a continuous wave is transmittedby one antenna and a second receives the radio energy reflected from an object.Doppler radar as weather radarDoppler radar uses the Doppler Effect to return additional information from a radarsystem. The Doppler Effect shifts the frequency of the radar beam due to movement ofthe "target", allowing for the direct and highly accurate measurement of speeds. Doppler
    • radars were originally developed for military radar systems, but have since become a partof almost all radar systems, including weather radar and radar guns for traffic police andsportsMillimeter cloud radarThe millimeter wave cloud radar (MMCR) is a remote sensing instrument that transmits aradar pulse directly overhead to determine the tops and bottoms of clouds. It can alsoserve as a type of Doppler radar in measuring up and down particle movements within acloud. Values that the radar measures are Doppler velocity, radar reflectivity, and spectralwidth.NEXRADNEXRAD Radar at NSSLNEXRAD or Nexrad (the next-generation radar) is a network of Doppler radarsoperated by the National Weather Service, an agency of NOAA, the National Oceanicand Atmospheric Administration, in the United States. NEXRAD detects precipitationand atmospheric movement or wind. It returns data which when processed can bedisplayed in a mosaic map which shows patterns of precipitation and its movement. Theradar system operates in two basic modes, selectable by the operator: a slow-scanning
    • "clear-air mode" for analyzing air movements when there is little or no activity in thearea, and a "precipitation mode" with a faster scan time to do traditional storm tracking.Passive radarPassive Radar is type of radar system which uses one or more receivers, but lacks anactive transmitter. The system detects ambient radio signals emanating from nearby radiotransmitters. The receiver is either bistatic or multistatic, since it is positioned elsewhere.The system is not restricted to one receiver—several receiver systems may be operated inconjunction with one or many transmitters.Pulse-Doppler radarRadar gun traffic and sports radarsU.S. Army soldier uses a radar gun to catch speedingviolators at Tallil Air Base, Iraq.A radar gun is a small Doppler radar used to detect thespeed of objects. A radar gun does not return position orpower information. It relies on the Doppler Effect applied toa radar beam to measure the speed of objects it is pointed at.Radar guns may be hand-held or vehicle-mounted. Common uses include traffic speedlaw enforcement, and measuring the speed of balls in sports.Secondary surveillance radar (SSR).
    • Synthetic aperture radar The surface of Venus, as imaged by the Magellan probe using SAR,Synthetic aperture radar (SAR) is a form of radar in which sophisticated post-processing of radar data is used to produce a very narrow effective beam. It can only beused by moving instruments over relatively immobile targets, but it has seen wideapplications in remote sensing and mapping.X-band radar