经典的《Radar Systems Analysis and Design Using Matlab》。 第一章。 讲解详实，图文并茂，深入浅出，有理有据。 配合MATLAB源代码。array radar, and the early warning PAve PAWS multifunction UHF phasedarray radar. Because of the very large wavelength and the sensitivity require-ments for very long range measurements, large apertures are needed in suchadar systemsU.S. Navy ROTHR2.6-km Receiving ArrayFigure 1. 1. U. s Navy Over The horizon Radar. Photograph obtainedvia the internetFigure 1.2. Fylingdales bmews-United Kingdom Photograpobtained via the interneto 2000 by Chapman Hall/crCRadars in the L-band are primarily ground based and ship based systems thatare used in long range military and air traffic control search operations. Mostground and ship based medium range radars operate in the s-band For exam-ple, the Airport Surveillance Radar(ASR) used for air traffic control, and theship based U.S. Navy AEGIS(Fig. 1.3) multifunction phased array are S-bandradars. The Airborne Warning And Control System(AWACS shown in Fig1. 4 and the national weather service Next Generation doppler weather radar(NEXRAD) are also S-band radars. However, most weather detection radarsystems are C-band radars. Medium range search and fire control militaryradars and metric instrumentation radars are also c-bandFigure 1.3. U S Navy AEGIS. Photograph obtained via the Internet.Figure 1.4. U.s. Air Force AWACS Photograph obtained via the Internet.o 2000 by Chapman Hall/crCThe X-band is used for radar systems where the size of the antenna consti-tutes a physical limitation; this includes most military multimode airborneradars. Radar systems that require fine target detection capabilities and yet can-not tolerate the atmospheric attenuation of higher frequency bands may also bX-band. The higher frequency bands(Ku, K, and Ka) suffer severe weatherand atmospheric attenuation. Therefore, radars utilizing these frequency bandsare limited to short range applications, such as the police traffic radars, shortrange terrain avoidance and terrain following radars. Milli-Meter Wave(MMW) radars are mainly limited to very short range radio Frequency (rF)seekers and experimental radar systems1. 2 RangeFigure 1. 5 shows a simplified pulsed radar block diagram. The time controlbox generates the synchronization timing signals required throughout the sys-tem. A modulated signal is generated and sent to the antenna by the modulator/transmitter block Switching the antenna between the transmitting and receiv-ing modes is controlled by the duplexer. The duplexer allows one antenna to beused to both transmit and receive During transmission it directs the radar elec-tromagnetic energy towards the antenna. Alternatively, on reception, it directsthe received radar echoes to the receiver. The receiver amplifies the radarreturns and prepares them for signal processing. Extraction of target information is performed by the signal processor block. The targets range, R, is computed by measuring the time delay, At; it takes a pulse to travel the two-wayath between theand the target. Since eleetic waves travel atthe speed of light, c=3X 10m/sec, thenRDuplexerTimeReceiveFigure 1.5. A simplified pulsed radar block diagram.o 2000 by Chapman Hall/crC△tRwherer is in meters and At is in seconds. The factor of is needed toaccount for the two-way time delayIn general, a pulsed radar transmits and receives a train of pulses, as illustrated by Fig. 1.6. The Inter Pulse Period (pp)is T, and the pulse width is tThe IPP is often referred to as the Pulse repetition Interval (PRI). The inverseof the Pri is the PRF, which is denoted by f,(12)Ptransmitted pulsesIPPpulse 1pulse 2pulse 3time咄k2llse 3received pulsesechoechotim eFigure 1.6. Train of transmitted and received pulsesDuring each PRi the radar radiates energy only for t seconds and listens fortarget returns for the rest of the Pri. The radar transmitting duty cycle(factor)d is defined as the ratio d = t/t. The radar average transmitted power isPa=P1×d(1.3)where p, denotes the radar peak transmitted power. The pulse energy isEn =Pt= Pat= Pov/fThe range corresponding to the two-way time delay t is known as the radarunambiguous range, R. Consider the case shown in Fig. 1.7. Echo 1 represents the radar return from a target at range R1 =cAt/2 due to pulse l. echo2 could be interpreted as the return from the same target due to pulse 2, or itmay be the return from a faraway target at range /2 ue to pulse i again. Inthis caseR(T+△t)0/Ro 2000 by Chapman Hall/crC1/PRltransm itted pulsespulse Ipulsc 2timc or ranecho lecho 2 time or rangereceived pulses△△RFigure 1.7. Illustrating range ambiguityClearly, range ambiguity is associated with echo 2. Therefore, once a pulse istransmitted the radar must wait a sufficient length of time so that returns fromtargets at maximum range are back before the next pulse is emitted. It followsthat the maximum unambiguous range must correspond to half of the PRl,MATLAB Function"pulsetrain. mThe MaTLAB function "pulse_train. m"computes the duty cycle, averagetransmitted power, pulse energy, and the pulse repetition frequency. It is givenin Listing 1.1 in Section 1. 8; its syntax is as followslat pav ep prf ru =pulsetrain( tau, Pri, p_peakwhereSymbolDescriptionUnitsStatuspulse widthsecondsprtPRIsecondsinputPpeakpeak powerWattsinputd tduty cvclenoneoutputDavaverage transmitted powerWattsoutputpulse energyJoulesoutputprfPRFHzoutputruunambiguous rangeKoutnuto 2000 by Chapman Hall/crCExample 1.1: A certain airborne pulsed radar has peak power P,= 10Kwand uses two PRFS, frI 10KHz and f2= 30KHz. What are the requpulse widths for each PRF so that the average transmitted power is constantand is equal to 1500 Watts Compute the pulse energy in each caseSolution: Since Pay is constant, then both PRFs have the same duty cycleMore precisely15000.1510×10The pulse repetition intervals areO. ms10×1030×10It follows that0.15×T1=15μS0.15×72=5sn1=P=10×10×15×10°=0.15ol!es210×103×5×106=0.05 Joules.1.3. Range resolutionRange resolution, denoted as AR, is a radar metric that describes its abilityto detect targets in close proximity to each other as distinct objects. Radar systems are normally designed to operate between a minimum range Rmin, andmaximum range rThe distance between R andR is divided intomlnnaxM range bins(gates ), each of width ARRMmax△R(16)Targets separated by at least AR will be completely resolved in range, as illustrated in fig.1. 8. Targets within the same range bin can be resolved in crossrange(azimuth)utilizing signal processing techniqueso 2000 by Chapman Hall/crCConsider two targets located at ranges R and R2, corresponding to timedelays t, and t2, respectively. Denote the difference between those two ranges△R△R=R2R△R中△rangeCluste中RRFigure 1.8. Resolving targets in range and cross range.Now, try to answer the following question: What is the minimum St suchthat target I at R, and target 2 at r2 will appear completely resolved in range(different range bins ) In other words, what is the minimum AR?First, assume that the two targets are separated by ct/4, t is the pulsewidth. In this case, when the pulse trailing edge strikes target 2 the leadingedge would have traveled backwards a distance ct, and the returned pulsewould be composed of returns from both targets (i.e, unresolved return),asshown in Fig. 1.9a. However, if the two targets are at least ct/2 apart, then asthe pulse trailing edge strikes the first target the leading edge will start to returnfrom target 2, and two distinct returned pulses will be produced, as illustratedby Fig. 1.9b. Thus, AR should be greater or equal to cT/2. And since the radarbandwidth b is equal to 1/t, then△R2BIn general, radar users and designers alike seek to minimize AR in order toenhance the radar performance. As suggested by Eq(1. 8), in order to achievefine range resolution one must minimize the pulse width. However, this willreduce the average transmitted power and increase the operating bandwidthAchieving fine range resolution while maintaining adequate average transmit-ted power can be accomplished by using pulse compression techniqueso 2000 by Chapman Hall/crCR, Rincident pulseCt℃4a)reflected pulsereturnreturnttlt2ttl tgtshaded area has returnsfrom both targeRreflected pulsesreturnreturnt1tgtCTCTlgl1 lg12Figure 1.9.(a) Two unresolved targets (b)Two resolved targetso 2000 by Chapman hall/CRC