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Principles of SAR and
Characteristics of SAR images
Sentinel Asia System Operation Training
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Table of Contents
1. About Remote Sensing
2. Basics of SAR
3. Characteristics of SAR images
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1. About Remote Sensing
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What is Remote Sensing？
Platform
Satellite
Sun
atmosphere
Radiation
Radiation
Radiation
Reflection
Reflection
Surface Objects
Sea
Citation: Material of Professor Okamoto,Osaka Prefecture University, (Edited by author)
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Applications of Remote Sensing
•
•
•
•
Global Change: Global Warming、Ozone Hole、Air・Water・Soil pollution、Acid rain
Resource Investigation: Mineral Resource (Oil・Metal etc.)、Agricultural Produce、
Forest Resource、Oceanic Resource
Monitoring : Decrease of forest land and Desertification、Change in ecosystem、
Reconnaissance: Mapping、Information gathering、Early Warning
Ozone hole
Volcanic activity
Typhoon・Rainfall
Land Use
Glacier
Land form
Water pollution
Vegetation
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Sea Temperature and
Plankton Density
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Classification of Remote Sensing
• Method of Data Acquisition
- Active、Passive
• Type of Electromagnetic wavelength
- Visible spectrum、Infrared、Microwave
ALOS
→SAR (PALSAR)
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Difference in Satellite and Aircraft
2 types of Platforms to Remote sensing:
Satellite, aircraft
Characteristics of Satellite
・Can observe an extremely wide area
(Satellite: 10-few100 km, cf. aircraft: few km)
・Can Observe the Earth’s surface periodically
・Near real time downlink of data is possible
・Once launched, operating cost is low,
proficient operation may be econonomical
Satellite - Wide observation
Aircraft - Detailed observation
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Difference between Active and Passive
There are two major concepts to retrieve information on Earth;
Passive
aktiv
Active
• Optical System use sunlight and thus are passive sensors
• Radar use their own source of radiation thus are active sensors
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Difference in Wavelength
Synthetic Aperature Radar(SAR) : Microwave
Optical sensors: Visible spectrum to Thermal Infrared
Optical
Cannot observe the surface
in cloudy weather(captures clouds)
or night-time
Microwave
All-weather、night-time
observation is possible
What is the difference？
Key: particle size and wavelength
Microwave has “longer wavelength” than optical
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Comparing SAR image and Spectral Image
Left：PALSAR
Acquisition Date: Aug 11, 2009
14:35:47 UTC,
Mode:FBS6
Polarization: HH
Right：Google Earth Image
©METI, JAXA
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Image (c) 2009 TerraMetrics
Image (c) 2009 GeoEye
Data SIO. NOAA, U.S. Navy, NGA, GEBCO
Image (c) 2009 Digital Globe
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2. Basics of SAR
• Principles of SAR
• SAR Satellites and Characteristics
• Band, Resolution, Polarization
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What is SAR?
• SAR
：Synthetic Aperature Radar
– An active sensor which emits Microwave to
the surface on slant range, and detects the
backscattering
• Radar (RAdio Detecting And Ranging)
Synthetic Aperature
Antenna
Real Aperature
Antenna
– Emits electromagnetic (EM) waves and
detects EM waves reflected from target
– Determines distance to target from the
returning time of the EM waves
• Why “Synthetic Aperature” ？
– Higher resolution of image with larger
antenna
– However antenna size is limited
⇒ By moving smaller antennas, synthesizing a
bigger antenna is possible
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Resolution
acquired by
Synthetic Aperature
Resolution acquired by
Real Aperature
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Principles in SAR observation
• SAR emits microwave radiation on a slant range on a
perpendicular dimension （Range）to flight path（Azimuth）
• Adjustable incidence angle（angle between the range of
microwave and vertical range)
gh
i
l
F
ath
p
t
Pulse Width
Off-Nadir
Width acquired in
an image
Incidence angle
Near range
Target
Range
Azimuth
Far range
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Development of SAR
1950’sーSynthetic Aperature Radar technology is developed
and expanded in military obectives.
1991ーEuropean Space Agency (ESA)launches ERS-1
Scientific Objectives
 1992 NASDA(Japan)launches JERS-1
 1995 ESA launches ERS-2
Resolution is 10 m～30 m
 1995 CSA(Canada)launches RADARSAT
 2002 ESA launches Envisat
 2006 JAXA(Japan) launches ALOS
Commercial Objectives
2007ーISA(Italy) launches COSMO SkyMed
Resolution is in order of
DLR(Germany) launches TerraSAR-X a couple of meters
CSA(Canada) launches RADARSAT-2
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Development Process of SAR Spatial Resolution
・There
・There have
have been
been several
several steps
steps in
in the
the
innovation
innovation of
of satellite
satellite SAR
SAR technology
technology
・From
・From these
these innovations,
innovations, application
application
of
of satellite
satellite SAR
SAR technology
technology has
has
broadened
broadened to
to many
many fields
fields
TerraSAR-X
（DLR）
X/1m/HH・VV・HV・VH
ALOS
（JAXA）
L/7m/HH・VV・HV・VH
High Resolution 1m（X-band、300MHz）
ENVISAT
（ESA）
C/30m/HH・VV
Observation by Multi-polarization
becomes possible
Sensing mode becomes variable
（Observation width、resolution、
incidence angle）
•Basic Research
•Military
Development
1950～60’s
JERS-1
（JAXA）
L/18m/HH
ERS-1,2
（ESA）
C/30m/VV
SEASAT
（NASA）
L/25m/HH
1970’s
RADARSAT
（CSA）
C/6m/HH
1980’s
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1990’s
2000’s
Satellite Name
（Developer）
Band/Resolution/
Polarization
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Comparison of SAR Satellite Specifications
Satellite Name
ERS-2
JERS-1
Radarsat-1
Envisat
(ASAR)
ALOS
(PALSAR)
TerraSAR-X
COSMOSkymed
Radarsat-2
Country
Europe
Japan
Canada
Europe
Japan
Germany
Italy
Canada
Launch year
1995.4
1992.2
1995.11
2002.3
2006.1
2007.6
2007.6
2007.12
Orbit Altitude
785km
568km
793-821km
780-820km
691km
514km
620km
798km
Orbit Slant
Angle
98.5deg
97.7deg
98.6deg
98.55deg
97.16deg
97.44deg
97.86deg
98.6deg
Revisiting date
35
44
24
35
46
11
16
24
Frequency
5.3GHz
(C-band)
1.275GHz
(L-band)
5.3GHz
(C-band)
5.3GHz
(C-band)
1.270GHz
(L-band)
9.95GHz
(X-band)
9.6GHz
(X-band)
5.405GHz
(C-band)
Wavelength
5.7cm
23.5cm
5.7cm
5.7cm
23.5cm
3.1cm
3.1cm
5.7cm
Polarization
VV
HH
HH
Single、
Dual
Single、
Dual、
Quad
Single、
Dual、
Quad
Single、
Dual
Single、
Dual、
Quad
Incidence
Angle
23deg
38.7deg
10 -60deg
15 -45deg
8 -60deg
15 -60deg
25 -47deg
20 -60deg
Pulse Width
100km
75km
50 -500km
58 -405km
70 -350km
10 -100km
10 -200km
20 -500km
Resolution
30m
18m
9 -147m
30 -1000m
10 -100m
1 -16m
1 -100m
3 -100m
Antenna Size
1 x10m
2.2 x12m
1.5 x15m
1.3 x10m
3.1 x8.9m
0.7 x4.8m
1.4 x5.7m
1.5 x15m
Citation: RESTEC SAR Basic Course (Edited by author)
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Band（Wavelength）
• ALOS PALSAR
: L-Band(15～30cm)
– Radarsat-2 ： C-Band
– TerraSAR-X : X-Band
L -Band
Microwave
Wavelengths used in
other satellites
ALOS PALSAR： 15～30cm
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Wavelength and Penetration
• Penetration to objects differ according to wavelength
– Short wavelength electric waves reflect from object surfaces
– Long wavelength electric waves penetrate accordingly into objects
(Particles which are smaller than 1/4 of the wavelength are penetrated)
Earth and sea
can be observed using
all bands
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What is Polarization？
• Relationship between amplitude of the periodically changing
electric field and Nadir track
– H：Horizontal Polarization（amplitude is horizontal to nadir track）
– V：Vertical Polarization（amplitude is vertical to nadir track）
• Combination of transmission and reception:
HH、HV、VH、VV
– Ex. HV: transmission in H, reception in V
receive
receive
receive
V
V
H
V V
receive
H H
H
transmit
transmit
transmit
VV Polarization
HH Polarization
transmit
HV Polarization
VH Polarization
• Polarization characteristics differ according to objects and
shapes
⇒ Multi polarization has higher possibility in acquiring data than
single polarization
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Data Extraction by Polarization
©DLR
Multi polarization observation of southern France by Airborne
X-band SAR (Red: Farmland, Green：Forest, Blue: Bare land)
HH-VV HV HH+VV
・Satellite based Qual-Pol observation can only be done by ALOS PALSAR
・TerraSAR-X is the first high resolution satellite
・Detailed analysis on land coverage is anticipated
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Vegetation Classification of Kushiro Marshland
Intense HH backscattering in alder colony
Sphagnum Marshland
Intense
backscattering
Intense backscattering in
herb colony
Feature which cannot
be seen in herb colony
Alder
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Alder、Alder-Reed、Sedge Reed-Sedge
Reed、Sphagnum、Buck-Bean、Water
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What is Observation Modes？
According to the objective, there are 3 beam modes
ALOS PALSAR
Reference: ERSDAC <http://www.palsar.ersdac.or.jp/e/about/sensor.html>
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PALSAR Sensor
Main Characteristics of PALSAR
Mode
Fine
ScanSAR
Center Frequency
Polarimetric
1270MHz(L-band)
Bandwidth
28MHz
14MHz
14,28MHz
14MHz
Polarization
HH or VV
HH+HV or VV+VH
HH or VV
HH+HV+VH+VV
Incidence angle
8 ~ 60deg.
8 ~ 60deg.
18 ~ 43deg.
8 ~ 30deg.
Range Resolution
7 ~ 44m
14 ~ 88m
100m
24 ~ 89m
Swath
40 ~ 70km
40 ~ 70km
250 ~ 350km
20 ~ 65km
Quantization
5bits
5bits
5bits
3 or 5bits
Data rate
240Mbps
240Mbps
120Mbps, 240Mbps
240Mbps
Reference: ERSDAC <http://www.palsar.ersdac.or.jp/e/about/sensor.html>
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Fine Mode
•Most commonly used under regular
operation.
•Maximum ground resolution of 7m
•Approx. 18m resolution) of SAR
onboard JERS-1
©METI, JAXA
Reference: <http://www.disasterscharter.org/graphics/dis/
CALLID_122/bp_palsar-merapi060516e.jpg>
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ScanSAR Mode
• Off-nadir
angle from 3 to 5 times (scan by
the swath of 70km)
•Cover wide area from 250km (70x3) to
350km (70x5) (for comparison, swath
width of SAR onboard JERS-1 was about
75km)
•Resolution inferior to high-resolution
mode.
©METI, JAXA
Reference: <http://www.eorc.jaxa.jp/ALOS/img_up/jpal_chicago.htm>
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Polarimetric Mode
©METI, JAXA Reference: <http://www.eorc.jaxa.jp/ALOS/en/img_up/pal_polarization.htm>
•Both horizontal and vertical polarization
•Can also simultaneously receive horizontal and vertical polarization per each polarized
transmission, which is called multi polarimetry.
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Process Levels
Example: ALOS Palsar
Level
Definition
Remark
1.0
Not image data with directly recorded slant
range. Special software is necessary to use
this data.
JERS-1: Level 0
RADARSAT: Signal Data
1.1
Slant range image data with coordinate
information. 1.1 is a 1.0 data that has been
single-look azimuthaly compressed.
JERS-1: Level 1.1
RADARSAT: Single Look
Complex
TerraSAR-X: Single Look
Slant Range Complex
1.5
Multi-azimuthally processed image data with
range compression that is projected to ground
range, and farther geometrically projected.
JERS-1: Level 2.1
TerraSAR-X: Multilook
Ground Range Detected
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Level 1.1 Data (Single Look Complex)
SLC image
• Since it is a complex data, not only
microwave scatter intensity, but
topological data is store in each
pixel.
• Each pixel is positioned in slant
range. Thus differs from pixel
disposition of ground range data.
• Spatial resolution of range and
azimuth are not equal, thus length
and breadth ratio do not fit.
• Conversion to backscatter
coefficient is possible.
©METI, JAXA
Reference:<http://www.palsar.ersdac.or.jp/product/p_product.html>
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Level 1.5 Data (Multi Looking Intensity)
MLI image
• Foreshortening created by landform are not
corrected
• Image is in real-value(16 bit uncoded
integer) . Image is create by complex numbers
thus differs from the complex image and does
not have topological data.
• Images which have length and breadth
according to the flight path are called
georeferenced images, while images which
have length and breadth according to north
direction are called geocoded images.
• Pixel size is adjusted in range and azimuth
©METI, JAXA
• convertion to backscatter coefficient is
possible
©JAXA
Reference: <http://www.opengis.co.jp/htm/gamma/PALSAR_Processing_060502.pdf>
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Geometrical Characteristics of SAR
SAR Antenna
Slant range
Satellite
Even when data is
sampled in even intervals,
when projected to ground
range, intervals are not
equal!
Near range
Far range
short
long
Ground range
・Georeferenced, geocoded SAR data is projected to ground range
・Most SLC SAR data is in slant range
・Higher resolution achieved in far range when displayed in ground range
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Ortho Correction of SAR image
Microwave
θi
fs=h cotθi
h
Fs Amount of Foreshortening
Distortion occurring from
foreshortening can be calculated
from the incidence angle (θi)
and altitude (h).
Thus correction is possible
(However, the earth’s curvature
must be considered, so the
formula becomes complex)
Incidence angle can be
calculated from satellite
parameters( orbit, position）
However, altitude in unknown,
thus DEM (Digital Elevation
Model) are widely used.
Generally, ortho correction of a SAR image indicates
correction of foreshortening distortion
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DEM Data and Geoid Data
Altitude data is necessary for ortho correction.
In Mapready, the altitude data will be achieved from SRTM3 data, and EGM96
Geoid data.
SRTM-3
Latitude and altitude data produced from the Shuttle Radar Topography Mission.
Data resolution is 3 second mesh (approx. 90m) from 60 deg. north to 56 deg. South.
Files are divided in 1 deg. mesh according to WGS84. 1pixel overlaps with neighboring mesh
The latitude・longitude in the file name is of the pixel center on the left bottom of the image.
●ftp://e0srp01u.ecs.nasa.gov/srtm/version2/SRTM3/
EGM96 Geoid
Geoid data produced from the Earth Gravity Model 1996 released by NGA(National
Geospatial-Intelligence Agency) (USA)
Data which covers the globe into 15 minute mesh according to WGS84 is accessible (Data
File:WW15MGH.DAC)
＊Latest geoid data(EGM2008) has also been released
●http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm96/binary/bunarygeoid.html
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What is Spatial Resolution？
The ability to distinguish and separate
two neighboring objects
e.g., Digital Camera
「Spatial Resolution」
High Resolution
「Pixel Size」
2006
ＡＬＯＳ：10 m
©Infoterra GmbH
1m
©METI, JAXA
5cm
©Infoterra GmbH
1990’s
ＥＲＳ：30 m
JERS-1：18 m
etc.
20cm
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3m
©Infoterra GmbH
18 m
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Example image
©Infoterra GmbH
Tsukuba (Japan)
Acquisition date/time: 23 Oct 2007/05:35 (JST) and 29 May 2008/05:52 (JST)
Acquisition mode: High resolution SpotLight / Polarization: HH (Single) / Incidence angle:52.70 and
StripMap / Polarization: VV (Single) / Incidence angle:23.99
Product: EEC (SE) / Resolution: 1.50m and 3.13m
The above highlighted areas were acquired by High resolution SpotLight mode.
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3. Characteristics of SAR image
• Shading in image
• Geometric characteristics of SAR image
• Speckle
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SAR Signal and Resulting Image
• Scattering mechanism of the incident wave
– Specular reflection from surface（e.g.calm water surface） ⇒ dark pixel
– Diffuse reflection from a volume (e.g. vegetation)
⇒ intermediate and texture
– Corner reflection (e.g. among buildings)⇒ bright pixel
Antenna
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Object Characteristics
•
Strength of measured signal and resulting grayvalue and texture depends on:
– Surface roughness in relation to wavelength（3.1cm：TerraSAR-X）
Radiometrically smooth surface scatter
the wave away from the antenna
No return to the antenna
⇒ Black in the image
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Radiometrically rough surfaces scatter
the waves mostly away from the
antenna
Little return to the antenna
⇒ Gray in image
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Comparison of ALOS and other Satellites
（Mt.Fuji）
Optical
SAR
ALOS/AVNIR-2
ALOS/PALSAR
Comparison is
difficult due to
difference in
filming date and
scale.Classifying
snow, vegetation
and bare land is
possible
Shading in image
differs by land
coverage( sand
and vegetation),
and geological
differences.
TerraSAR-X
EROS-B
High ground
resolution,
thus small
details (e.g.
mountain
trail) can be
deciphered
Land coverage
and snow, can be
confirmed due to
high resolution.
Since image has
central
perspective,
crater can be
observed.
ALOS/AVNIR-2: scale 1：40,000
Others: scale 1：20,000
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ALOS/PALSAR image
Bare Land
（Sand）
Vegetation
（Forest）
Difference in shading in image due to different reflecting features:
land coverage (sandy land and vegetation), geological aspects
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TerraSAR-X image
Fuji Subaru
Line (road)
Mountain
Trail
High ground resolution, thus small details (e.g. mountain trail)
can be deciphered
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ALOS/AVNIR-2 image
・Since filming date (season) and scale differs from other
satellite images, comparison is subtle
・Classifying snow, vegetation and bare land is possible
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EROS-B image
・High resolution, thus land coverage and snow, can be confirmed
・Since image has central perspective, inside of the crater can be observed
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Side-looking System
• Geometries in Radar versus Optical data
– Optical System：Nadir looking geometry creates central perspective
– SAR： Side looking system creates slant range geometry
Radar Sensor
Camera
Optical Sensor
Radar Antenna
NADIR
NADIR
Imag
e
Imag
e
Viewing Geometry
Spectral ： Objects are folded towards the
outside of the image
SAR ： Objects are folded towards
the radar
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Geometric Distortion of SAR Images
• Foreshortening（E）
– High building (places) appear closer to antenna than its original position
• Layover（B）
– Hilltops appear closer than actually closer positioned objects
• Shadowing（Places further from antenna than E）
– Shadows form in areas on the further side of hills away from the antenna
B
Ａ
E
C
D
Position in image
B A
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C
D E
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Example of Geometric Distortion
TerraSAR-X Product
Before Geometric（ortho）
correction
（Slant Range image）
Beam
TerraSAR-X Product
After Geometric (ortho)
correction
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Summary
Comparison of Characteristics of SAR and Spectral
SAR System
Spectral System
Active Sensor
Passive Sensor
Side-looking Geometry
Sub satellite Geometry
Wavelength in cm units
Wavelength in nano meter units
All-weather, all-day observation
possible
Measure difference in returning
time of signal
Observation in day time, in
cloudless areas
No difference in returning time of
signal
Slant range resolution
Visual range resolution
Usage of polarization
Image contents：Intensity +
Topology
Polarization not effective
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Image contents：Intensity
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