RADAR AND SYNTHETIC APERTURE RADAR SYSTEM

JITENDER KUMAR

07-ECE-236

OVERVIEW

• PRINCIPLES OF IMAGING RADAR

• RADAR INTERFEROMETRY FOR HEIGHT MAPPING

• SIMULTANEOUS ACQUISITION

• REPEAT TRACK

• DIFFERENTIAL INTERFEROMETRY FOR CHANGE DETECTION

PRINCIPLES OF RADARHOW DOES RADAR WORK?

TRANSMITTER

RECEIVER

CIRCULATOR

RADAR PULSE

"TARGET"

• RADAR = Radio Detection And Ranging

• Since radar pulses propagate at the speed of light, the difference to the “target” is proportional to the time it takes between the transmit event and reception of the radar echo

PRINCIPLES OF IMAGING RADARTHE RADAR EQUATION

• In order to improve the signal-to-noise ratio for a fixed radar frequency, one has (among others) the following options:

– Increase the transmitted power. This is usually limited by the power available from the spacecraft or aircraft.

– Increase the antenna gain. This requires larger antennas, severely affecting the launch mass and volume.

– Increase the pulse length. This means poorer resolution.

– Decrease bandwidth. This also means poorer resolution.

– Fly lower. Increases atmospheric drag, requiring more fuel for orbit maintenance.

• Signal modulation is a way to increase the radar pulse length without decreasing the radar range resolution

– All civilian spaceborne SARs, and most civilian airborne SARs use linear FM chirps as the modulation scheme.

PRINCIPLES OF RADAR IMAGINGSYNTHETIC APERTURE RADAR

Because the radar is moving relative to the target,the received signal will be shifted in frequencyrelative to the transmitted frequency by an amount

Targets ahead of the radar will have positiveDoppler shifts, and those behind the radar havenegative Doppler shifts.

• Range Resolution:

• Azimuth Resolution

• Swath Width:

f d 2v

sin

Xr c

2BW sin

Xa v

2 f DM

L

2

S h

W cos 2

fd

+fDM

-fDM

time

TARGET

L

v

BOTH RANGE AND AZIMUTH RESOLUTIONS ARE INDEPENDENT OF DISTANCE TO TARGET!

PRINCIPLES OF RADAR IMAGING POINT TARGET RESPONSE

• The radar system transmits a series of chirp pulses:

• The target will be in view of the radar antenna for a limited time period. During this period, the distance to the target is

• Usually, so that

W t A t exp i2 fct Bt2 2 A t

1 for nT 2 t nT 2

0 otherwise

r t r02 v2t 2 h2 D2 v2t2

vt r0

r t r0 v2t2 2r0

PointTarget

Radar

vt

r0

h

D

r t

Geometry

PRINCIPLES OF RADAR IMAGING RANGE-DOPPLER PROCESSING

• The phase of the range compressed signal is

• The last approximation on the right is valid when the antenna beamwidth is very narrow, and is usually a good approximation for most higher frequency airborne SAR systems

• The expression above is that of a chirp signal with a bandwidth of where T is half the time that the target is in the field of view of the antenna

• Note that the bandwidth of the azimuth chirp is a function of the range to the target.

• The range-Doppler processing algorithm uses this fact to first perform matched filter range compression, followed by matched filter azimuth compression

4r

4

r02 v 2 2

4

r0 2r0

v 2 2

B 2v2T r0

PRINCIPLES OF RADAR IMAGING CLASSICAL SAR PROCESSING GEOMETRY

insert sphere

Range S phere

Doppler Cone

VelocityVector

Assumed ReferencePlane

S catterer is assumed at the intersection of RangeS phere, Doppler Cone and Reference Plane

AircraftPosition

PRINCIPLES OF IMAGING RADARSAR IMAGE PROJECTION

A three-dimensional image is projectedonto a two-dimensional plane, causingcharacteristic image distortions:

• b’ appears closer than a’ in radar image LAYOVER

• d’ and e’ are closer together in radarimage

FORESHORTENING

• h to i not illuminated by the radar RADAR SHADOW

a c d f g i

b

e

b’

a’c’

d’e’

g’h’

i’

f’

RADARIMAGE PLANE

TYPES OF IMAGING RADARS

Spatial InformationImaging Radar

Spatia

l Inf

orm

atio

n

Imag

ing

Radar

Spatial Information

Imaging Radar

Elevation InformationInterferometer

Spectral InformationSpectrometers

Structural In

formation

Polarimeter

Imaging RadarSpectrometer

Imag

ing

Rad

arIn

terf

erom

eter

Imaging Radar

Spectrometer

Imaging R

adar

Polarim

eter

Imaging Radar

PolarimeterIm

agin

g Rad

ar

Inte

rfero

met

er

Mul

ti-fre

quen

cy

Pol

arim

eter

Multi-frequency

Interferometer

Multi-frequency

Imaging Radar

Multi-frequencyImaging Polarimeter

ImagingPolarimetric

Interferometer

Multi-frequencyImaging Interferometer

Multi-frequencyImaging Polarimetric

Interferometer

• Transverse electromagnetic waves are characterized mathematically as 2-dimensional complex vectors. When a scatterer is illuminated by an electromagnetic wave, electrical currents are generated inside the scatterer. These currents give rise to the scattered waves that are reradiated.

• Mathematically, the scatterer can be characterized by a 2x2 complex scattering matrix that describes how the scatterer transforms the incident vector into the scattered vector.

• The elements of the scattering matrix are functions of frequency and the scattering and illuminating geometries.

SAR POLARIMETRYSCATTERER AS POLARIZATION TRANSFORMER

INCIDENT WAVE

SCATTERER

SCATTERED WAVES

POLARIMETER IMPLEMENTATION

TIMING

Transmission:

Horizontal

Vertical

Reception:

Horizontal

Vertical

HH HH HHHV HV

VH VV VH VV VH

Transmitter

Receiver

Receiver

BLOCK DIAGRAM

Horizontal

Vertical

POLARIZATION SIGNATURE

• The polarization signature (also known as the polarization response) is a convenient graphical way to display the received power as a function of polarization.

• Usually displayed assuming identical transmit and receive polarizations (co-polarized) or orthogonal transmit and receive polarizations (cross-polarized).

RADAR INTERFEROMETRYHOW IS IT DONE?

BB

SIMULTANEOUS BASELINETwo radars acquire data atthe same time

REPEAT TRACKTwo radars acquire data from different vantage points at different times

RADAR INTERFEROMETRYCOMPARISON OF TECHNIQUES

IMPLEMENTATION ADVANTAGES DISADVANTAGES

Simultaneous Baseline • Known baseline • Difficult to get adequatebaseline in space

• No temporal decorrelation • High data rate from tworadars

• Typically better performance • Typically higher cost

Repeat Track • Lower data rate from oneradar

• Temporal decorrelation

• Lower cost • Baseline not well known andmay be changing

• Depending on orbit, anybaseline can be realized

INTERFEROMETRIC SAR PROCESSING GEOMETRY

insert sphere

Range S phere

Doppler Cone

VelocityVector

Phase Cone

AircraftPosition

BaselineVector

S catterer is at intersection of RangeS phere, Doppler Cone and Phase

Cone

DIFFERENTIAL INTERFEROMETRYHOW DOES IT WORK?

• THREE-PASS REPEAT TRACK:

• Two different baselines:

• Incidence angle the same

• Absolute range the same

• Use parallel ray approximation to show thatif nothing changed,

(B1,1); (B2 , 2 )

2 1

B2 sin( 2 )

B1 sin(1 )

0

B2B1

DIFFERENTIAL INTERFEROMETRYERROR SOURCES

• Uncompensated differential motion

• Atmospheric effects

• Temporal decorrelation

• Layover

EMERGING SAR TECHNIQUESPOLARIMETRIC INTERFEROMETRY

• Polarimetric interferometry is implemented by measuring the full scattering matrix at each end of the interferometric baseline

• Currently there are no single baseline systems that can acquire this type of data

• During the last three days of the second SIR-C/X-SAR mission the system was operated in the repeat-pass interferometric mode, and some fully polarimetric interferometric data were acquired

• Using the full scattering matrix one can now solve for the optimum polarization to maximize the interferometric coherence

• This problem was first analyzed and reported by Cloude and Papathanassiou

• Using interferograms acquired with different polarization combinations, one can also for vector differential interferograms

• These vector differential interferograms have been shown to measure large elevation differences in forested areas, and cm-level elevation differences in agricultural fields

EMERGING SAR TECHNIQUES TOPOGRAPHY FROM POLARIMETRY

• By measuring the shift in the maximum of the polarization signature, the tilt of the surface in the azimuth direction can be estimated.

• In vegetated areas, P-Band data are used since a tilted surface will show a similar behavior if the trunk-ground interaction term is relatively strong

• The accuracy with which one can measure the surface tilt is determined by the signal to noise ratio

• Once the surface tilts (surface slopes) are known, the slopes are integrated in the azimuth direction to find the topography as a series of azimuth profiles

• Ground control points are needed to find the correct absolute height, and to tie different azimuth profiles together

• By using data acquired in a crossing flight pattern, the topography can be derived requiring only a single ground control point

• While the accuracy of this technique is not as good as that of interferometry, crude digital elevation maps can be produced.