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”

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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

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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

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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

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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.

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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.

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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

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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

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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

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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

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RISAT-1 Data Images

43

Gujarat RegionDescending Pass

Karachi RegionAscending Pass

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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

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HH Gamma0

HV Gamma0

HH/HV Gamma0

49

THANK YOU

50