2. Radar Basics_2011

Radar Basics

Radar

•  Detects and tracks objects of interest"•  Measures the location of objects remotely in range

and angle"•  Generates imagery of the ground (and other

objects)"•  Able to provide very wide coverage (e.g. 3600,

global)"•  Operates day and night and in all weathers"•  Can provide information about object type and

intent"

Schematic Diagram of a Simple Radar System

“I’m Listening”

Wavelength and Frequency

Phase and Superposition

Intensity of spherical waves

Polarization

For a wave travelling in the Z direction the electric field must lie in the x-y plane as defined by Ex and Ey ."If Ex =0, or Ey =0 then the polarization is linear. "If Ex=Ey and they oscillate in phase, the field will oscillate linearly along a 450 line in the x-y plane."If Ex=Ey and are npi/2 put of phase the polarization is circular and if Ex is not equal to Ey the polarization is elliptical."

Diffraction

Beams

Refraction and Reflection

Radar Types

Primary Air Traffic Control Radar at Heathrow Airport , London, UK

Air Traffic Control Radar (ACTR) ALE 3X5 Maccarese, Italy

Courtesy of AMS A. Farina, June 2003

Civil marine band radar used "by shipping throughout the world. "All commercial craft are legally obliged to carry such radar systems"

Civil Marine Radar

Martello – UK Air Long Range Defence Radar System

US E3-D Sentry Airborne Early Warning Radar System

Captor Radar for TYPHOON Aircraft Airborne Intercept Radar

Courtesy of BAE SYSTEMS"

BYSON – UK Experimental Radar

AEGIS AN-SPY1 Air Defence

MESAR-1 Experimental Phased Array Naval Air Defence

Early Bistatic Radar Experiments at UCL

Griffiths, H.D. and Carter, S.M., ʻProvision of moving target indication in an independent bistatic radar receiverʼ; The Radio and Electronic Engineer, Vol.54, No.7/8, pp336-342, July/August 1984 ."

94 GHz SAR Experimental System sometimes used in Missile Seekers

ERS-1 Earth Resources Satellite

SEASAT Shuttle SAR Image

Terra SAR - X

Airborne SAR Image of the Penatgon

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

Stealth aircraft

Typical Applications

HF VHF and UHF L-band S-band C-band X-band Ku- and Ka-band V- , W- and mm-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 with lower resolution and accuracy. More useful over the oceans."Long range, line-of-sight surveillance with low to medium resolution and accuracy and freedom from weather effects."Long-range surveillance with medium resolution and slight weather effects (200 nmi)"Short-range surveillance (60 nmi), long-range tracking with medium accuracy. Subject to moderate weather effects in heavy rain or snow."Short-range surveillance, long-range tracking with high accuracy. 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 of clouds."

Radar Frequency Bands

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 Ku

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

K Ka RADAR BANDS

ECM BANDS

Atmospheric Attenuation – Weather

Atmospheric attenuation – clear air

mm-wave sub mm-wave Infra-red Vis. UV

micro-wave

Extra Attenuation – Foul Weather

99.8%

Atte

nuat

ion

/ dB

per

km

Tran

smis

sion

ove

r 1km

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

1

10

100

1000

0.1

0.01

10%

80%

98%

10 % -8

10 %

mm-wave

sub mm-wave

Infra-red

Vis. UV micro-wave

94GHz

35GHz

Fog (50m vis)

Heavy rain (25 mm/hr)

Drizzle (0.25 mm/hr)

-100

Courtesy of AMS A. 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

Block Diagram of a Typical Radar

RF AMPLIFIER

MODULATOR POWER SUPPLY

DIPLEXER AND FILTER

EXCITER

RF PREAMPLIFIER

AND MIXER

IF AMPLIFIER AND

QUADRATURE DETECTOR

ADC and SIGNAL

PROCESSING

DATA PROCESSING AND DISPLAY

SYNCHRONISER

ANTENNA

ANTENNA

RECEIVER

TRANSMITTER

MICROWAVE EXCITER AND SYNCHRONISER

Radar Ranging

point !target!

r

radar!

pulse !

Two-way propagation delay"""hence "

2 rtc

=

2ctr =

Radar range is sometimes quoted in nautical miles (1 nmi = 1.85 km), and velocity in knots (1 kt = 1 nmi/hr) "

Range Resolution

point !targets!

radar!

pulse"duration = τ

r + Δr

r

received energy

time"

echo"duration = τ

The separation in time between the echoes from the targets is""""" "Set this = t, then""The radar echo is usually processed in"samples of range extent equal to the range"resolution, and known as range bins."

( )2 2 2 r r r rc c c+Δ Δ

− =

2cr τ

Δ =

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/t""""So we can write the range resolution as "

2Δ =

crB

0( ) cos 2 2

f t A t tτ τω= − ≤ ≤

( ) ( )( )

0

0

sin 22 2

Fω ω ττ

ωω ω τ

⎛ ⎞+= ⎜ ⎟⎜ ⎟+⎝ ⎠

Angular Resolution

(radians)B dλ

θ ≈

r antenna"

length = d"

azimuth resolution

Br rdλ

θ= ≈

Pulse Repetition Frequency

time

Pulses are usually transmitted at a regular interval, known as the"pulse repetition interval (PRI)"""""""""""The reciprocal of the PRI is the pulse repetition frequency (PRF)"

PRI

1PRF PRI

=

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 the original pulse.""The maximum unambiguous range is therefore or""

PRI

. PRI2

c2. PRFc

Maximum Unambiguous Range

Scanning radar

Many (but not all) radars scan in azimuth.""If the scan period is T, then the time (dwell) for which the beam illuminates a given target is :"""""Hence the number of pulses illuminating the target is :""""""The rotation rate of the antenna (in r.p.m.) is"

Bo . θ

θB(with in degrees)360

T

Bo . .

360θ PRFT

60T