Radar Systems.pdf - www.alljntuworld.in JNTU World JN TU W or SYSTEMS ld RADAR 1 Downloaded From JNTU World(http\/www.alljntuworld.in www.alljntuworld.in

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Unformatted text preview: JNTU World JN TU W or SYSTEMS ld RADAR 1 Downloaded From JNTU World ( ) JNTU World SYLLABUS ld UNIT-I W UNIT-II or Introduction, Nature of RADAR, Maximum Unambiguous range, Radar Waveforms, Block schematics of pulse radar and Operation, simple form of radar equation, RADAR frequencies, Applications of RADARS. Prediction of Range Performance, Minimum Detectable Signal, Receiver Noise, Modified radar range equation, Illustrative problems. TU Radar Equation: SNR, Envelop Detector, False alarm time and Probability, Integration of Radar Pulses, Radar Cross Section of Targets (simple Targets-sphere, conesphere),Transmitter Power, PRF and Range Ambiguities, System Losses(qualitative treatment), Illustrative Problems . UNIT-III JN CW and Frequency Modulated Radar: Doppler Effect, CW Radar – Block Diagram, Isolation between Transmitter and Receiver, Non-zero IF Receiver, Receiver bandwidth requirements, Applications of CW radar, Illustrative Problems. UNIT-IV FM-CW Radar, Range and Doppler measurement, Block Diagram and Characteristics (Approaching/Receding targets), FM-CW Altimeter, Measurement errors, 2 Downloaded From JNTU World ( ) JNTU World Multiple Frequency CW Radar. UNIT-V UNIT-VI W or ld MTI and Pulse Doppler radar: Introduction, Principle, MTI Radar with - Power Amplifier Transmitter and Power Oscillator Transmitter, Delay Line Cancellers – Filter Characteristics, Blind Speeds, Double Cancellation, Staggered PRFs. Range Gated Doppler Filters. MTI Radar Parameters, Limitations to MTI Performance, Non-coherent MTI, MTI versus Pulse Doppler radar. TU Tracking Radar: Tracking with Radar, Sequential Lobing, Conical Scan, Monopulse Radar – Amplitude Comparison and Phase Comparison Monopulse. Low angle tracking, tracking in range, acquisition, ComparisonofTrackers. JN UNIT-VII Detection of Radar Signals in Noise: Introduction, Matched filter receiverResponse Characteristics and Derivation, Correlation Function and Cross-Correlation receiver, Efficiency of Non-matched Filters, matched Filter with Non-white Noise. UNIT-VIII 3 Downloaded From JNTU World ( ) JNTU World JN TU W or ld Radar Receivers: Noise Figure and Noise temperature, Displays-types, DuplexersBranch type and Balanced type, Circulators as Duplexers. Introduction to Phased Array Antennas-Basic Concepts, Radiation Pattern, Beam Steering and Beam Width Changes, Series versus Parallel Feeds, Applications, Advantages and Limitations. 4 Downloaded From JNTU World ( ) JNTU World or ld JN TU W UNIT-I 5 Downloaded From JNTU World ( ) JNTU World JN TU W or ld NATURE OF RADAR 6 Downloaded From JNTU World ( ) JNTU World INTRODUCTION: ld The name Radar stands for Radio Detection and Ranging Two distinguishing characteristics: or Radar is a remote sensing technique: Capable of gathering information about objects located at remote distances from the sensing device. W 1. Employs EM waves that fall into the microwave portion of the electromagnetic spectrum (1 mm < l < 75 cm) Active technique: radiation is emitted by radar – radiation scattered by objects is detected by radar. JN TU 2. 7 Downloaded From JNTU World ( ) JNTU World JN TU W or ld 8 Downloaded From JNTU World ( ) JNTU World Radar is an electromagnetic system for the detection and location of objects (Radio Detection and Ranging). Radar operates by transmitting a particular type of waveform and detecting the nature of the signals reflected back from objects. ld Radar can’ t resolve detail or color as well as the human eye (an optical frequency passive scatter meter). or Radar can see in conditions which do not permit the eye to see such as darkness, haze, rain, smoke. Radar can also measure the distances to objects. The elemental radar system consists of a transmitter unit, an antenna for emitting electromagnetic radiation and receiving the echo, an JN TU W Energy detecting receiver and a processor. 9 Downloaded From JNTU World ( ) JNTU World ld or A portion of the transmitted signal is intercepted by a reflecting object (target) and is reradiated in all directions. The antenna collects the returned energy in the backscatter direction and delivers it to the receiver. The distance to the receiver is determined by measuring the time taken for the electromagnetic signal to travel to the target and back. W The direction of the target is determined by the angle of arrival (AOA) of the reflected signal. Also if there is relative motion between the radar and the target, there is a shift in frequency of the reflected signal (Doppler Effect) which is a measure of the radial component of the relative velocity. This can be used to distinguish between moving targets and stationary ones. JN TU Radar was first developed to warn of the approach of hostile aircraft and for directing anti aircraft weapons. Modern radars can provide AOA, Doppler, and MTI etc. 10 Downloaded From JNTU World ( ) JNTU World JN TU W or ld RADAR RANGE MEASUREMENT 11 Downloaded From JNTU World ( ) get c • Target range = where c = 2 eed of light round trip JN TU = W or ld T JNTU World 12 Downloaded From JNTU World ( ) time JNTU World The simplest radar waveform is a train of narrow (0.1μs to 10μs) rectangular pulses modulating a sinusoidal carrier the distance to the target is determined from the time T R taken by the pulse to travel to the target and return and from the knowledge that electromagnetic energy travels at the speed of light. ld Since radio waves travel at the speed of light (v = c = 300,000 km/sec) W The range or distance, R = cTR/2 or Range = c×time/2 TU R (in km) = 0.15TR (μs) ; R (in nmi) = 0.081TR (μs) NOTE: 1 nmi = 6076 feet =1852 meters. 1 Radar mile = 2000 yards = 6000 feet Radar mile is commonly used unit of distance. JN NOTE: Electromagnetic energy travels through air at approximately the speed of light:- 13 Downloaded From JNTU World ( ) JNTU World 1. 300,000 kilometers per second. 2. 186,000 statute miles per second. ld 3. 162,000 nautical miles per second. or Once the pulse is transmitted by the radar a sufficient length of time must elapse before the next pulse to allow echoes from targets at the maximum range to be detected. Thus the maximum rate at which pulses can be transmitted is determined by the maximum range at which targets are expected. This rate is called the pulse repetition rate (PRF). W If the PRF is too high echo signals from some targets may arrive after the transmission of the next pulse. This leads to ambiguous range measurements. Such pulses are called second time around pulses. TU The range beyond which second time around pulses occur is called the maximum unambiguous range. RUNAMBIG = c/2f P Where fP is the PRF in Hz. JN More advanced signal waveforms then the above are often used, for example the carrier maybe frequency modulated (FM or chirp) or phase modulated (pseudorandom bi phase) too permit the echo signals to be compressed in time after reception. This achieves high range resolution without the need for short pulses and hence allows the use of the higher energy of longer pulses. This technique is called pulse compression. Also CW waveforms can be used by taking advantage of the Doppler shift to separate the received echo from the transmitted signal. Note: unmodulated CW waveforms do not permit the measurement of range. 14 Downloaded From JNTU World ( ) JNTU World What is done by Radar? ld Radar can see the objects in or day or night rain or shine cloud or clutter TU fog or frost W land or air earth or planets stationary or moving and JN Good or bad weather. In brief, Radar can see the objects hidden any where in the globe or planets except hidden behind good conductors. 15 Downloaded From JNTU World ( ) JNTU World INFORMATION GIVEN BY THE RADAR: or The position of the object The distance of objects from the location of radar The size of the object Whether the object is stationary or moving Velocity of the object Distinguish friendly and enemy aircrafts The images of scenes at long range in good and adverse weather conditions Target recognition Weather target is moving towards the radar or moving away The direction of movement of targets Classification of materials W ld Radar gives the following information: TU NATURE AND TYPES OF RADARS: JN The common types of radars are: Speed trap Radars Missile tracking Radars Early warning Radars 16 Downloaded From JNTU World ( ) JNTU World Airport control Radars Navigation Radars Ground mapping Radars Astronomy Radars Weather forecast Radars Gun fire control Radars Remote sensing Radars Tracking Radars Search Radars IFF (Identification Friend or Foe) TU W or ld Synthetic aperture Radars Missile control Radars MTI (Moving Target Indication) Radars JN Navy Radars Doppler Radars Mesosphere, Stratosphere and Troposphere (MST) Radars Over-The-Horizon (OTH) Radars Mono pulse Radars 17 Downloaded From JNTU World ( ) JNTU World Phased array Radars Instrumentation Radars Gun direction Radars Airborne weather Radars JN TU W or ld 18 Downloaded From JNTU World ( ) JNTU World PULSE CHARACTERISTICS OF RADAR SYSTEMS: ld There are different pulse characteristics and factors that govern them in a Radar system Carrier or Pulse width W Pulse Repetition Frequency (PRF) U n a m b i g u o u s Range TU NOTE: ECHO is a reflected EM wave from a target and it is received by a Radar receiver. CARRIER: The carrier is used in a Radar system is an RF(radio frequency) signal with microwave frequencies. JN Carrier is usually modulated to allow the system to capture the required data. In simple ranging Radars, the carrier will be pulse modulated but in continuous wave systems such Doppler radar modulation is not required. 19 Downloaded From JNTU World ( ) JNTU World In pulse modulation, the carrier is simply switched ON & OFF in synchronization. ld PULSE WIDTH: The pulse width of the transmitted signal determines the dead zone. When the Radar transmitter is active, the receiver input is blanked to avoid the damage of amplifiers. For example, a Radar echo will take approximately 10.8 µsec to return from 1 standard mile away target. W PRF = 1/PRT or PULSE REPETITION FREQUENCY (PRF): PRF is the number of pulses transmitted per second. PRF is equal to the reciprocal of pulse repetition time (PRT). It is measured in Hertz Pulse Interval Time or Pulse Reset Time (PRT) is the time interval between two pulses. It is expressed in milliseconds. TU Pulse Reset Time = Pulse Repetition Time – Pulse Width JN UNAMBIGUOUS RANGE: In simple systems, echoes from targets must be detected and processed before the next transmitter pulse is generated if range ambiguity is to be avoided. 20 Downloaded From JNTU World ( ) JNTU World W or ld Range ambiguity occurs when the time taken for an echo to return from a target is greater than the pulse repetition period (T). TU Echoes that arrive after the transmission of the next pulse are called as second-timearound echoes. The range beyond which targets appear as second-time-around echoes is called as the Maximum JN Unambiguous Range and is given by RUNAMBIG = c/2fP c = velocity of propagation Where, TP = fP 21 Downloaded From JNTU World ( ) JNTU World fP is the PRF(PULSE REPETITION FREQUENCY) in Hz ld TYPES OF BASIC RADARS: TU FM-CW W CW or Monostatic and Bistatic Pulsed radar JN Monostatic radar uses the same antenna for transmit and receive. Its typical geometry is shown in the below fig. 22 Downloaded From JNTU World ( ) JNTU World or ld W Bistatic radars use transmitting and receiving antennas placed in different locations. JN TU CW radars, in which the two antennas are used, are not considered to be bistatic radars as the distance between the antennas is not considerable. The bistatic radar geometry is shown in below fig. 23 Downloaded From JNTU World ( ) JNTU World JN TU W or ld 24 Downloaded From JNTU World ( ) JNTU World RADAR WAVE FORMS: ld The most common Radar waveform is a train of narrow, rectangular shape pulses modulating a sine-wave carrier. JN TU W or The figure shows a pulse waveform, which can be utilized by the typical Radar. 25 Downloaded From JNTU World ( ) JNTU World JN TU W or ld 26 Downloaded From JNTU World ( ) JNTU World From the given Radar waveform: Pulse Width τ = 1 µsec. or Pulse Repetition Period TP = 1 msec. ld Peak power pt = 1 Mwatt W A maximum unambiguous range of 150 km was provided by the PRF fP = 1000 Hz. TU RUNAMBIG = c/2fP ==> 150×103 = 3×108 / 2fP ==> fP = 1000 Hz. JN Then, the average power Pavg of a repetitive pulse train wave form is given by Pavg = pt τ/TP ==> Pavg = pt τ fP In this case, Pavg = 1 Kwatt For a Radar wave form, the ratio of the total time that the Radar is radiating to the total time it could have radiated is known as duty cycle. 27 Downloaded From JNTU World ( ) JNTU World Duty Cycle = τ/TP = τ fP = Pavg /pt or The energy of the pulse is given by, E = τ pt = 1 Joule. ld Duty Cycle = τ/TP = 0.001 The Radar waveform can be extended in space over a distance of 300 meters using a pulse width of 1 µsec. W i.e., Distance = c τ = 300 m. JN TU Half of the above distance (i.e. c τ/2) can be used to recognize the two equal targets which are being resolved in range. In this case, a separation of 150m between two equal size targets can be used to resolve them. 28 Downloaded From JNTU World ( ) JNTU World Name Symbol Units Typical values Transmitted h m Frequency Interpulse Period Peak Transmitted PRF T sec-1 Milli sec Power Average Power Received Power Pt Pavg Pr 1000 sec-1 1 milli sec MW kW mW 1 MW 1 kW (Pavg = Pt PRF) 10-6 mW TU W Pulse Duration Pulse Length Pulse Repetition 1000-12500 Mhz 3-10 cm 1 sec 150-300 m (h=c ld MHz, G hz cm sec or ft Frequency Wavelength JN The Radar Range Equation: The radar range equation relates the range of the radar to the characteristics of the transmitter, receiver, antenna, target and the environment. It is used as a tool to help in specifying radar subsystem specifications in the design phase of a program. If the transmitter delivers PT Watts into an isotropic antenna, then the power density (w/m2) at a distance R from the radar is 29 Downloaded From JNTU World ( ) JNTU World Pt/4πR2 ld Here the 4πR2 represents the surface area of the sphere at distance R JN TU W or Radars employ directional antennas to channel the radiated power Pt in a particular direction. The gain G of an antenna is the measure of the increased power radiated in the direction of the target, compared to the power that would have been radiated from an isotropic antenna 30 Downloaded From JNTU World ( ) JNTU World ∴ Power density from a directional antenna = PtG/4πR2 The target intercepts a portion of the incident power and redirects it in various directions. or Hence the Power density of the echo signals at the radar = ld The measure of the amount of incident power by the target and redirected back in the direction of the radar is called the cross section σ. as seen by the radar. W Note: the radar cross-section σ has the units of area. It can be thought of as the size of the target TU The receiving antenna effectively intercepts the power of the echo signal at the radar over a certain area called the effective area Ae. JN Since the power density (Watts/m2) is intercepted across an area Ae, the power delivered to the receiver is Pr = (PtGσAe) /(4πR2)2 ==> R4 = (PtGσAe) /(4π)2 Pr 31 Downloaded From JNTU World ( ) JNTU World R = [(PtGσAe) /(4π)2 Pr]1/4 or ld Now the maximum range Rmax is the distance beyond which the target cannot be detected due to insufficient received power Pr, the minimum power which the receiver can detect is called the minimum detectable signal Smin. Setting, Pr = Smin and rearranging the above equation gives These are related by: W Note here that we have both the antenna gain on transmit and its effective area on receive. JN TU As long as the radar uses the same antenna for transmission and reception we have 32 Downloaded From JNTU World ( ) JNTU World ld Example: Use the radar range equation to determine the required transmit power for the TRACS Now, W From or radar given: Prmin =10-13 Watts, G=2000, λ=0.23m, PRF=524, σ=2.0 m2 TU = 3.1 MW JN Note 1: these three forms of the equation for Rmax varywith different powers of λ. This results from implicit assumptions about the independence of G or Ae from λ. Note 2: the introduction of additional constraints (such as the requirement to scan a specific volume of space in a given time) can yield other λ dependence. 33 Downloaded From JNTU World ( ) JNTU World or RADAR BLOCK DIAGRAM AND OPERATION: ld Note 3: The observed maximum range is often much smaller than that predicted from the above equation due to the exclusion of factors such as rainfall attenuation, clutter, noise figure etc. W The Transmitter may be an oscillator (magnetron) that is pulsed on and off bya modulator to generate the pulse train. the magnetron is the most widely used oscillator TU typical power required to detect a target at 200 NM is MW peak power and several kW average power typical pulse lengths are several μs JN typical PRFs are several hundreds of pulses per second 34 Downloaded From JNTU World ( ) JNTU World W or ld TU The waveform travels to the antenna where it is radiated. The receiver must be protected from damage resulting from the high power of the transmitter. This is done by the duplexer. duplexer also channels the return echo signals to the receiver and not to the transmitter duplexer consists of 2 gas discharge tubes called the TR (transmit/receive) and the and an JN ATR (anti transmit/receive) cell The TR protects the receiver during transmission and the ATR directs the echo to the receiver during reception. solid state ferrite circulators and receiver protectors with gas plasma (radioactive keep alive) tubes are also used in duplexers 35 Downloaded From JNTU World ( ) JNTU World The receiver is usually a superheterodyne type. The LNA is not always desirable. Although it provides better sensitivity, it reduces the dy namic range of operation of the mix er. A receiver with just a mixer front end has greater dynamic range, is less susceptible to overload and is less vulnerable to electronic interference. ld The mixer and Local Oscillator (LO) convert the RF frequency to the IF frequency. JN TU W MHz. or The IF is typically 300MHz, 140Mz, 60 MHz, 30 MHz with bandwidths of 1 MHz to 10 36 Downloaded From JNTU World ( ) JNTU World The IF strip should be designed to give a matched filter output. This requires its H(f) to maximize the signal to noise power ratio at the output. This occurs if the |H(f)| (magnitude of the frequency response of the IF strip is equal to the signal spectrum of the echo signal |S(f)|, and the ARG(H(f)) (phase of the frequency response) is the negative of the ARG(S(f)). ld i.e. H(f) and S(f) should be complex conjugates or For radar with rectangular pulses, a conventional IF filter characteristic approximates a JN TU W matched filter if its bandwidth B and the pulse width τ satisfy the relationship 37 Downloaded From JNTU World ( ) JNTU World JN TU W or ld 38 Downloaded From JNTU World ( ) JNTU World The pulse modulation is extracted by the second detector and amplified by video amplifiers to levels at which they can be displayed (or A to D’d to a digital processor). The display is usually a CRT; timing signals are applied to the display to provide zero range information. Angle information is supplied from the pointing direction of the antenna. ld The most common type of CRT display is the plan position indicator (PPI) which maps the location of the target in azimuth and range in polar coordinates The PPI is intensitymodulated bythe amplitude of the receiver output and the CRT or electron beam sweeps outward from the centre corresponding to range. Also the beam rotates in angle in synchronization with the antenna pointing angle. W A B scope display uses rectangular coordinates to display range vs angle i.e. the x axis is angle and the y axis is range. Since both the PP...
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