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Synthetic Aperture Radar (SAR)

Basics and Theory

Dr. M Jahanzeb Malik

B.E. (Civil Engg.), NED University, Pakistan

M.Sc (Geo information science and Earth Observation), ITC-UT, The Netherlands

PhD (Remote Sensing and Land Surface Modeling), ITC-UT, The Netherlands

Remote Sensing (RS)

Electromagnetic

spectrum: Wave and

frequencies used in RS

Why microwave RS…?

Independence of the Sun as asource of illumination: all-hour

Ability to penetrate clouds, haze,dust (and to some extent rain):all-weather

Ability to penetrate moredeeply into vegetation, snow,soils then optical waves

Microwave RS complementsOptical RS

MODIS

Aqua

5-Aug, 10

R,G,B

5(NIR),1(R),4(G)

PALSAR

ALOS

L-Band

5-Aug, 10

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Microwave RS complements Optical RS

ALOS: PALSAR, HH

ALOS: AVNIR, (R,G,B): (NIR,R,G)

Nawabshah/ Dadu, Sindh, Pakistan

Microwave RS complements Optical RS

Envisat, ASAR, WSM, C-Band, VV

01-Mar, 2006

Tibet, China

Basic operation of RADAR

RADAR: RAdio Detection And Ranging

Spaceborne radar RS

Radar Altimeters: measure the round trip time delay totargets to determine their distance from the sensor (e.g.,SIRAL on CryoSat-2)

Radar Scattromters: make measurements of the amount ofenergy backscattered from targets (e.g., Seawinds onQuikSCAT)

Synthetic Aperture Radars (SAR): measure the round triptime and amount of energy backscattered from targets (e.g.,TerraSAR-X, COSMO-SkyMed,RADARSAT)

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Imaging geometry of Side Looking RADAR

or pulse width

Bandwidth (B) = 1/τ

Common angles in radar imaging

slant range: the range along the radar line of sight, and

ground range: range from the nadir track along a smooth surface (the ground) to the

scatterer.

The incidence angle is the angle between the radar beam and ground surface

The look angle is the angle at which the radar looks at the surface

What does RADAR measure? Backscattering coefficient

units of m²

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Spatial resolution

What is spatial resolution…?

the size of the smallest possible feature that can be

detected

RS in visible region:

For a homogeneous feature to be detected, its size

generally has to be equal to or larger than the IFOV.

If the feature is smaller than this, it cannot be detectable

as the average brightness of all features in that IFOV will

be recorded

Road

IFOV IFOV

Spatial resolution

SLAR Case:

Slant range resolution

Ground range resolution

Along-track (or azimuth)

resolution

=pulse duration

Range resolution

In designing the signal pattern for a radar sensor, there is

usually a strong requirement to have as much energy as

possible in each pulse in order to enhance the signal-to-

noise ratio (SNR)

This can be done by increasing the transmitted peak

power or by using a longer pulse

peak power is usually strongly limited by the available

power sources

an increased pulse length, which leads to a worse range

resolution

Spatial resolution vs range

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Example: SLAR

X-band radar: 9.65 GHz (0.03 m); bandwidth: 150MHz

(pulse duration: 6.67 ns); speed of light: 2.9x108 m/s;

Antenna length: 9 m; height: 3000 m; incidence angle: 20

deg

Slant range resolution: 1 m

Ground range resolution: 3 m

Azimuth resolution: 11 m

Height: 500 km; Azimuth resolution: around 2 km

For this reason, SLAR are not commonly used in spaceborne remote sensing

SAR

The main difference between

SLAR and SAR is the way in

which the azimuth resolution is

achieved.

The longer synthetic array

allows a larger Doppler

bandwidth and, hence, a finer

surface resolution.

The range resolution derived for

a real aperture radar is still valid

here.

SAR: azimuth resolution

The achievable azimuth resolution of a SAR is

approximately equal to one-half the length of the actual

(real) antenna and does not depend on

platform altitude (distance or range)

Wavelength

It shows that a smaller antenna gives better resolution

Power of the antenna is proportional to the square of its

aperture

So, Penalty in SNR must be paid to push antenna aperture

to very small dimension

SAR basic principle

Shorter pulse duration (Te ) higher bandwidth (Be) higher range resolution

Now, SAR bandwidth is selectable, e.g., PALSAR 2 (14/28/42/84 MHz), RADARSAT2 (11.6-

100 MHz), TerraSAR-X (150/300 MHz)

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Spatial resolutions

ALOS PALSAR

(L-Band; ~23 cm)

TerraSAR-X

(X-Band; ~3 cm)

Spatial resolutions

ALOS PALSAR

(L-Band; ~23 cm)

TerraSAR-X

(X-Band; ~3 cm)

Topographic effects on reflectivity and map

geometry

For slopes facing the radar, the ground

range resolution will be poorer than that for slopes facing away

from the radar.

Geometric issues

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Geometric issues Geometric issues

Envisat, ASAR,

WSM, C-Band, VV

13-Feb, 2006

Tibet, China

Backscattering coefficients

𝜎𝑂=𝜎

𝐴𝐿

𝛽𝑂=𝜎

𝐴𝛽

Backscattering coefficient

Example: for an object with an RCS of 10m2, and an image

pixel size of 10mx10m, = -10dB.

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Backscattering coefficient

The value depends on:

Physical and electrical properties of the material

Shape, size, orientation, arrangement

Dielectric constant

Sensor parameters

Wavelength

Polarization

Incidence angle

Site parameters

Surface roughness

Topographic relief

Backscattering coefficient

Airborne SAR systems Spaceborne SAR systems

The Envisat mission ended on Apr 2012, following the unexpected loss of contact with the satellite,

and RADARSAT-1 in Mar, 2013.

2014ALOS-2/PalSAR-2 8 – 70 deg.

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Surface Interaction with the Radar Beam

Surface Scattering

Specular Scattering

Bragg Scattering

Volume Scattering

Main scattering mechanism

Main scattering mechanism How trees are seen by RADARS

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Surface scattering

surfaces appear “rougher” at larger angles

Surface Roughness

Roughness is a relative concept depending upon

wavelength and incidence angle.

According to the Rayleigh criterion, a surface is

considered smooth if:

and considered rough if:

Commonly used frequency bands Advanced SAR modes

Stripmap: The classic mode for side looking SAR

instruments

ScanSAR (Wide Swath Mode): Increased swath width

with loss in azimuth resolution

Spotlight Mode: Improved azimuth resolution; no

continious imaging can be achieved

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Spackles

Inherent to coherent systems (i.e., phase information is

preserved)

Spackles make interpretation more difficult

Spackles : salt and paper effect

Speckle reduction

Multi-look processing

By averaging the looks incoherently pixel by pixel the speckle reduced image is obtained.

The speckle reduced image has an improved radiometric resolution, but the geometric resolution will be worse.

Adaptive and non-adaptive filters

A moving window filter changes the intensity of the central pixel depending on the intensities of all the pixels within the window

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Summary: SAR Signal Properties Summary: SAR Modes

SAR Main Properties and Applications

Thank you