RadarEquationforpointTarget雷达气象学讲义课件.ppt

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1、Chapter 3:Principle of Radar Observation雷達系統與觀測原理,Radar system:Transmitter,Receiver,Antenna,and Display of dataScattering theory,Mie scattering,The stabilized local oscillator(STALO)generates a continuous wave(cw)signal of nearly perfect sinusoid form(coherent signal)which is modulated(pulse on and

2、off 脈衝觸發器)and amplified by a klystron to produce intense microwave power.Klystron(調速管)amplifier is phase coherent and Magnetron(磁控管)amplifier is phase incoherent oscillator.Radar pulses are phase coherent from pulse to pulse if the phase angle for each pulse is fixed.,MOPA:master oscillator and powe

3、r amplifier:Transmitter,Block diagram of a coherent pulse radar,The choice of the transmitted frequency f0 results from a compromise,taking into account the conditions of propagation in the atmosphere and the technological requirements involved in implementing the equipment.The emitted wave:S(t)=At

4、cos0tThe received signal:Pr(t)=a cos(0t+t),t=2(2r/),Transmitted frequency,W-band,90-100GHz,0.33-0.3 cm,for example:a 94GHz cloud radar,3.2mm wavelength,by Lhermitte(1987)K-band,18-26.5GHz,1.6-1.13 cm(H2O absorption band 22.3GHZ sits in the center,thus was abolished,then use above and under K bands),

5、Ka-band,26.5-40GHz,1.13-0.75 cm(US air force for cloud observations),Ku-band,12.5-18GHz,2.4-1.6 cmX-band,8-12.5GHz,3.7-2.4 cm(installed on board B-17 bomber)after WW2 donated to meteorological institutions in the world(wavelength 3.2cm,APQ13 and APS15)C-band,4-8GHz,7.5-3.7 cm S-band,2-4GHz,15-10 cmL

6、-band,1-2GHz,30-15 cmUHF,0.3-1 GHz,1 to 0.3 m(clear air backscattering motion)VHF,0.1-0.3GHz,3 to 1 m,Block diagram of a coherent pulse radar,I(t)=A0 cos(d t)Q(t)=A0 sin(d t)Or E(t)=A0 exp id tFor the pulse n,En=A0 exp i r,n with r=tan-1Q/IFor two samples n and n-1 corresponding to two successive pu

7、lses,we haver/t=r,n-r,n-1/PRT=d,The electron gun produces a flow of electrons.The bunching cavities regulate the speed of the electrons so that they arrive in bunches at the output cavity.The bunches of electrons excite microwaves in the output cavity of the klystron.The microwaves flow into the wav

8、eguide,which transports them to the accelerator.The electrons are absorbed in the beam stop.,In a klystron,Klystron gallery in Stanford U.,Stanford University-Klystron gallery,In a magnetron,The nucleus of the high-voltage system is the magnetron tube.The magnetron is a diode-type electron tube whic

9、h is used to produce the required 2450 MHz of microwave energy.It is classed as a diode because it has no grid as does an ordinary electron tube.A magnetic field imposed on the space between the anode(plate)and the cathode serves as the grid.While the external configurations of different magnetrons

10、will vary,the basic internal structures are the same.These include the anode,the filament/cathode,the antenna,and the magnets,Solid state transmitter,Solid State FM Exiter/Amplifier/Transimitters from 500 w to 20 kw Tube(Zero Bias triode)type,long life economical FM Transmitter from 1 kw to 32.5 kw

11、STL systems(Composite Studio Transmitter Links)Solid State AM Transimitter from 250 watts to100 kw,Spectral purity:the absence of power at frequencies other than the intended ones.發射訊號頻率純度對地面雜波去除以及大範圍強回波區內偵測弱回波訊號（龍捲）都非常重要。Ground clutter cancellation:suppressing echoes from stationary objects on the

12、ground.Microwave pulsesPulse repetition time(frequency):PRT(PRF)Ts(fs)Pulse duration(pulse width)1 s(defined as the time between instances when the power is one-quarter of the peak).Listening period:Ts-,receiving signal,Power density of an idealized pulse S(r,)U(t-r/c)，where U(t-r/c)=1,r/c t(r/c+)an

13、d U(t-r/c)=0,otherwise.,Antenna 天線,天線由輻射器(radiator)與反射器(reflector)組合而成輻射器是由波導管(wave-guide)以及波導管擴展而成之喇叭口(horn feed)組合而成經放大之高能微波脈衝經由波導管引至喇叭口放射至反射器上反射器為一拋物面之圓形天線,喇叭口位於此反射器之焦點上,產生窄波束之波鋒,Antenna,There are several critical parameters that affect an antennas performance and can be adjusted during the desig

14、n process.These are resonant frequency,impedance,gain,aperture or radiation pattern,polarization,efficiency and bandwidth.Transmit antennas may also have a maximum power rating,and receive antennas differ in their noise rejection properties.,Resonant frequency,The resonant frequency is related to th

15、e electrical length of the antenna.This is usually the physical length of the wire multiplied by the ratio of the speed of wave propagation in the wire.Typically an antenna is tuned for a specific frequency,and is effective for a range of frequencies usually centered on that resonant frequency.,Impe

16、dance and SWR,Impedance is similar to refractive index in optics.As the electric wave travels through the different parts of the antenna system(radio,feed line,antenna,free space)it may encounter differences in impedance.At each interface,some fraction of the waves energy will reflect back to the so

17、urce,forming a standing wave in the feed line.The ratio of maximum power to minimum power in the wave can be measured and is called the standing wave ratio(SWR).A SWR of 1:1 is ideal.A SWR of 1.5:1 is considered to be marginally acceptable in low power applications where power loss is more critical,

18、although an SWR as high as 6:1 may still be usable with the right equipment.Minimizing impedance differences at each interface(impedance matching)will reduce SWR and maximize power transfer through each part of the antenna system.,Antenna gain:dBi or dBd.,It is important to note that antenna gain is

19、 different than amplifier gain.Antennas do not have a power source that allows the antenna to create additional energy to boost the signal.An antenna is similar to a reflective lens in principle-it takes the energy available from the source and focuses it over a wider or narrower area.Antenna gain i

20、s then a measure of the amount of focus that an antenna can apply to the incoming signal relative to one of two reference dispersion patterns.MaxStream specifies all antenna gains in dBi.dBi is the amount of focus applied by an antenna with respect to an Isotropic Radiator(a dispersion pattern that

21、radiates the energy equally in all directions onto an imaginary sphere surrounding a point source).Thus an antenna with 2.1 dBi of gain focuses the energy so that some areas on an imaginary sphere surrounding the antenna will have 2.1 dB more signal strength than the strength of the strongest spot o

22、n the sphere around an Isotropic Radiator.dBd refers to the antenna gain with respect to a reference dipole antenna.A reference dipole antenna is defined to have 2.15 dBi of gain.So converting between dBi and dBd is as simple as adding or subtracting 2.15 according to these formulas:dBi=dBd+2.15 dBd

23、=dBi-2.15Specifying antenna gain in dBd means that the antenna in question has the ability to focus the energy x dB more than a dipole.,Isotropic and dipole antenna,The isotropic radiator is a purely theoretical antenna that radiates equally in all directions.It is considered to be a point in space

24、with no dimensions and no mass.This antenna cannot physically exist,but is useful as a theoretical model for comparison with all other antennas.Most antennas gains are measured with reference to an isotropic radiator,and are rated in dBi(decibels with respect to an isotropic radiator).The dipole ant

25、enna is simply two wires pointed in opposite directions arranged either horizontally or vertically,with one end of each wire connected to the radio and the other end hanging free in space.Since this is the simplest practical antenna,it is also used as reference model for other antennas;gain with res

26、pect to a dipole is labeled as dBd.Generally,the dipole is considered to be omnidirectional in the plane perpendicular to the axis of the antenna,but it has deep nulls in the directions of the axis.Variations of the dipole include the folded dipole,the half wave antenna,the groundplane antenna,the w

27、hip,and the J-pole.,Beam Width,Because higher gain antennas achieve the extra power by focusing in on a smaller area it is important to remember that the greater the gain,the smaller the area covered as measured in degrees of beam width(think of an adjustable beam flashlight).In many cases a high ga

28、in antenna is a detriment to the system performance because the system needs to have reception over a large area.,Dipole antenna radiation pattern,Isotropic antenna(dBi),half-wavelength dipole antenna(actually no gain antenna,dBd),omni vs directional antenna,parabolic antenna,The vast majority of an

29、tennas are simple vertical rods a quarter of a wavelength long.Such antennas are simple in construction,usually inexpensive,and both radiate in and receive from all horizontal directions(omnidirectional).One limitation of this antenna is that it does not radiate or receive in the direction in which

30、the rod points.This region is called the antenna blind cone or null.,Aperture is the shape of the beam cross section in the direction of highest gain,and is two-dimensional.(Sometimes aperture is expressed as the radius of the circle that approximates this cross section or the angle of the cone.)and

31、 radiation pattern,Bandwidth and polarization,The bandwidth of an antenna is the range of frequencies over which it is effective,usually centered around the resonant frequency.The bandwidth of an antenna may be increased by several techniques,including using thicker wires,replacing wires with cages

32、to simulate a thicker wire,tapering antenna components(like in a feed horn),and combining multiple antennas into a single assembly and allowing the natural impedance to select the correct antenna.Small antennas are usually preferred for convenience,but there is a fundamental limit relating bandwidth

33、,size and efficiency.The polarization of an antenna or orientation of the radio wave is determined by the electric field or E-plane.The ionosphere changes the polarization of signals unpredictably,so for signals which will be reflected by the ionosphere,polarization is not crucial.However,for line-o

34、f-sight communications,it can make a tremendous difference in signal quality to have the transmitter and receiver using the same polarization.Polarizations commonly considered are linear,such as vertical and horizontal,and circular,which is divided into right-hand and left-hand circular.,Antenna eff

35、iciency,EfficiencyEfficiency is the ratio of power actually radiated to the power put into the antenna terminals.A dummy load may have a SWR of 1:1 but an efficiency of 0,as it absorbs all power and radiates heat but not RF energy,showing that SWR alone is not an effective measure of an antennas eff

36、iciency.Radiation in an antenna is caused by radiation resistance which can only be measured as part of total resistance including loss resistance.Loss resistance usually results in heat generation rather than radiation,and therefore,reduces efficency.Overview of antenna parametersOf the parameters

37、above,SWR is most easily measured.Impedance can be measured with specialized equipment,as it relates to the complex SWR.Measuring radiation pattern requires a sophisticated setup including significant clear space(enough to get into the antennas far field)or an anechoic chamber designed for antenna m

38、easurements,careful study of experiment geometry,and specialised measurement equipment that rotates the antenna during the measurements.Bandwidth depends on the overall effectiveness of the antenna,so all of these parameters must be understood to understand bandwidth.However,typically bandwidth is m

39、easured by only looking at SWR,i.e.,by finding the frequency range over which the SWR is less than a given value.Bandwidth over which an antenna exhibits a particular radiation pattern might also be considered.,Transmission and reception,All of these parameters are expressed in terms of a transmissi

40、on antenna,but are identically applicable to a receiving antenna,due to reciprocity.Impedance,however,is not applied in an obvious way;for impedance,the impedance at the load(where the power is consumed)is most critical.For a transmitting antenna,this is the antenna itself.For a receiving antenna,th

41、is is at the(radio)receiver rather than at the antenna.Antennas used for transmission have a maximum power rating,beyond which heating,arcing or sparking may occur in the components,which may cause them to be damaged or destroyed.Raising this maximum power rating usually requires larger and heavier

42、components,which may require larger and heavier supporting structures.Of course,this is only a concern for transmitting antennas;the power received by an antenna rarely exceeds the microwatt range.Antennas designed specificially for reception might be optimized for noise rejection capabilities.This

43、can be done by selecting a narrow bandwidth so that noise from other frequencies is rejected,or selecting a specific radiation pattern to reject noise from a specific direction,or by selecting a polarization different from the noise polarization,or by selecting an antenna that favors either the elec

44、tric or magnetic field.For instance,an antenna to be used for reception of low frequencies(below about ten megahertz)will be subject to both man made noise from motors and other machinery,and from natural sources such as lightning.Successfully rejecting these forms of noise is an important antenna f

45、eature.A small coil of wire with many turns is more able to reject such noise than a vertical antenna.However,the vertical will radiate much more effectively on transmit,where extraneous signals are not a concern.,Parabolic antenna,Radar scan mode,Elevation angleeAzimuth angleRange rHorizontal dista

46、nce dHeight hZenith z,Various typical antenna reflectors and associated beams,Most weather radars use symmetrical beams with widths of 1 to 3 degrees.Narrow vertical and wider horizontal beams is used to measure heights and vertical dimensions of small targets.Search radar for detecting aircraft gen

47、erally use wide vertical beams.The precision of the measurements of azimuthal and elevation angles to a target is largely a function of the shape of the beam produced by the antenna.The shape of the beam is governed by the shape and size of the antenna reflector and the wavelength of the radar set.,

48、Reflector and horn feed,3-dB beam width,當脈衝離開天線以天線直徑大小平行電磁波束行進。由於繞射緣故，此一平行電磁波束在到達第一個繞射零點(距離為 r=Da2/)後開始發散成圓錐狀。雷達波束的束寬(beam width)定義為能流密度不小於最大值的一半之波束寬，或稱之為3-dB束寬。由於雷達天線發射出之雷達束的能流密度可以 S(r,)表示。天線無法將能量完全集中在一個窄束中，因此脈衝能流密度分佈呈現主瓣(main lobe)和幾個副瓣(side lobe)現象。現有一般雷達設計，副瓣的能流密度沒有大於主瓣百分之一的。事實上，所有副瓣能流密度的總和僅佔主瓣的

49、百分之幾而已。,天線-反射器經常成拋物圓形,能流密度(輻射強度)與離束軸(beam axis)距離()成 1-4(/Da)2 2的關係。在此情形下,常態化能流密度形狀(或稱為輻射形狀函數radiation pattern)與偏角的關係可寫成：2()=S()/S(0)=8J2(Dasin)/(Dasin)/22 其中為離束軸之偏角大小,為雷達發射電磁波波長,Da為天線直徑,J2 是Bessel function of second order。雷達發射之微波脈衝可視為厚度為c(脈衝寬)的球形殼以光速往外傳播並擴張。因此,在一封閉球體內所發射之脈衝能量（Pt)為常數,但是散射物所接收到的能流密度大

50、小與距離平方成反比。此外由於天線本身,傳輸線,天線罩等造成的電磁波能量減損,進入天線之脈衝能量 Pt 要大於實際發射出的 Pt。,Antenna beam pattern,Beam pattern 束形,天線(雷達發射之微波脈衝)束形,脈衝之主瓣以及多個副瓣清楚可見。,Beam pattern,With a parabolic reflector and Gaussian illumination(氣象常用)Width of the beam at 3 dB,1/2=70/DaWidth of the main lobe(between nulls)null=170/DaLevel of fi