Radar Systems for NTU, 1 Nov 2014

RADAR SYSTEMSfor

NANYANG TECHNOLOGICAL UNIVERSITY

LEE Kar Heng, Ph.DTBSS-Scilab Singapore Center

A TBSS Group Company

Introduction

• RADAR- Radio Detection and Ranging

• Theory of reflection, absorption and scattering

• Higher the frequency better the result

• Location parameters: Range, height, direction, direction of motion, relative velocity

• Maritime, Aviation and Land navigational aids• Height measurement (radar altimeter)• Instrument landing (in poor visibility)• Space applications (planetary observations)• Radars for determining speed of moving targets

(Police radars Law enforcement and Highway safety)

• Remote sensing (weather monitoring)• Air traffic control (ATC) and aircraft safety• Vessel traffic safety

Applications

Military

• Detection and ranging of targets in all weathers

• Weapon control – aiming guns at target

• Early warning on approaching aircrafts or ships

• Direct guided missiles

• Search submarines, land masses and buoys

Applications

• Target distance is calculated from the total time(tdelay) taken by the pulse to travel to the targetand back

• c = 3 x 108 m/s, speed of light

Radar Ranging

A Monostatic Radar

Scan Pattern

Generator

Antenna Duplexer

Waveform

GeneratorTransmitter

ReceiverSignal

Processor

Data

Extractor

Data

Processor

Radar

Display

Radar Block Diagram

TX RX

Radar Display

• Antenna is highly directive with large gain

• Duplexer switches automatically

• Tx remains silent during Rx period

• Tx pulse is high power, short duration

• Rx has sensitivity to receive weak echo signals and is be highly immune to noise

Radar Block Diagram

Band Designation ITU Nominal

Frequency Range

Specific radar bands based

on ITU assignment

HF 3 – 30 MHz

VHF 30 – 300 MHz 138-144, 216-225 MHz

UHF 300 – 1000 MHz 420-450, 590-942 MHz

L 1 – 2 GHz 1215-1400 MHz

S 2 – 4 GHz 2300-2500, 2700-3700MHz

C 4 – 8 GHz 5250-5925 MHz

X 8 – 12 GHz 8500-10680 MHz

Ku 1 2– 18 GHz 13.4-14, 15.7-17.7 GHz

K 18 – 27 GHz 24.05-24.25 GHz

Ka 27 – 40 GHz 33.4-36 GHz

Radar Frequency Band Designations

• Tx transmits a train of narrow rectangularshaped pulses modulating a sine wave carrier

• The range to the target is determined bymeasuring the time taken by the pulse totravel to the target and return to the radar

Pulsed Radar

Average Power

• In each cycle (Pulse Repetition Time), the radar only radiates from t sec

• The average transmitted power is

where Pt = peak transmitted power and

PRF = Pulse Repetition Frequency,

. .av t tP P P PRFPRT

tt

1PRF

PRT

Range Ambiguity

• The range that corresponds to the 2-way time delay is the radar unambiguous range, Ru

• Consider detection of 2 targets,

Transmitted

Pulses

Received

Pulses

Pulse 1 Pulse 2

echo 1 echo 2

PRT

t

tdelay

tdelay

Ru

(R )1

R2

Range Ambiguity

• Echo 1 is the return from target at range R1,

• Echo 2 is the return from the same target at range R1, from the 2nd transmission

• Echo 2 can also be taken as an echo from a different target from the 1st transmission

12

delayctR

2 12

delayctR R

2

2

delayc PRT tR

ERROR!

Range Ambiguity

• The maximum unambiguous range is

,max2 2

u

cPRT cR

PRF

Range Resolution

• Range resolution, R, is the radar ability to detect targets in close proximity as 2 distinct targets

• 2 close proximity targets must be separated by at least R to be completely resolved in range

• Consider 2 targets located at ranges R1 and R2, corresponding to time delays t1 and t2respectively, the difference between the 2 ranges is

2 1 2 12 2

c cR R R t t t

Range Resolution

• To distinguish the 2 targets, they must be separated by at least 1 pulse width t,

where B = radar bandwidth2 2

c cR

B

t

Received

Pulses

return

target 1

c

return

target 2

t ct

target 1 target 2

ct

R R1 2

• The number of wavelengths contained in the two way path between the radar and the target,

• Total phase shift,

• Radar is able to give radial velocity of a moving target from Doppler Effect

• Doppler effect causes a shift in frequency of the received echo signal from a moving target

• Doppler frequency shift

• Let R be the Range of the target

Doppler Effect

RADAR TOWER

INBOUND

ECHO

RADAR

ANTENNA

TRANSMIT

PULSE

OUTBOUND

ECHO

2Rn

42

Rn

• When the target is moving, R and φ change continuously

• The rate of change of φ is angular frequency

where vr = radial velocity of the target towards the radar

• The Doppler frequency shift,

where

Doppler Effect

442 r

d d

vd dRf

dt dt

22 r ord

v fvf

c

cosrv v

Pulse Repetitive Frequency

• For a single pulse, the maximum unambiguous range, Ru,max, is determined by the PRF,

• High PRF is unambiguous in Doppler but highly ambiguous in Range since it meets the Nyquistsampling criteria for Doppler shift of all targets design to detect but there is little time between pulses for ranging

,max2 u

cPRF

R


• Medium PRF radar may be ambiguous in both Doppler and range since it samples too fast for echoes from long range but too slow to Nyquistsample the Doppler shift of all targets

• Medium PRF however has the best of both worlds, a compromise performance between unambiguity in Doppler and range

• Low PRF is unambiguous in range but high ambiguous in Doppler since it waits until the last transmitted pulse arrives before the next transmission


• Different PRF is used in different application such as Low PRF or Medium PRF is suitable for detection of vessels while High PRF is used in detecting air targets

• Careful selection of PRF will also result in better tracking performance since one must be able to detect a target before tracking it

• The radar range equation (or The Range Equation or The Radar Equation) provides an indication on the ability of the radar to detect the presence of a target

• It is used in the radar system design

• Radar range equation relates the target range to the characteristics of the Tx, Rx, antenna, target and the environment

• The equation is based on the Friis Transmission equation

Radar Range Equation


• Radiation from an isotropic radiation source


• Consider a radar with isotropic radiation, i.e., radiation in all directions as from the surface of a minute sphere, the power density per unit area is given by

where Pisotropic = radiation power density of the

isotropic radiation [W/m2]

Pt = peak power in the transmitted pulse

R = distance from the transmitter to the target

4R2 = surface area of an imaginary sphere (the isotropic isolator) with radius R

24

tisotropic

PP

R


• Radiation from a directional radiation source


• For a directional radar, the power density per unit area is given by

where Pdirectional = radiation power density of the

isotropic radiation [W/m2]

Gt = directional gain of the antenna measured in the target direction

24

t tdirectional

PGP

R


• A target at distance R intercepts the transmitted energy, part of the energy will be reflected by the target

• The re-radiated power due to the reflection

where s = target radar cross section, the target EM size viewed by the radar

, arg 24

t tt t et directional

PGP P

Rs s


• The reflected power from the target received by the radar,

, arg

2

22

4

4

t t et

reflected

t t

PP

R

PG

R

s

RADAR

ANTENNA

Rs

ISOTROPIC

RADIATION

DIRECTIONAL

RADIATION

2

tisotropic

R4

PP

2

ttldirectiona

R4

GPP

2

ttetargt,t

R4

GPP

s

22

ttreflected

R4

GPP

s

G, Ae


• The power received by the radar by the antenna depends on the effective aperture,

but

hence, the radar equation,

• Since same antenna is used for receiving and transmitting,

2

24

t t er reflected e

PG AP P A

Rs

2

4

re

GA

2

3 44

t t rr

PG GP

R

s

2 2

3 44

tr

PGP

R

s

The Complete Radar Equation

• The simple form of the radar range equation is useful in 1st order calculations

• For more accurate and realistic calculations, the following effects must be considered– Propagation medium and path

– Atmospheric noise

– System losses

– Thermal noise introduced within the radar

– Signal processing losses

– Other losses associated with particular configurations and applications


• A realistic operational scenario includes propagation medium and environment

• A loss factor, L, accounts for all system, medium and propagation losses


• For a radar with system temperature 290K, the system noise

where N and S indicate the noise and signal power levels and subscripts i and o represent the antenna input and receiver output

• The equivalent thermal noise,

where k = Boltzman’s constant,

T = temperature, B = bandwidth

i i i o in

o o o o i

SNR S N N NF

SNR S N S S

iN kTB


• Overall signal losses include internal loss, LI and external loss, LE

• The effective input signal power,

• The output signal power

where A = radar received power gain and

L = LILE, overall loss factor

ri

E

PS

L

i r ro

I I E

S P PS A A A

L L L L


• The output noise power,

since o n iN AF N

iN kTB

o nN AF kTB


• The signal to noise ratio at output,

since

the complete radar equation

o r ro

o n n

S AP L PSNR

N AF kTB F kTBL

2 2

3 44

tr

PGP

R

s

2 2

3 44

to

n

PGSNR

R F kTBL

s

• Factors affecting detection range of a radar

– Transmitter power, PT

– Frequency, fo,

– Target radar cross section, s

– Minimum received signal power, Pr

– Antenna, G

• The difference between the signal and noise power levels determine the detection performance

• A good receiver with low Fn is necessary


Radar Design Considerations

DESIGN PARAMETERS DESIGN CONSIDERATIONS

Average Power (Pt) Peak transmit power

Allowable duty cycle

Antenna Gain (G) Aperture size

Beamwidth

Wavelength () Operating frequency

Aperture size

System Noise Temperature

(T)

Low noise figure

Signal processing gain

System Losses (L)

Ohmic losses

Signal processing losses

Atmospheric loss and clutter

Maximum Detection Range

• The maximum detection range of a target with a specified radar cross section, s, is

• The maximum range is obtained when the signal-to-noise ratio of a target is minimum,

1

42 2

34

t

n o

PGR

F kTBL SNR

s

1

42 2

max 3

,min4

t

n o

PGR

F kTBL SNR

s

Radar Cross Section

• Radar Cross Section (RCS) describes the amount of scattered power from a target towards the radar, when the target is illuminated by the RF energy

• EM waves, with any specified polarization, are normally diffracted or scattered in all directions when incident wave hit on a target

• The intensity of the backscattered energy that has the same polarization as the radar’s receiving antenna is used to define the target RCS

Radar Cross Section

• The target’s RCS fluctuates as a function of radar aspect angle

• 2 unity scatters of 1 m2 are aligned and placed along the radar line of sight contributing to the zero aspect angle at a range R

• The composite RCS is consisted of the superposition of the two individual radar cross sections which is 2 m2

• When the aspect angle varies, the composite RCS is modified by the phase between the 2 scatters

1m

Radar line of sight

Radar line of sight

1m

Radar line of sight

Radar line of sight

Radar Cross Section

• The radiation field of an antenna is composed of electric and magnetic lines of force

• These lines of force are always right angles to each other

• The electric field determines the direction of polarization of the wave

• When a single-wire antenna is used to extract energy from a passing radio wave, maximum pickup will be resulted when the antenna is oriented in the same direction as the electric field

Radar Cross Section

• 3 types of polarizations:

• When polarization changes, target RCS changes and it affects the detection performance

Vertical Horizontal Circular

Radar Cross Section

• The maximum detection ranges of the 3 targets with different RCS when a fixed SNR is used

• It can be concluded that RCS plays a very important role to determine the maximum range detected

SNR (dB) Range (km)

136.1 (Default RCS)

12 76.52 (RCS - delta1)

181.5 (RCS + delta2)

85.86 (Default RCS)

20 48.28 (RCS - delta1)

114.5 (RCS + delta2)

Radar Cross Section

• Maximum detection range against SNR plot with different RCS

Probability of Detection

• The probability of detecting a target is affected by its randomness

• The probability of detection, PD, defines the probability of detection a given target at a given range when the antenna beam passes the target

• PD is defined as the probability that a sample of the signal exceeded the threshold when noise plus signal are present in the radar


• Mathematically,

where erfc = complimentary error function 0.5 0.5D faP erfc InP SNR


• Simulation resulting showing plots of the probability of detection, PD, versus the single pulse SNR, with the probability of false alarm, Pfa


• From the plot, it can be concluded that the amount of SNR needed to achieve a fixed amount of probability of detection is greater when the probability of false alarm gets smaller

Target Types

• Target fluctuates when a radar system detecting a fluctuating target indicates that the probability of detection will decrease, or the SNR will reduce due to the RCS of the fluctuating target varies irregularly

• Fluctuating targets can be categorized into for different Swerling types

Target Types

• Swerling types:

• Probability of detection for different types of Swerling targets:

– Type 1

– Type 2

– Type 3

– Type 4

Target Types

1

111 , 1 1 , 1

11

p

T p

n

V n SNRT

D I T p I p

p

p

VP V n n e

n SNR

n SNR

1 ,1

TD I p

VP n

SNR

1

01 , 11 2 2 !

p Tn V

TD I T p

p p

V eP V n K

n SNR n

2

0 1 2

11 .... 1

2 2 2! 2 2

p p

p

n np p

D p n

n nSNR SNR SNR SNRP n

Target Types

• PD for different Swerling targets with different number of pulses

Swerling I Swerling II

Swerling III Swerling IV

Target Types

• The plot shows PD of all the Swerling targets when the SNR changes

• Swerling target type 4 give the largest PD when the SNR is larger than 4 dB

Target Types

• In general, the Swerling 2 and 4 target types give the largest for a given SNR

• Swerling 1 target gives the lowest and Swerling3 is somewhere between the other three

• The target RCs for Swerling 1 and 2 fluctuate considerably, thus both noise and RCS fluctuation affects the threshold crossing

• Swerling 3 and 4 consist of predominant scatter, therefore, the threshold crossing for these target types are affected by RCS fluctuation, but not to the extent of Swerling 1 and 2 targets

Radar Systems for NTU, 1 Nov 2014

Engineering

Transcript of Radar Systems for NTU, 1 Nov 2014