This document provides an overview of synthetic aperture radar (SAR) basics and theory. It discusses key aspects of SAR including how it works, imaging geometry, spatial resolution, backscatter coefficients, common frequency bands, and advanced modes. SAR uses microwave radiation and can image the Earth's surface in all weather and light conditions, providing complementary data to optical remote sensing. It discusses concepts such as range and azimuth resolution, factors that influence backscatter, and challenges like speckle that SAR addresses through techniques like multi-look processing.

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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 a
source of illumination:all-hour
 Ability to penetrate clouds, haze,
dust (and to some extent rain):
all-weather
 Ability to penetrate more
deeply into vegetation, snow,
soils then optical waves
 Microwave RS complements
Optical 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 to
targets to determine their distance from the sensor (e.g.,
SIRAL on CryoSat-2)
 Radar Scattromters: make measurements of the amount of
energy backscattered from targets (e.g., Seawinds on
QuikSCAT)
 Synthetic Aperture Radars (SAR): measure the round trip
time 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
IFOVIFOV
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
𝜎 𝑂=
𝜎
𝐴 𝐿
𝛽 𝑂=
𝜎
𝐴 𝛽
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