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3. Radar Basics - 2

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ERS-1 Earth Resources Satellite

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Terra SAR - X

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SEASAT Shuttle SAR Image

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Airborne SAR Image of thePenatgon

source: Sandia Corporation -http//www.sandia.gov

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Stealth aircraft

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Courtesy of AMSA. Farina, June 2003

RAN 20 S, radar electronic cabinet including: a) RF Generator (double

conversion); RF-IF receiver; b) Signal processor (target and weather); c) Data

processor (target and weather);

Radar Equipment Rack

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Block Diagram of a Typical Radar

RFAMPLIFIER

MODULATOR POWERSUPPLY

DIPLEXERAND FILTER

EXCITER

RFPREAMPLIFIER

AND MIXER

IF AMPLIFIERAND

QUADRATUREDETECTOR

ADC andSIGNAL

PROCESSING

DATAPROCESSINGAND DISPLAY

SYNCHRONISER

ANTENNA

ANTENNA

RECEIVER

TRANSMITTER

MICROWAVE EXCITER ANDSYNCHRONISER

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Typical Applications

HF

VHF and UHF

L-band

S-band

C-band

X-band

Ku- and Ka-

band

V- , W- andmm-wave

bands

3 - 30 MHz

30 - 1000 MHz

1 - 2 GHz

2 - 4 GHz

4 - 8 GHz

8 - 12 GHz

12 - 40 GHz

40 - 100+ GHz

Over-the-horizon radar, combining very long range withlower resolution and accuracy. More useful over theoceans.Long range, line-of-sight surveillance with low to mediumresolution and accuracy and freedom from weather effects.Long-range surveillance with medium resolution and slightweather effects (400 km)Short-range surveillance (120 km), long-range tracking

with medium accuracy. Subject to moderate weathereffects in heavy rain or snow.Short-range surveillance, long-range tracking with highaccuracy. Subject to increased weather effects in light to

medium rain.Short-range surveillance in clear weather or light rain;

long-range tracking with high accuracy in clear weather,

reduced to short range in rain.

Short-range tracking, real and synthetic aperture imaging,

especially when antenna size is limited and when all-weather operation is not required or ranges are short.Limited to short ranges in a relatively clear atmosphere,very short ranges in rain. Generally for tracking and smart

seekers with very small antennas. Remote sensing ofclouds.

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Radar Frequency Bands

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Radar Letter Band Designations

frequency (GHz)

wavelength (cm)

0.125 0.25 0.5 1 2 4 8 16 32 64 128

240 120 60 30 15 7.5 3.75 1.87 0.94 0.47 0.23

L S C X K u

A B C D E G I J K MF H L

K KaRADAR BAND

ECM BANDS

A h i A i

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Atmospheric Attenuation Weather

h i i l

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Atmospheric attenuation cleaair

mm-wavesub mm-

wave Infra-red Vis. UVmicro-wave

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Extra Attenuation Foul Weather

99.8%

A

ttenuation/dBp

erkm

Transmissionover1km

10GHz 100GHz 1THz 10THz 100THz 1000THz

1

10

100

1000

0.1

0.01

10%

80%

98%

10 %-8

10 %

mm-wave

submm-wave

Infra-red

Vis. UVmicro-wave

94GHz

35GHz

Fog

(50m vis)

Heavy rain

(25 mm/hr)

Drizzle

(0.25 mm/hr)

-100

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Radar Ranging

pointtarget

rradar

pulse

Two-way propagation delay

hence

2

rt

c

=

2

ctr =

Radar range is sometimes quoted in nautical miles (1 nmi = 1.85 km), and

velocity in knots (1 kt = 1 nmi/hr)

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Range Resolution

pointtargets

radarpulse

duration =!

r + "r

r

receivedenergy

time

echoduration = !

The separation in time between the echoes from the targets is

Set this = , thenThe radar echo is usually processed insamples of range extent equal to the rangeresolution, and known as range bins.

( )2 2 2

r r r r

c c c

+! !" =

2

c

r

!

" =!

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Range Resolution (ii)

The spectrum of a rectangular pulse of length t

is a sinc function centered on w0

whose bandwidth (at -3.9 dB) is B= 1/

So we can write the range resolution as2

! =c

rB

0( ) cos2 2

f t A t t! !

"= # $ $

( ) ( )

( )0

0

sin 2

2 2F

! ! ""

!

! ! "

# $+= % &% &

+' (

!

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Angular Resolution

(radians)B

d

!" #

r

antennalength = d

azimuth resolution

B

r rd

!"= #

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Pulse Repetition Frequency

time

Pulses are usually transmitted at a regular interval, known as thepulse repetition interval(PRI)

The reciprocal of the PRI is the pulse repetition frequency(PRF)PRI

1PRF

PRI=

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Maximum Unambiguous Range

time

Associated with a given PRF is a maximum unambiguous range,since any echo which arrives after the instant of transmission of

the next pulse cannot unambiguously be associated with theoriginal pulse.The maximum unambiguous range is therefore or

PRI

. PRI

2

c

2. PRF

c

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Maximum Unambiguous Range

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Scanning radar

Many (but not all) radars scan in azimuth.If the scan period is T, then the time (dwell) for which the beamilluminates a given target is :

Hence the number of pulses illuminating the target is :

The rotation rate of the antenna (in r.p.m.) is

B

o

.!

!B(with in degrees)360

T

B

o

. .

360

!PRFT

60

T