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12-01-2017
1
by
RISAT-1 SAC DP Team
presented by:
RAGHAV MEHRAraghavmehra@sac.isro.gov.in
MICROWAVE DATA PROCESSING DIVISION
SIGNAL AND IMAGE PROCESSING GROUP
SPACE APPLICATIONS CENTRE
Polarimetric SAR Data Processing and Analysis
(with special emphasis on RISAT-1)
Space Applications Centre, Ahmedabad
20-21 December 2016
Contents
Radar Imaging Principles
RISAT – 1 Specifications
RISAT – 1 SAR Data Processing
SAR Image Formation
Geocoding
Calibration
Formatting
RISAT – 1 Processing Environment
RISAT – 1 Data Images
2
Real Aperture Radar
Slant Range Resolution
B
cc
22
Ground Range Resolution
)sin(2 i
c
Azimuth Resolution
L
R
L : Antenna Lengthτ : Effective Pulse Widthh : Satellite Altitudeλ : WavelengthB: Chirp Bandwidth
3
Synthetic Aperture Radar
Slant Range Resolution Ground Range Resolution
)sin(2 i
c
Azimuth Resolution
2
L
Adequate Gain Effective Transmitted
Power
High Resolution High SNR
“Pulse Compression”
B
cc
22
Linear Frequency Modulated
Signal “Chirp”
Antenna Length “L”
4
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2
Azimuth Compression
Simulated EchoRange
Azi
mu
th
Ech
o P
ow
er
Range Compression
Range
Azi
mu
th
RCM Correction
Target Imaged
by SAR
Range
Azi
mu
th
Range
Azi
mu
th
5
RISAT-1 Specifications
6
Parameter Specification
Orbit Circular Polar Sun Synchronous
Orbit altitude 536 km
Orbit inclination 97.552°
Orbit period 95.49 min
Operating Frequency C-Band (5.35 GHz)
Operating Modes FRS1, FRS2, MRS, CRS & HRS
No. of orbits per day 14
Equator crossing 6.00 a.m./6.00 p.m.
Data rate 2 × 160 Mbps (total 640 Mbps in
two chains)
SSR 240 Gbits (End of Life)
Pointing accuracy 0.05°
Drift rate 5.0 × 10–5 °/sec
Attitude knowledge 0.02°
7
Imaging Capabilities
Either side of the flight track (Right Look & Left Look direction)
Incidence Angle range from 12 deg to 55 deg covered in beam numbers 0 to 60 (for right look direction) and beam numbers 64 to 124 (for left look direction).
For a given StripMap mode (FRS1 & FRS2), Data can be acquired using any beam.
Each MRS scene is composed of 6 alternate beams and CRS scene is composed of 12 alternate beams.
High Resolution Sliding Spotlight Mode with capability to cover an azimuth extent of 100 Kms 8
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3
Imaging Modes FRS-1 FRS-2 MRS CRS HRS
Swath (km) 25 25 115 223 10
PolarizationSingle, Dual,
CircularQuad
Single, Dual,
Circular
Single, Dual,
Circular
Single, Dual,
Circular
Resolution
(Azimuth/Slant Range) (m) 3.3/2.2 10/4.68 24/8.8 48/8.8 1/0.7
Chirp Bandwidth 75 MHz 37.5 MHz 18.75 MHz 18.75 MHz 225 MHz
Level 0 RAW Signal Product
BAQ Decoded I/Q Samples and CEOS formatting
Level-1 Geo-Tagged Product
Ground/Slant Range Product along with Grid File
Level-2 Terrain Geo-Referenced Product
UTM/Polyconic Projection using GTOPO/SRTM along with Grid FileLevel-2A Enhanced Terrain Geo-Referenced Product
UTM/Polyconic Projection using finer resolution GTOPO/SRTM DEM along with Grid File , Local Incidence Angle Map, Layover Mask
Levels of Processing
9
RISAT-1 SAR Data Processing
10
Processing Steps
• Raw Data Extraction
• Imbalance Correction
• Range Compression
• Doppler Parameter Estimation
• Range Cell Migration Correction
• Azimuth Compression
• Multilooking
• Scalloping Correction (For ScanSAR)
• Antenna Pattern Correction
• Grid Generation• Level-1 Ground Range/SLC
Image Generation
• Level-2 Geocoded Image Generation
• Calibration
• Formatting
Geocoded ScanSAR ImageRange Doppler Algorithm (RDA) hasbeen used for SAR Data focusing.
11
Azimuth / Along Track Ran
ge /
Acr
oss
Tra
ck
Data Buffer
12
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4
Azimuth / Along Track
Ran
ge /
Acr
oss
Tra
ck
Data Buffer
Scan lines corresponds to along track positioning of target i.e. Doppler history Pixels corresponds to across track range positioning i.e. Ranging
SAR Raw Data Extraction
Average Range Spectrum Average Azimuth Spectrum
rel
d
v
Rf
2
Azimuth Coordinate
13
cos(2πf0τ)
–sin(2πf0τ)
Measured Voltage from Receiver
I Channel
Q Channel
I Q Channel Imbalance Correction
Power Imbalance Correction
Phase Imbalance Correction
𝑰′ = 𝑰 −𝑴𝒊 𝑸′ = 𝑸 −𝑴𝐪 ∗𝑺𝑫𝒊
𝑺𝑫𝒒
𝑸′ =𝑸 − 𝑰 𝐬𝐢𝐧 ∆𝛉
𝐜𝐨𝐬 ∆𝛉
𝐬𝐢𝐧 ∆𝜽 =𝑪 𝑰𝑸
𝑪 𝑰𝟐 =𝑪 𝑰𝑸
𝑪 𝑸𝟐
where
15.2
15.6
16
16.4
16.8
17.2
1 6 11 16 21 26 31
Stan
dard
Dev
iati
on i
n Co
unt
1 K block size with 512 overlap
Standard Deviation Vs Block Number(Mode-FRS1 Orbit-2181 Scene-25 Pol-HH)
sd_i
sd_q
-0.24
-0.18
-0.12
-0.06
0
0.06
0.12
0.18
1 6 11 16 21 26 31
Mea
n V
alue
s in
Cou
nt
1 K block size with 512 overlap
Mean Values Vs Block Number(Mode-FRS1 Orbit-2181 Scene-25 Pol-HH)
Mean I
Mean Q
-1.94
-1.92
-1.9
-1.88
-1.86
-1.84
-1.82
1 6 11 16 21 26 31
Phas
e Im
bala
nce
(in d
eg)
1 K block size with 512 overlap
Phase Imbalance Vs Block Number(Mode-FRS1 Orbit-2181 Scene-25 Pol-HH)
14
Azimuth / Along Track Ran
ge /
Acr
oss
Tra
ck
Data Buffer
Range Compression Correlation between Transmitted Chirp signal and received echo signal in
range direction
𝒈 𝒕 = 𝑨𝒕 𝐞𝐱𝐩 −𝒋𝟒𝝅𝑹 𝒕
𝝀 𝒔𝒊𝒏𝒄 𝑿
Range Compressed Data
𝒘𝒉𝒆𝒓𝒆 𝑿 = 𝝅𝒌𝝉𝒑 𝒕 − 𝒕𝒏
15
Azimuth / Along Track Ran
ge /
Acr
oss
Tra
ck
Data Buffer
Range Cell Migration Correction Target trajectory is straightened in Range – Doppler domain by using the Range
Cell Migration function for each target.
Range Cell Migration Function
∆𝑅 = −𝜆𝑓𝑑𝑐
2 𝑡 − 𝑡𝑐 −
𝜆𝑓𝑅4
𝑡 − 𝑡𝑐 2
16
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5
Azimuth / Along Track Ran
ge /
Acr
oss
Tra
ck
Data Buffer
SAR Signal Processed Image SAR Slant Range Image For each target approx. 7billion complex multiplications are required. Typical FRS scene is 25kmx25km i.e. 64 million targets. Near Real-Time Processing
Azimuth Compression Correlation between RCM corrected data and Azimuth replica function
𝑅 𝑡 = 𝑅0 −𝜆𝑓𝑑𝑐
2 𝑡 − 𝑡0 −
𝜆𝑓𝑅4
𝑡 − 𝑡0 2
Azimuth replica function is given as :
ℎ 𝑡 = 𝑒𝑥𝑝 4𝜋𝑅 𝑡
𝜆 where
17Multilook Compressed ImageSingle Look Compressed Image
Speckle NoiseSpeckle noise is an inherent property of SAR images. It is the result of constructive anddestructive interference of the returns from many targets within a single resolution cell. It can bereduced by multi-look processing.
Multilook ProcessingIn this, Azimuth spectrum is divided inmultiple segments effectively dividingthe viewing aperture in sub-apertures.Then each of the segment is used toform the image and then the images areadded incoherently to produce amultilook image.
18
Radiometric Correction Across Swath – Antenna Pattern CorrectionProcessed SAR images has a radiometric variationacross swath because of antenna pattern effect. Toget a uniformly illuminated SAR image, thisradiometric variation is reduced by using followingformulation :
19Without Antenna Pattern Correction After Correction without using DEM After Correction using DEM
Terrain impact on Antenna Pattern Correction
Gain experienced
with Height h
Gain experienced
with zero Height
20
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6
ScanSAR Mode SAR Image Formation
Azimuth
1
2
3
4
7
Ra
ng
e
sw6
sw5
sw4
sw3
sw2
sw1
4
5
6
Azimuth
Ra
ng
e
beam0 Signal Processed
burst k
burst k+1
burst k+2
Processing of one ScanSAR beam
BlendedBursts
After ScallopingCorrection
Radiometric Correction
Across Swath
21 Mosaiced ScanSAR Ground Range data
Signal Processed ScanSAR beamsbm0 bm1 bm2 bm3 bm4 bm5
Corrected ScanSAR beams
bm0 bm1 bm2 bm3 bm4 bm5
ScanSAR Mode SAR Image Formation
22
Sliding Spotlight (HRS) mode
Sliding Spotlight Imaging Geometry
23
N SARWider Swath and moderate/high Azimuth Resolution SAR images can be obtained by SweepSAR Concept.
12-01-2017
7
SweepSAR Imaging Concept
SAR GEOLOCATION ALGORITHM
Coordinate System for SAR viewing Geometry
Iso- Range and Iso- Doppler lines specifying location of target
SAR Range Equation :
SAR Doppler Equation :
where,
fdc = Doppler centroid frequency
& = position and velocity vector of target
& = position and velocity vector of
satellite at scan line s(i)
= wavelength
= Minimum Range
= Range Sampling
p
)(is
p
)(is
0R
r
|)(|
))())(((2),(),(1
isp
ispispjifjiF dc
|)(|),( 02 ispjrRjiF
26
GEOCODING
Digital Elevation Model(DEM)
Signal Processed Image (Slant)
Geocoded Image (Map Projected)
Geographic Coordinates
(latitude, longitude)Map Projection
Inverse Map ProjectionSAR Geolocation
27
Geometric Distortions
Each SAR image is affected byForeshortening, Layover andShadowing effects due to thelocal topography of the terrain.
ForeshorteningIncidence Angle (η) > terrain slope (⍺)
LayoverIncidence Angle(η)<=terrain slope (⍺)
ShadowingTerrain slope (⍺) away from the radar is greater than Incidence Angle(η).
These effects are prominent in moderately undulating terrain and highly undulatingterrains and make the image interpretation difficult. Hence these areas should beavoided while doing any analysis in hilly terrain area.
28
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8
Geo-Referenced Image
Foreshortened Region Mask Ortho-Corrected ImageOrtho-Rectification
SAR Image Ortho-Rectification
An accurate DEM information is required in order togenerate the Ortho – rectified image. Use of accurateDEM generates heavy distortions in image. Hence amask is generated marking the unusable and usableareas.An Ortho – rectified product has less internaldistortions and good geo – location accuracy.
29
SAR Data Calibration
30
Calibration means characterizing the performance of end to end SAR system in terms of its ability to measure amplitude and phase of the backscattered signal.
Natural Calibration Targets-Amazon Rain Forests
Corner Reflectors Inter Sensor Calibration
RISAT-1
RADARSAT-2
Calibration
Radiometric CalibrationAmplitude and Phase
Geometric Calibration
nS
aSsl
P
E
eATd PA
LLR
GGGPP .
..
...
)4(0
2
3
2
),().,(2
azel
A
razel
A
tAGGG
3RRsl
Pd = Received Power for Distributed Targets
Pt = Transmitted Power
GA = Transmitted & Received Antenna Gain
GE= Electronic Gain in Radar Receiver
GP = Processor Constant
Rsl = Range Spread Loss
Ls = System Loss term
La = Atmospheric Attenuation
As = Scattering Area
Pn = Additive Noise
R = Range
Gt = Transmit Gain
Gr = Receiver Gain
pr = Image Pixel Dimension in Range
pa = Image Pixel Dimension in Azimuthiair
ar
s
ppA
cos.sin
.
Radar Equation (SAR)
where
03
)sin(.
)()(..
ir
elreltT
d
R
GGPKP Finally,
Antenna Pattern for a beam from Payload Team
Application of DEM ensures appropriate swath selection and correct radiometric correction
where ),()( HRfpixeleachel
)(ˆ).(),(az
A
azel
A
elazelGGG
az
az
dGGazazA
)()(ˆ
2
2
3
0 .).(
)sin(. DN
nG
RC
TRel
ir
&
TBkFPn
... Source : University of Zurick
Calibration Model for RISAT-1 Data Products
Following Radar Equation has been used to derive the Calibration Constant for RISAT-1 DataProducts. Principle of Amazon Rain Forests Gamma0 uniformity over different incidenceangles and hence Amazon C-Band Gamma0 values for different polarizations have beenused for calibration. Model has been validated with several Amazon , Corner Reflectorsacquisitions and Inter-Sensor C-Band Sigma0 comparison.
RISAT-1 Data Products Calibration
𝜎0 = 𝛾0 cos 𝜃𝑖𝜎0 = 𝛽0 sin 𝜃𝑖
Projected Area= a cos 𝜃𝐼 θ = I0 cos θI0 at θ =0
12-01-2017
9
RISAT-1 Data Products Calibration
It is based on Uniformity of Gamma0 over Amazon Rain Forests for different Incidence Angles
Beam- 72
Inc. Ang.- 20.02 º
Beam- 78
Inc. Ang.- 25.50 º
Beam- 92
Inc. Ang.- 36.77 º
Beam- 100
Inc. Ang.- 42.30 º
Beam- 115
Inc. Ang.- 50.96 º
FRS-1 Amazon Acquisitions
Beam- 6-16
Inc. Ang.- 22.69 º
Beam- 42-52
Inc. Ang.- 48.68 º
Beam- 35-57
Inc. Ang.- 48.20 º
Beam- 65-87
Inc. Ang.- 23.52 º
MRS & CRS Amazon Acquisitions
Pol. Amazon γ0 Pol. Amazon γ0
HH -6.47 ± 0.3 dB HV -12.47 ± 0.3 dB
VV -6.11 ± 0.3 dB VH -12.11 ± 0.3 dB
RH -6.35 ± 0.3 dB RV -6.25 ± 0.3 dB
Reference Amazon Gamma0
σ0 = γ0 cos (incidence angle)
RISAT-1 Sigma0 Computation
σ○ (dB) = 20log10 (DNp) – KdB
+ 10log10 (Sin (ip)/Sin (icentre))
KdB = Calibration Constant33
Verification of Gamma0 Profiles for Amazon Acquisitions
MRS : HH Polarization CRS : HH Polarization
FRS-1 : RV PolarizationMRS : RV Polarization
34
Point Target Response for Corner Reflectors
-30-28-26-24-22-20-18-16-14-12-10
PS
LR
(d
B)
Orbit_CR_Id
Azimuth PSLR (dB) RH HH Point Targets
Slant Range Ground Range
-30
-28
-26
-24
-22
-20
-18
-16
-14
-12
-10
PS
LR
(d
B)
Orbit_CR_Id
Range PSLR (dB) RH HH Point Targets
Slant Range Ground Range
PSLR-25
-20
-15
-10
-5
ISL
R (
dB
)
Orbit_CR_Id
Azimuth ISLR (dB) RH HH Point Targets
Slant Range Ground Range
-25
-20
-15
-10
-5
ISL
R (
dB
)
Orbit_CR_Id
Range ISLR (dB) RH HH Point Targets
Slant Range Ground Range
ISLR
____ ____
1
1.5
2
2.5
3
3.5
4
4.5
5
Ran
ge R
eso
luti
on
(m
ete
rs)
Orbit_CR_Id
Range Resolution (meters) RH HH Point Targets
Slant Range Ground Range
2
3
4
5
6
7
8
Azi
mu
th R
eso
luti
on
(m
ete
rs)
Orbit_CR_Id
Azimuth Resolution (meters) RH HH Point Targets
Slant Range Ground RangeResolution
35
I
3 mII
3 m
DOP 23/09/14 Bea
m
39 DOP 16/10/14 Beam 9
RH RV RH RV
CR Id I
3 m
II
3 m
I
3 m
II
3 m
I
3 m
II
3 m
I
3 m
II
3 m
RCS (dBm2) 51.8 51.2 51.2 50.9 51.1 50.7 51.2 50.7
Theor’lRCS
(dBm2)
50.3 50.3 50.3 50.3 50.3 50.3 50.3 50.3
Comparison of Theoretical RCS with Computed/Measured RCS for Corner Reflectors
36
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10
Inter Sensor Sigma0 Comparison of RISAT1 and RADARSAT-2 SAR Data
Sensor Polrzn. Imaging Mode DOP Inci. Angle Beam No.’s
RISAT-1 RH/RV FRS-1 09/03/2014 31.340 85
RADARSAT-2 Quad Fine Quad Pol 11/03/2014 26.85 0- 28.670 FQ8
Region No.
Average Gamma0
RISAT-1
(RH pol)
RADAR SAT-2
(HH pol)
Reg-1 -14.42 dB -15.01 dB
Reg-2 -15.93 dB -16.58 dB
Reg-3 -15.29 dB -15.84 dB
Reg-4 -3.70 dB -3.95 dB
Reg-5 -2.01 dB -2.64 dB
Reg-6 -14.91 dB -15.15 dB
Region -1
Region -2
Region -3
Region -4
Region -5
Region -6
37
RISAT-1 Data Products Formats :: CEOS and GEO-TIFF
Level-2A Enhanced Geo-Referenced
Local Incidence Angle Map
Layover Mask
38
RISAT-1 Processing Environment
39
Data
Reception
Facility
ADP VADSVersion
Control
Server
WFM
Server
Gigabit Ethernet
Storage Area Network
Data Acquisition
Electronics
Station Automation
Systems
Data
Ingest
RISAT-1
Data
Processing
PQC DQE
Gigabit Ethernet
Data Exchange Gateway
FTP PPS User Services
Internet
IMGEOS
40
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11
RISAT-1 Data Products Throughput
Mode Swath Acquisition Time Processing Time
FRS-1 25 km x 25 km 4.3 seconds 15 seconds
MRS 115 km x 115 km 17 seconds 55 seconds
CRS 223 km x 223 km 35 seconds 3 minutes
FRS-2 25 km x 25 km 4.3 seconds 25 seconds
Distributed processing using 6 high-end servers
Master-Slave architecture
Accelerated signal processing using specialized APIs (Intel MKL)
Multi-threading at core processing level
Computing Aspects
41
Imaging Mode RAW
(CEOS)
L1-SLC
(CEOS)
L1-Ground Range
(CEOS)
L2-Geo
Referenced
(CEOS &
GeoTIFF)
(Proj: UTM &
Polyconic)
L2A- Enhanced
Geo
Referenced
(CEOS &
GeoTIFF)
(Proj: UTM &
Polyconic)
FRS-1 Available Available Available Available Available
FRS-2 Available Available Not Applicable Not Applicable Not Applicable
MRS Available Not Applicable Available Available Available
CRS Available Not Applicable Available Available Available
No of Products for Each Mode
FRS1 – 11 FRS2 – 2
MRS – 10 CRS – 10
42
RISAT-1 Data Images
43
Gujarat RegionDescending Pass
Karachi RegionAscending Pass
44
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12
Bhubaneswar RegionDescending Pass
Tamli Nadu RegionDescending Pass
Chilka
Lake
45
HRS Image 1 m Resolution
FRS-1 Image 3 m Resolution
MRS Image 25 m Resolution
All India Radio (AIR), Bengaluru Viewed in Different Modes of RISAT-1
AIR
Look Direction
Look Direction Look Direction
Red : Even Bounce Green : Depolarized Blue : Odd BounceRed : HH
Green: HV
Blue: HH/HV
46
NRSC, Shadnagar, TelanganaNamma Metro Station, Pennya, Bengaluru
47
Howrah Bridge Kolkata
RISAT-1 CARTOSAT-1
48
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13
HH Gamma0
HV Gamma0
HH/HV Gamma0
49
THANK YOU
50
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