RADAR AND RADAR AND NAVIGATIONAL AIDSNAVIGATIONAL AIDS

SPK.BABUSPK.BABUASST. PROFASST. PROF

PREC, VALLAMPREC, VALLAM

What is RADAR?What is RADAR? The word radar is an abbreviation for The word radar is an abbreviation for RARAdio dio DDetection etection AAnd nd

RRanginganging Radar is an Radar is an electromagnetic systemselectromagnetic systems used for used for detection detection

and location of objectsand location of objects such as aircraft, ship, vehicles, such as aircraft, ship, vehicles, people, natural environment etc.people, natural environment etc.

Radar systems use modulated waveforms and directive Radar systems use modulated waveforms and directive antennas to antennas to transmit electromagnetic energytransmit electromagnetic energy into a into a specific volume in space to search for targets.specific volume in space to search for targets.

Objects (targets) within a search volume will Objects (targets) within a search volume will reflect reflect portions of this energyportions of this energy (radar returns or echoes) back to (radar returns or echoes) back to the radar. the radar.

These echoesThese echoes are then processed (Afterwards they are are then processed (Afterwards they are called as called as video signalsvideo signals) by the radar receiver to extract ) by the radar receiver to extract target information such as target information such as range, velocity, angular range, velocity, angular position,position, and other target identifying characteristics. and other target identifying characteristics.

Radar PrinciplesRadar Principles TransmitterTransmitter generates and transmits generates and transmits

electromagnetic wave (sine or pulse).electromagnetic wave (sine or pulse). A portion of it is A portion of it is reflected backreflected back by the target by the target

(object you want to identify). (object you want to identify). The radiated portion is collected by the radar The radiated portion is collected by the radar

antenna and processed. antenna and processed. One antennaOne antenna can be used for both transmission can be used for both transmission

and reception.and reception.

RADAR BLOCK DIAGRAM AND OPERATION

transmitter -transmitter - oscillator, - magnetron, -" pulsed"-(turned on and on) oscillator, - magnetron, -" pulsed"-(turned on and on) by the modulator to generate a repetitive train of pulses.by the modulator to generate a repetitive train of pulses.

Example: Example: to detect aircraftto detect aircraft at ranges of 100 or 200 nmi you need at ranges of 100 or 200 nmi you need peak powerpeak power of the order of a megawatt, of the order of a megawatt, a pulse widtha pulse width of several microseconds, of several microseconds, pulse repetition frequencypulse repetition frequency of several hundred pulses per second. of several hundred pulses per second.

The waveform generated by the transmitter travels via a The waveform generated by the transmitter travels via a transmission linetransmission line to the antenna, where it is radiated into space. to the antenna, where it is radiated into space.

A single antennaA single antenna is generally used for both transmitting and is generally used for both transmitting and receiving. receiving.

DuplexerDuplexer The The duplexer protects the receiverduplexer protects the receiver from high from high

transmission power.transmission power. consist of consist of two gas-discharge devicestwo gas-discharge devices, one known , one known

as a TR (transmit-receive) and the other an ATRas a TR (transmit-receive) and the other an ATR(anti-transmit-receive). (anti-transmit-receive). The The TR protects the receiverTR protects the receiver during during transmission transmission

and the and the ATR directs the echo signalATR directs the echo signal to the to the receiver receiver during receptionduring reception..

Solid-state ferrite circulators and receiver Solid-state ferrite circulators and receiver protectorsprotectors with gas-plasma TR devices and/or with gas-plasma TR devices and/or diode limiters are also employed as duplexers.diode limiters are also employed as duplexers.

ReceiverReceiver The receiver is usually of the The receiver is usually of the superheterodyne type. superheterodyne type. The first stage The first stage low-noise RF amplifierlow-noise RF amplifier, such as a , such as a parametric parametric

amplifier or a low-noise transistor. amplifier or a low-noise transistor. The mixer and local oscillator (LO) convert the RF signal to an The mixer and local oscillator (LO) convert the RF signal to an

intermediate frequency(IF). intermediate frequency(IF). • A typical" IF amplifier for an air-surveillance radar might have a center A typical" IF amplifier for an air-surveillance radar might have a center

frequency of 30 or 60 MHz and a bandwidth of the order of one frequency of 30 or 60 MHz and a bandwidth of the order of one megahertz. megahertz.

The IF amplifier should be designed The IF amplifier should be designed as a matched filteras a matched filter - - frequency-response function - maximize thefrequency-response function - maximize the

sigtial-to-mean-noise-power ratio sigtial-to-mean-noise-power ratio This occurs when the This occurs when the

• magnitude magnitude of the of the frequency-response functionfrequency-response function = the = the magnitudemagnitude of the of the echo signal spectrumecho signal spectrum

• the the phase spectrumphase spectrum of the matched filter = of the matched filter = negative negative of the phase of the phase spectrum of the spectrum of the echo signalecho signal

In a radar whose signal waveform approximates a rectangular In a radar whose signal waveform approximates a rectangular pulse, the conventional IF filter bandpass characteristic pulse, the conventional IF filter bandpass characteristic approximates a matched filter when the product of the IF approximates a matched filter when the product of the IF bandwidth B and the pulse width r is of the order of unity, that is, bandwidth B and the pulse width r is of the order of unity, that is, Br=- 1.Br=- 1.

22ndnd Detector & Display Detector & Display Demodulates the pulse modulationDemodulates the pulse modulation Amplified by the video amplifierAmplified by the video amplifier Displayed on a CRT- This is a special Displayed on a CRT- This is a special

CRT called PPI-CRT called PPI-plan position indicator

DisplayDisplay The most common form of cathode-ray tube display is the The most common form of cathode-ray tube display is the

plan position indicator, or PPIplan position indicator, or PPI which which maps in polar maps in polar coordinatescoordinates the location of the target in azimuth and the location of the target in azimuth and range. range.

This is an This is an intensity-modulated displayintensity-modulated display in which the in which the amplitude of the receiver output modulates the electron-amplitude of the receiver output modulates the electron-beam intensity (z axis) as the electron beam is made to beam intensity (z axis) as the electron beam is made to sweep outward from the center of the tube.sweep outward from the center of the tube.

The beam rotates in angle in response to the antenna The beam rotates in angle in response to the antenna position.position.

B-scope displayB-scope display is similar to the PPI except that it utilizes is similar to the PPI except that it utilizes rectangularrectangular, rather than polar, coordinates to display , rather than polar, coordinates to display range vs. angle. range vs. angle.

Both the B-scope and the PPIBoth the B-scope and the PPI, being intensity modulated, , being intensity modulated, have have limited dynamic rangelimited dynamic range. .

A Scope-A Scope- which plots target .amplitude axis) vs. range which plots target .amplitude axis) vs. range axis), for some fixed direction. This is a deflection-axis), for some fixed direction. This is a deflection-modulated display. It is more suited for tracking-radar modulated display. It is more suited for tracking-radar application than for surveillance radar.application than for surveillance radar.

Radar FrequenciesRadar Frequencies

Applications of RadarApplications of Radar

Air Traffic controlAir Traffic control Aircraft Aircraft

navigationnavigation Ship safetyShip safety Remote SensingRemote Sensing Law enforcementLaw enforcement MilitaryMilitary

Range- Distance from you and the Range- Distance from you and the target! target!

The range of the object is found by The range of the object is found by the time the pulse takes to travel to the time the pulse takes to travel to and from the detected object and from the detected object

Maximum unambiguous rangeMaximum unambiguous range We send a train of pulses. We send a train of pulses. Not a single pulseNot a single pulse. A pulse is sent . A pulse is sent

and reflected back as echoand reflected back as echo We have We have to wait to get for the echoto wait to get for the echo before we send the before we send the

next onenext one If not the If not the echo for the first pulse we sent will become echo echo for the first pulse we sent will become echo

for the second one.for the second one. This is called This is called second time around pulsesecond time around pulse Due to thisDue to this the the target may look neartarget may look near (as you get the echo (as you get the echo

immediately for the send pulse: this echo was for the first immediately for the send pulse: this echo was for the first pulse!!!!)pulse!!!!)

So the pulse repetition frequency (PRF) is important and it So the pulse repetition frequency (PRF) is important and it determines the maximum unambiguous rangedetermines the maximum unambiguous range

RRunun = cTp/2 = c/ 2fp = cTp/2 = c/ 2fp

To increase the rangeTo increase the range Transmitted power is Transmitted power is increasedincreased Gain of the antenna should be Gain of the antenna should be highhigh Large antennaLarge antenna for reception for reception The receiver should The receiver should be sensitivebe sensitive to to

weak signals (should pick them)weak signals (should pick them) Range equationRange equation is an important is an important

derivation we are going to do to derivation we are going to do to calculate the range. calculate the range.

Radar EquationRadar Equation The radar equation relates the range of a radar to

the characteristics of the transmitter, receiver, antenna, target, and environment.

It is useful for determining the maximum distance from the radar to the target

it can serve both as a tool for understanding radar operation and as a basis for radar design.

However it cannot give the precise value. Why??• Statistical nature of noise and signalStatistical nature of noise and signal• Fluctuation & uncertainty of the targetFluctuation & uncertainty of the target• Propagation effect of the wavePropagation effect of the wave• LossesLosses

Are affecting the calculation We will consider each one separately.

The Radar Equation- The Radar Equation- Considerations of various Considerations of various

parametersparameters As I pointed out earlier there are many factors As I pointed out earlier there are many factors

which we have to consider in the range equation. which we have to consider in the range equation. They affect the calculation. They affect the calculation.

But we But we can model them and mathematicallycan model them and mathematically find find the model and the model and insert in the range equationinsert in the range equation..

Noise Noise is in the system and it is random. So we is in the system and it is random. So we have to have probability.have to have probability.

If affects the detection. So we have to If affects the detection. So we have to get SNRget SNR Pulse repetition frequencyPulse repetition frequency Cross section of the targetCross section of the target exposed to the wave exposed to the wave

we sendwe send

Detection of Signals in NoiseDetection of Signals in Noise ((MINIMUM DETECTABLE SIGNAL)

Noise affects the systemsNoise affects the systems The The weakest signal which can be weakest signal which can be

detected by radardetected by radar is called as is called as minimum detectable signal and it is minimum detectable signal and it is included in the radar equation.included in the radar equation.

> target is present> target is present

---------- received signal----- received signal----- Threshold-- Threshold--

< target is absent< target is absent

This is called threshold detectionThis is called threshold detection

Threshold detection: two problemsThreshold detection: two problems

false alarm:false alarm: if the threshold is set as low then the noise may be if the threshold is set as low then the noise may be detected as targetdetected as target

Missed detection:Missed detection: If the threshold is set high then real target will If the threshold is set high then real target will be missed. be missed. • However we use matched filter which will increase the SNR.However we use matched filter which will increase the SNR.• So we conclude that SNR is an important factor and more analysis is So we conclude that SNR is an important factor and more analysis is

needed.needed.

Receiver Noise and the Signal-to-Noise RatioReceiver Noise and the Signal-to-Noise Ratio My aim is to find My aim is to find SSminmin and and insert it in the radar equationinsert it in the radar equation Most receivers are super heterodyne.Most receivers are super heterodyne. The noise is mostly internal. It is thermal or Johnson noise. Its The noise is mostly internal. It is thermal or Johnson noise. Its

value is 1.38 X 10value is 1.38 X 10-23-23 J/deg. Noise figure is to be derived and J/deg. Noise figure is to be derived and defined belowdefined below

Probabilities of Detection Probabilities of Detection

and and Probability of False AlarmProbability of False Alarm

Probability Density FunctionsProbability Density Functions Below are some of the probability density functions we use in the Below are some of the probability density functions we use in the

radar.radar.

Examples of PDFs: a: Uniform b: Gaussian c: Raleigh voltage d: Examples of PDFs: a: Uniform b: Gaussian c: Raleigh voltage d: Exponential(Raleigh power)Exponential(Raleigh power)

False alarm timeFalse alarm time

Will false alarm affect our Will false alarm affect our detection?detection?

Integration of Radar PulsesIntegration of Radar Pulses This is the method of using This is the method of using more than one more than one

pulse in detectionpulse in detection Using Using n pulsesn pulses instead of instead of one pulseone pulse increases increases

the SNRthe SNR

Fitting the gain we got into radar Fitting the gain we got into radar equationequation

Note:Note: We have two We have two levels of detection one levels of detection one is called is called predetection predetection and other is and other is post post detection. detection.

This integration of This integration of pulses give pulses give more more efficiency in efficiency in predetection.predetection.

This is the efficiency we gainThis is the efficiency we gain

Previously the radar equation Previously the radar equation waswas

Now substituting we getNow substituting we get

Radar Cross Section of Targets Radar Cross Section of Targets

σ σ is the radar cross section.is the radar cross section. It is the It is the area coveredarea covered and it will and it will be proportional to the magnitude be proportional to the magnitude

wave reflectedwave reflected. So we can define this parameter in dB. So we can define this parameter in dB Since the wave is covering a volume/ area we can define this Since the wave is covering a volume/ area we can define this

parameter asparameter as

This power reflected can be found by solving Maxwell’s equation.This power reflected can be found by solving Maxwell’s equation. Radar wavelength and dimension of the targetRadar wavelength and dimension of the target are two important are two important

parameters which define thisparameters which define this• If the If the wavelength is larger than the dimensionwavelength is larger than the dimension of the target of the target Raleigh Raleigh

scattering occursscattering occurs. Example rain. Example rain• if the if the Wavelength is smaller than the targetWavelength is smaller than the target the the scattering lies in the scattering lies in the

optical regionoptical region. Example aircraft. Example aircraft If the wavelength and the object is of same dimension then it is in If the wavelength and the object is of same dimension then it is in

the the resonance regionresonance region

22 ||44/_

_rER

incidentpower

reflectedpower

Sphere:Sphere:

Here the σ is directly proportional to fHere the σ is directly proportional to f44 in resonance region in resonance region σ nearly equal in resonance regionσ nearly equal in resonance region σ very small. No scattering only a small bright spot is seen.σ very small. No scattering only a small bright spot is seen.

(if you take a photograph of a polished metal ball, you will (if you take a photograph of a polished metal ball, you will see only a small bright spot: not the ball)see only a small bright spot: not the ball)

At resonance it oscillates.Attains maximum at =1

a2

=1

A thin rodA thin rod

Here the reflected wave(scattered Here the reflected wave(scattered wave/echo) depends on the angle at wave/echo) depends on the angle at which the rod is viewedwhich the rod is viewed

Square flat plateSquare flat plate

Large Cone SphereLarge Cone Sphere

Complex targetsComplex targets Aircrafts, buildings, missilesAircrafts, buildings, missiles are some examples are some examples It It depends on the viewing angle and frequencydepends on the viewing angle and frequency

(wavelength)(wavelength) One complex target will have One complex target will have two or more surfacestwo or more surfaces which which

scatters the incoming waveformscatters the incoming waveform They can They can be each modeled as cone, sphere, flat or rod be each modeled as cone, sphere, flat or rod

shaped.shaped. So we can So we can calculate them individually and add them calculate them individually and add them

together.together. That is the reason we studied the above separatelyThat is the reason we studied the above separately We should also note that the phase of each signal reflected We should also note that the phase of each signal reflected

will be different and depend on the anglewill be different and depend on the angle Here we see two scatters showing the change in the Here we see two scatters showing the change in the

magnitude with respect to change in the viewing angle magnitude with respect to change in the viewing angle

An AircraftAn Aircraft The scattering shown as polar plot.The scattering shown as polar plot. We see more signals are reflected back when viewed along We see more signals are reflected back when viewed along

• Sideways:Sideways: Flat surface Flat surface• Nose and tail.Nose and tail. Cone and sphere Cone and sphere

Engine will Engine will modulate the echo signal.modulate the echo signal. The The fluctuations fluctuations in the reflected signal are in the reflected signal are lowlow when you use when you use

microwave frequencymicrowave frequency and and highhigh when you use when you use low frequency.low frequency.

A missileA missile

Radar cross Section Radar cross Section Fluctuations Fluctuations

The The fluctuationsfluctuations are due to the following are due to the following reasonsreasons• Change in Change in viewing angleviewing angle• Individual Individual scattersscatters• EchoEcho from each scatter will have different from each scatter will have different

amplitude and phaseamplitude and phase• MultipleMultiple reflections reflections• ShadowingShadowing one scatter by another one scatter by another• Phase shiftPhase shift of more than 2*pi causes amplitude of more than 2*pi causes amplitude

changeschanges Take more scansTake more scans and see the results. and see the results.

This is one of the solutions.This is one of the solutions.

Four Swirling modelFour Swirling model • To study the fluctuations in detail To study the fluctuations in detail Four Swirling modelFour Swirling model is is

usedused Case 1:Case 1:

• Each scan give constant amplitude but different correlationEach scan give constant amplitude but different correlation• This is called slow fluctuations (Scan to Scan fluctuations)This is called slow fluctuations (Scan to Scan fluctuations)• Raleigh scatters belong to this categoryRaleigh scatters belong to this category

Case 2Case 2• Each pulse give different outputEach pulse give different output• Each of them independentEach of them independent

Case 3:Case 3:• Same as case 1 but different pdfSame as case 1 but different pdf

Case 4:Case 4:• Fluctuations is pulse to pulse and same pdf as case 3Fluctuations is pulse to pulse and same pdf as case 3• The fluctuation affect the probability of detectionThe fluctuation affect the probability of detection• Fluctuations are usually small.Fluctuations are usually small.

We see the four cases using a diagram and we see that We see the four cases using a diagram and we see that case 1 will create more problemscase 1 will create more problems

Inference from all the modelsInference from all the models The probability-density function assumed in cases 1 and 2 applies

to a complex target consisting of arbitrary independent scatterers of approximately equal echoing areas.

Although, in theory, the number of independent scatterers must be essentially infinite, in practice the number may be as few as four or five.

The probability-density function assumed in cases 3 and 4 is more indicative of targets that can be represented as one large reflector together with other small reflectors.

In all the above cases, the value of cross section to be substituted in the radar equation is the average cross section σ

The signal-to-noise ratio needed to achieve a specified probability of detection without exceeding a specified false-alarm probability can be calculated for each model of target behavior.

For purposes of comparison, the no fluctuating cross section will he called case 5.

A comparisonA comparison

Additional SNR required for a Additional SNR required for a particular performanceparticular performance

PULSE REPETITION FREQUENCY AND RANGE AMBIGUITIES

Suppose you are trying to Suppose you are trying to detect the object A.detect the object A. It is in unambiguous rangeIt is in unambiguous range. So we get the echo after the pulse . So we get the echo after the pulse

repetition.repetition. If suppose If suppose there are other two objects B and Cthere are other two objects B and C which which we are not we are not

interestedinterested, at distance above the unambiguous and twice., at distance above the unambiguous and twice. The The echoes from these objectsechoes from these objects will also be seen on the A scope. will also be seen on the A scope. Since Since we are integrating the pulseswe are integrating the pulses, we see pulses in position P1, , we see pulses in position P1,

P2, P3 respectively.P2, P3 respectively.

SolutionSolution One method of distinguishing multiple-time-

around echoes from unambiguous echoes is to operate with a varying pulse repetition frequency.

The echo signal from an unambiguous range target will appear at the same place on the A-scope on each sweep no matter whether the prf is modulated or not.

However, echoes from multiple-time-around targets will be spread over a finite range

SYSTEM LOSSES losses that occur throughout the radar system. The losses reduce the signal-to-noise ratio at the

receiver output. They may be of two kinds, Depending upon

whether or not they can be predicted The antenna beam-shape loss, collapsing loss,

and losses in the microwave plumbing are examples of losses which can be calculated.

Losses not readily subject to calculation and which

are less predictable include 'those due to field degradation and to operator fatigue or lack of operator motivation.

Plumbing loss. i. Transmission line losses

• For low frequency the losses is high• For high frequency the losses is high• So keep the distance between antenna and receiver

short as possible• The loss is taken twice as the signal is transmitted and

received ii. Duplexer loss

• It is a gas filled section used to protect the receiver form the transmitter.

• The transmitter uses very high power and receiver we get only a small from from the reflected

• So we use a waveguide shutter which produces loss it can be upto 2dB

2. Antenna losses2. Antenna losses

Signal processing loss Signal processing loss

Other losses-Other losses-

Equipment degradation Equipment degradation Field degradation. Operator loss

Propagation effectPropagation effect

The radar environment The radar environment includes terrain and sea surfaces, includes terrain and sea surfaces, the atmosphere (including precipitation), and the the atmosphere (including precipitation), and the ionosphere.ionosphere. These may degrade radar observations and These may degrade radar observations and performance by producing performance by producing clutter and other spuriousclutter and other spurious returns, signal attenuation, and bending of the radar-signal returns, signal attenuation, and bending of the radar-signal path.path.

• • Terrain and sea surfaces,Terrain and sea surfaces, which may produce target which may produce target masking, radar clutter, and multipath interference;masking, radar clutter, and multipath interference;

• • Precipitation,Precipitation, principally rain, which may produce signal principally rain, which may produce signal attenuation and clutter returns;attenuation and clutter returns;

• • The troposphere,The troposphere, which may produce refraction that which may produce refraction that bends the radar signal path, signal attenuation, and a lens bends the radar signal path, signal attenuation, and a lens loss;loss;

• • The ionosphere,The ionosphere, which may produce refraction that bends which may produce refraction that bends the radar signal path, signal fluctuation and attenuation, the radar signal path, signal fluctuation and attenuation, waveform dispersion, and rotation of signal polarization.waveform dispersion, and rotation of signal polarization.