sar - systems calibration - prof. dr. wolfgang keydel - zur...
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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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2Institut für Hochfrequenztechnk und RadarsystemeQue
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2Institut für Hochfrequenztechnk und Radar
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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5Institut für Hochfrequenztechnk und Radar
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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6Institut für Hochfrequenztechnk und RadarsystemeQue
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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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8Institut für Hochfrequenztechnk und RadarsystemeQue
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8Institut für Hochfrequenztechnk und Radar
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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9Institut für Hochfrequenztechnk und RadarsystemeQue
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9Institut für Hochfrequenztechnk und Radar
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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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14Institut für Hochfrequenztechnk und Radar
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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22Institut für Hochfrequenztechnk und Radar
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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30Institut für Hochfrequenztechnk und Radar
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31Institut für Hochfrequenztechnk und Radar
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, like- antenna 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