SAR - Systems Calibration - Prof. Dr. Wolfgang Keydel - Zur …keydel.pixelplaat.de/uploads/File/erlangen 07-08... · · 2008-02-05•Development of calibration models ... B = Interferometric

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1Institut für Hochfrequenztechnk und RadarsystemeQue

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1Institut für Hochfrequenztechnk und Radar

SAR - Systems Calibration

Vorlesung: Hochauflösende RadarsystemeWS 2007/08,

Friedrich-Alexander-Universität Erlangen NürnbergLehrstuhl für Hochfrequenztechnik

Wolfgang KeydelDLR Oberpfaffenhofen, Institut für Hochfrequenztechnik und Radarsysteme

e-mail: [email protected], Web: http://www.keydel.com

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Calibration

Relates the SAR image intensity to radar backscattering coefficients σ & σ0providing information about the accuracy of this relationship.

Estimation and removal of all system-related influences results in pure object signatures.

GoalModel the relationship between

geophysical parameters & measured backscattering coefficients.

Quantitative analyses & the development & interpretation of models in different geophysical applications require calibrated data.

Quality of these RCS maps is essentially defined by the capability to determine the radiometric characteristics of the radar system. These are the

absolute and relative radiometric accuracy and the radiometric stability.

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An overall system calibration includes:• Internal calibration• External calibration• Compensation & correction for system errors in amplitude & phase

• Geo-referenced transformation of SAR image data to backscatter coefficients within an estimated error

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Efforts performed by Calibration

Compensation of instrument fluctuationsperformed by in-flight verification of the instrument against pre-flight results.

This internal calibrationyields a stabilized radar instrument & defines the radiometric stability.

Compensation of the antenna patternin order to obtain a constant gain across the whole SAR image by

determination of the actual antenna patternleading to relatively calibrated SAR data products

Correction for the radiometric biasby measuring the radar system against standard ground targets

This external calibration yields to an absolute calibrated radar system and defines the absolute radiometric accuracy.

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Processing Chain

• Kinematic postprocessing of GPS flight path data including IMU data• Antenna data generation• Computation of motion irregularities• SAR focusing considering motion irregularities• Determination of Coregistration Error • Calculation of interferometric phase and coherence• Phase unwrapping• Absolute phase calibration• Phase to Cartesian coordinate conversion• Terrain geocoding of amplitude data• Radiometric calibration• Mosaic of geocoded products

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Radiometric Calibration Base: Radar Equation

Proper determination of θ irge = local incidence in range, θiaz = local incidence in azimuth;

δrge = image pixel dimension in range; δaz = image pixel dimension in azimuthThese quantities depend upon:

• real antenna position• real antenna pointing direction• pixel position on the ground

( ) ( )( )

( ) ( )azrge

iazirge0Haz

2AazHaz

2Aaz

0s

33ocPrrecel

3Haz

2AazHel

2Argetr

rec

sinsin and dGG

LR4GGGGP

P

Haz

Hazδδ

ϑϑσσθθθ

σπ

λθθ

θΔθ

θΔθ==

=

∫+

−

)

)

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External Calibration On-Ground Calibration Targets with known RCS

Corner Reflector

Transponder

2

4

34

λπσ a=

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σ= 42,9 m2

16,3 dBm2

σ= 153,3 m2

21,9 dBm2

σ= 402,3 m2

26,0 dBm2

σ= 1221 m2

30,8 dBm2

σ= 1676 m2

32,2 dBm2

40 cm55 cm

90 cm100 cm

70 cm

2

4

34

λπσ a=

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Example: Calibration Field for Scan SAR, ENVISAT/ASAR

Ascending

Passau

Basel

Descending

Strasbourg

Salzburg4

5

12

3

T T T TT T T T

T

wide swath405km

ENVISAT

asc

sub-swath65km - 108km

calibration field(CALIF)

Enclosed Area~ 405km

T: transponder

simplifiedelevationpattern

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Measured Azimuth Antenna Patterns of ASAR/ENVISAT

M . S c h w e r d t

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a)Uncalibrated Slant Range Amplitude Data, b) Terrain geocoded c) Calibration FactorF. Holecz 1, P. Pasquali 1, J. Moreira 2, D.Nüesch: RIGOROUS RADIOMETRIC CALIBRATION OF AIRBORNE AeS-1 InSARDATA Proc. IGARSS July 1998, Seattle, USA,

a b c

Radiometric Calibration Example, Air-borne SAR (AES)

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X-SAR/SRTM Error Budget

Performance Requirements

• Relative Height Accuracy(90 %) <6 m

• Absolute Height Accuracy(90 %) <16 m

X-SAR/SRTM Height Error Sources

•Baseline Tilt Angle

•Baseline Length

Instrument Phase

•Random Phase

Ambiguity Phase

• Atmosphere

• Position

• Calibration

• Slant Range

• Processing

14,4 m5,5 mTotal4,2 m4,o deg4,2 m4,0 degInstrument Phase2,6 m4,0 mm0,8 m1,3 mmBaseline Length13,4 m9 arcsec3,0 m2 arcsecBaseline Tilt AngleErrorAccuracyErrorAccuracyError Type

Absolute (11 Days)Relative (30 seconds)Height Error Examples (Middle of Swath)

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Calibration Concept

Ocean Sea Level

Calibration

•Estimation of systematic errors•Monitoring of system parameters and instrument performance•Characterization of instrument parameters•Development of calibration models (parameter drifts as a function of time and temperature )•Ocean as reference height (sea surface height model)

Known orbit with respect to WGS 84 ellipsoid

Ground Control

PointOcean Sea

Level Calibration

Measured height

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X-SAR/SRTM Calibration Phases

• Preflight concept definition phase including sensor characterization,calibration algorithm development and implementation

• Ground campaigns during the mission

• 6 months commissioning phase: generation of static and dynamic calibration files, analysis and modeling of parameter drifts with temperature and time

• Operational calibration and validation

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17Institut für Hochfrequenztechnk und Radar To Origin of WGS84

:

B = (

)

-A Pi Po )

= (P -WGS

GPSxP

WGSicsM

ICSocs

M+

ICSocsM

WGSicsM A- Pi+Gx

- Ei ( Eo+

Ei+

GPS Antenna 1

GPS Antenna 2

Inboard C-Band Phase Center

Inboard Coordinate System Origin

Outboard Coordinate System Origin

A

Pi

G2

Outboard C-Band Phase Center

B

Po

Inboard Area Centroid

Eo

Ei

To Origin of WGS84

B = Interferometric Baseline VectorP = Phase Center Position VectorA = ICS to OCS Origin VectorP = Location of GPS Antennas in WGS84G = GPS Antenna ICS/OCS LocationP =Antenna Area Cenroid LocationE = Offset Vectors between Area Centroids

Antenna Phase CentersM = Rotation Matrices to convert between

Coordinate Systems Responsibility Color Codes

---- Structure & Mechanics---- AODA---- C-RADAR---- Ground Data Processing

SRTM Baseline Measurement Breakdown

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6m Antenna & 6 LNA’s

Outboard Receive Channel

Radio Frequency Electronics

Inboard Receive Channel

12m Antenna

Combiner

Down Conversion

RF Adaptor Electronics

IF&Demodulator Electronics

9602 MHz

Mast Cables

I&Q Signals

1052 MHz

135 MHz 263 MHz

CAL Tone

X-SAR/SRTM Receive Channels with Calibration Tone

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Correction & Calibration of the X-SAR Data

Critical Parts of the X-SAR Radar electronics

• Phase variation of radar receive signalin six individual paths:antenna panel to XCB including LNA’s& phase shifters

• Down-conversion using 263 MHz signalgenerated in RFE (1052 MHz) & distributedover the mast to XDC, ± 4 deg phase variationat 263 MHz multiplied by 36 in down-conversion

•Phase variation of radar receive signal running at 135 MHz over the mast cable: ± 2deg over a temperature range between -10oC and -50oC

Antenna 2

XCB

XDC

RFE

RAE

IFD

Antenna 1

Boom135MHz

1052MHz

263MHz

WG

Secondary Channel Primary Channel

STALO

X Combiner Box

X Down Converter

IF & Demodulator Unit, RF Adaptor Electronics, RF Electronics

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X-SAR/SRTM Swaths over Bavaria (Calibration site)

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X-SAR/SRTM Calibration Sites

D1-D18 1.5 m Corner reflectors GermanyD19-D24 3m Corner reflectorsD25-D27 RU1-Russia-Kitab-1 RU2-Russia-Zelenchukskaya-2RU3-Russia-Zvenigorod-3RU4-Russia-Irkutsk-4RU5-Russia-Bear_Lakes-5CH1-5 Switzerland-1-5SA1-2 South-Africa-1-2 NO1-2 Norway-1-2 EG1-3 Aegypten-1-3 BK1-6 Baikal-1-6KG1-Kirgisien-1-4 IS1-2 MIRBATS-ISRAELIS3-673-HILL-ISRAELIS4-MITSPE-ZOHAR-ISRAELIS5-HIDDEN-HILL-ISRAELIS6-SEDE-ZIN/minhat-north-ISRA IS7-SEDE-ZIN/minhat-south-ISRA IS8-BEER-SHEVA/goral-ISRAELIS9-BEER-SHEVA/hatserim-ISRAEL

2

4

34

λπσ a=

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Remote Sensing Technology Institute

Courtesy JPL

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AODA System OverviewInstruments Function Accuracy Rate

X Y ZGPS2 GPSR &4 GPSA

C-band area centroids (ACS and OCS) state vectors- Position (WGS 84)- Velocity (WGS 84)

Time tag

1m0.05m/s

1m0.05m/s

1m0.05m/s 1 Hz

Star TrackerAssembly (STA)

Estimates inertial attitude of ICSSTA boresight orientation wrt inertial space

InertialReferenceUnit (IRU)

Propagation of inertial attitudes between STA updates

roll5.0

0.05arcsec

pitch36.0

0.05arsec

yaw5.0

0.0.5arcsec

1 Hz

ASTROS (ATT)& Optical TargetAssembly (OTA)

Estimates the relative attitude and position of the OSS ->C-InSAR baseline and support antenna alignment

Tracks 3 LED targets at 60 m distance0.8

arcsec- 0.8

arcsec4 Hz

ElectronicDistance Meter(EDM)

Distance measurement to OCS and X-SAR backstructure

- 2 for ICS to OCS vector length determination (red.)- 2 for ICS to inboard X-SAR area centroid Y & Z offsetdetermination

-0.5mm

0.5 mm -0.5mm

0.5Hz

Courtesy JPL

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Geometric Calibration

Differential Range DelaysUsing Data itself by Cross Correlation of Speckle Patterns between the

two Interferometric Channels

Common Range Delay, Time Tag/Velocity BiasesUsing Corner Reflectors, Calibration Sites: Oberpfaffenhofen, Mojave Desert, Australia

Baseline Length/Tilt, Orbit, Phase Offsets: Estimation from Short Ocean Data Takes before & after Ocean-land Crossing or from Ground Control Points

Residual Phase Errors (e.g. Multipath) & System Stability: Long Ocean Data Takes at the Beginning & End of the Mission

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Required data for X-SAR calibration/validation

DLR/UserERS-1altimeter dataTOPEX-POSIDON data

AODA

MPOS

X-SAR

DLR/User

X-SAR

DLR/User

DLR/User

GCP data base

High precision DEMs

Tide tables

Geoid undulations

AODA-PADR file

X-SAR HK data

Ocean datatakesCalibration test sites(CRs)

Cal tone

Maps

GDPS X-GDPS

NIMA/JPL

DLR/User

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INSTRUMENT CALIBRATION

Interferometric phase:

( )( )

(CT1attCTtestcpRAEtestcp1ant

,LNAXCBi,switchi,LNAant

outboom

i,2CT1CT

21int

615.36

61

−−−−

−−

−

Φ+Φ−Φ−Φ+

Φ−Φ−Φ−

Φ+

Φ−Φ+

Φ−Φ−=Φ

∑

∑

• caltone estimation

• phase variation of 263MHz signal on boom cablefrom phase detector

correction terms from preflight instrumentcharacterization

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X-SAR/SRTM Swaths over Bavaria (Calibration site)

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Ocean Calibration

• Height error:

truthgroundmeas hhh −−=δ

• Error contributions from baseline length/tilt, phase and orbit height offsets

[ ]⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

δΦδδαδ

δδδδ=δ Φα

H

b

h H,h,h,hb,h

• MAP estimation based on prior information provided by AODAproblem sufficient SNR at 55deg incidence angle ?

Two iterations to determine static and dynamic calibration file

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SRTM Calibration:measured Height over Ocean

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Features/Operation Modes of TerraSAR-X

large number of:

calibration majorchallenge

costsaffordable

single/dual polarisation

wide range of swathpositions (20° - 55°)

3 basic operation modes

4 Strip Map (26 look angles)

4 ScanSAR (WS à 4 SS)

4 Spotlight (123 look angles)left/right looking SARexperimental modes

4 wide band width

4 along track interferometry

activeantenna array

• operation modes

• antenna beams

26 elevation beams

123 elevation beams à100 azimuth patterns

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TerraSAR Calibration Concept

Internal Calibration PN-Gating Method In-Orbit Analysing ofIndividual TRM’s

Antenna Pattern Precise Model Estimation of ActualAntenna Pattern

Measurementsas much as possible

Reliable ReferenceSufficient Numberof Ground Targets

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Calibration

RadiometricCharacteristics

Antenna PatternCompensation

RelativeAccuracy

imagemagnitude

calibration physicalunits

InternalCalibration

Stability

BiasCorrection

AbsoluteAccuracy

TerraSARCalibration Conzept & Philosophy

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Template

side lobe region main lobe region

Challenge of Antenna Pattern Optimisation

drifted or failedT/R modules

antenna array T/R modules Elevation Pattern and

side lobe region main lobe region

bestbeam steering

optimization ofremaining modules

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TerraSAR Internal Calibration

3 Calibration Pulses:

– 1 Transmit Path1-2-4-5

– 2 Receive Path 6-4-2-1

– 3 Only RFE/DCE 6-5

controllingthe instrument

as a whole

individual modules?

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TerraSAR Internal Calibrationcritical part of TerraSAR-X : is the active antenna, X-Band Front-end (XFE) consisting of 384 transmit/receive (T/R) modules each feeding a radiating sub-array for horizontal and vertical polarisation.critical elements & parameters of the XFE are monitored. For thisthree different types of calibration pulses are applied, whereby sets of these pulses areneeded at the start and end of each data take. All calibration pulses have the same lengthand bandwidth (chirp) as is commanded for the mode.Transmit Signal Path (1-2-3): During imaging transmit pulse is routed from the RF electronics (1) through the transmit/receive (Tx/Rx) distribution network where it isdivided up and connected to the T/R modules (2). There it is amplified before beingappliedto the radiator sub-array (3) for transmission.• Receive Signal Path (3-2-1): The radar echo received by the antenna is passed to the T/R modules (3), where it is amplified. The signals from all the modules are combined in theTx/Rx network and fed to the RF Electronics. Fromthere it goes to the Digital ControlElectronics (DCE) where it is digitised and formatted, before being stored in the Solid State Mass Memory (SSMM)Correcting Calibration Path (6-5): Calibration Pulse 3 is for monitoring the receive paththrough RF Electronics and Digital Control Electronics, excluding the XFE. It is needed forcorrecting the Calibration Pulse 1 and 2.

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PN – Gating Principle

Overall signal:

Complex signal of jth T/R module:

Information extraction for one T/R module: correlate overall signal is with respective PN sequence :

this decoding process removes the PN modulationthe complex correlation peak is an estimation of amplitude & phase settings of the respective T/R module

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Superposition of all T/R Signals

s1 c1

s3 c3

sN cN

s2 c2

)t(c)t(s)t(S i

N

1i

ic

R/T

⋅=∑=

1 2 3 3 4 5bit t

correlationwith the

inherent

Module code ci

extraction of individual

module signalsi^

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PN-Gating Technique characterizing individual T/R modules while all modules are

operating, i.e. a characterization under most realistic conditions.

Phase of each T/R moduleindividually shifted about

±π/2between Cal-pulses

i.e. phase shift keying according

to a PN code sequenceEach T/R module

has a different PN codeor

code shift resp

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Standard Deviations versus Number of T/R ModulesS/N =20db, 30 Modules, Codelength=1023

For larger Number of T/R Modules code length wouldneed to be increasedcorrespondingly to achieve thesame errors. But using of an ideally orthogonal code(Walsh sequence f.e.) the cross correlation of two distinctsequences is 0. Thus the codelength can be reducedconsiderably down to 512 for384 T/R modules.

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Antenna Pattern ModelGoal:

Derive all reference pattern required for SAR data processing; i.e.providean accurate estimation of the actual antenna pattern with a minimum

number of costly in-flight antenna pattern measurements.

Pre-flight Characterization: calculating & analyzing antenna patterns of thedifferent operation modes as well as on-ground antenna pattern measurementslike those of a single radiation element or individual rows or panels.

In-Orbit Verification: in-flight antenna patterns measured in use of ground receivers & by rainforest measurements (expensive!).

Internal Calibration: PN-gating method, an antenna pattern can be calculatedwith the actual excitation coefficients of the individual modules.

Module Failure Analysis: in case of contingencies, for example failed T/R modules, an optimization of the excitation coefficients of the remaining modulesis intended in order to obtain the best beam of each operation mode.

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Antenna Pattern Model

Estimation ofActual Antenna Pattern

Antenna PatternModel

Reference Patternfor Processing

Synthesis

Analysis

On-GroundMeasurements

Pre-FlightCharacterisation

In-FlightMeasurement

RainforestMeasurement

In-Orbit Verification Internal Calibration

PN-Gating

ModuleStepping

Module Failure

AntennaOptimisation

Synthesis

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Essential TerraSAR-X calibration facilities :

• standard ground targets for the bias correction,• ground receivers for in-flight measurements,• different analysis and evaluation tools, likeantenna pattern model providing an estimation of the actual antenna pattern or

• an appropriated SAR processor, as there are two types of calibration data that have to be processed:

– SAR images covering deployed calibration targets,– special calibration products like PN-gating method

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Essential TerraSAR-X calibration facilitiesStandard ground targets for bias correction

Ground receivers for in-flight measurements

Different analysis & evaluation tools, likeantenna pattern model providing to estimate the actual antenna pattern- SAR Product Control Software (SARCON) for target analysis of different calibration targets

Appropriated SAR processor, for the two types of calibration data to beprocessed:– SAR images covering deployed calibration targets,– Special calibration products like these of the presented

PN-gating method.A major challenge for calibrating the TerraSAR-X instrument. Is

keeping the cost affordable

Documents

SAR - Systems Calibration - Prof. Dr. Wolfgang Keydel - Zur …keydel.pixelplaat.de/uploads/File/erlangen 07-08... · · 2008-02-05•Development of calibration models ... B = Interferometric