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TanDEM-X: A Satellite Formation for High Resolution SAR Interferometry
G. Krieger, A. Moreira, H. Fiedler, I. Hajnsek, M. Eineder, M. Zink, M. Werner
German Aerospace Center (DLR)
Microwaves and Radar Institute
Postfach 111682230 Weßling, Germany
e-mail: [email protected]
December 2005
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[email protected] 3German Aerospace Center Microwaves and Radar Institute
TanDEM-X Timeline
2015201420132012201120102009200820072006200520042003
German Call for Proposals for a Future Earth Observation Mission
Phase A Study
Selection of TanDEM-X for Phase A Study
Final Decision
TerraSAR-X Operation
TanDEM-X Operation
Phase B/C/D
All TanDEM-X mission objectives are achieved within 3 years of joint operation
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Investigations in Phase A Study
Derivation of User and Mission Requirements:
• Organisation of Science Team (currently 97 members)
• Scientific and Commercial User Survey
• Product Definition (DEM, radar data products, …)
• Performance Analyses (DEM, PolInSAR, GMTI, DBF, …)
• Mission Planning and Data Management
• PRF and Phase Synchronisation (design upgrade)
• Close Formation Flying (collision avoidance)
• Precise Baseline Determination (double difference GPS)
• Bi-Static and Interferometric Data Processing
• Interferometric Calibration (tie points, crossing orbits, …)
• …
Derivation of User and Mission Requirements:
• Organisation of Science Team (currently 97 members)
• Scientific and Commercial User Survey
• Product Definition (DEM, radar data products, …)
• Performance Analyses (DEM, PolInSAR, GMTI, DBF, …)
• Mission Planning and Data Management
• PRF and Phase Synchronisation (design upgrade)
• Close Formation Flying (collision avoidance)
• Precise Baseline Determination (double difference GPS)
• Bi-Static and Interferometric Data Processing
• Interferometric Calibration (tie points, crossing orbits, …)
• …
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•system parameters:Bcross = … mBalong = … m…
•deliverable data:SLC SAR imagesSAR raw dataTanDEM-X
interferogramsorbit state vectors
Product and Mission Definition
• predefined accuracies:∆h = 2 m @ ∆x = 12 m∆h = 4 m @ ∆x = 6 m∆h = 1 m @ ∆x = 25 m∆h = 0,5 m @ ∆x = 50 m
• deliverable data:standard DEMdetected SAR imagescoherence mapsheight error maps
Standard DEMStandard DEM(global HRTI)(global HRTI)
• customised accuracies:∆h = … m @ ∆x = … m
• temporal requirements different seasonsdifferent years
• deliverable data:customised DEMdetected SAR images coherence mapsheight error maps
CustomisedCustomised DEMDEM(local)(local)
Radar Data Radar Data ProductsProducts
7%
76%
17%
fullypartially not User Survey:
• application areas• basic user needs• technical requirements• …
Product Product andand
Mission Mission DefinitionDefinition
Compliance
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HRTI-3 DEM Definition
Spatial Resolution Absolute Vertical Accuracy (90%)
Relative Vertical Accuracy (point-to-point in 1° cell, 90%)
DTED-1 90m x 90m < 30 m < 20 m
DTED-2 30m x 30m < 18 m < 12 m
HRTI-3 12m x 12m < 10 m < 2 m
HRTI-4 6m x 6m < 5 m < 0.8 m
SRTM / X-SAR E-SAR
~ DTED-2 ~ HRTI-3
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linear phaseAntenna Tapering
12 m x 12 mPost Spacing 30 kmSwath Width< 1 kmAlong-Track Displ. 500 mBaseline (perp.)
Ulaby (X-Band, VV, Soil)
Sigma Nought Model (90% occurence)
4 bit (BAQ)Quantization1/10 pixelMis-Registration2266 HzProcessed Bandwidth
~3500 HzPRF (swath variant)
4.8 m x 0.7 mAntenna Size (Tx, Rx)4.1 dBLosses (rad., atm.,..)4.3 dBNoise Figure TRM18 %Duty Cycle
2260 WPeak Transmit Power≤150 MHzChirp Bandwidth0.031 mWavelength
Relative Height AccuracyValueParameter
Relative Height Accuracy (B = 500 m)
90% point-to-point errors
σ (standard deviation)
Bistatic Strip mapB = 500 m ∆x = 12 m
Bistatic Strip mapB = 500 m ∆x = 12 m
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linear phaseAntenna Tapering
12 m x 12 mPost Spacing 30 kmSwath Width< 1 kmAlong-Track Displ. 1000 mBaseline (perp.)
Ulaby (X-Band, VV, Soil)
Sigma Nought Model (90% occurence)
4 bit (BAQ)Quantization1/10 pixelMis-Registration2266 HzProcessed Bandwidth
~3500 HzPRF (swath variant)
4.8 m x 0.7 mAntenna Size (Tx, Rx)4.1 dBLosses (rad., atm.,..)4.3 dBNoise Figure TRM18 %Duty Cycle
2260 WPeak Transmit Power≤150 MHzChirp Bandwidth0.031 mWavelength
Relative Height AccuracyValueParameter
Relative Height Accuracy (B = 1000 m)
Bistatic Strip mapB = 1000 m ∆x = 12 m
Bistatic Strip mapB = 1000 m ∆x = 12 m
Bistatic Strip mapB = 1000 m ∆x = 12 m
Bistatic Strip mapB = 1000 m ∆x = 12 m
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Phase Unwrapping
TanDEM-X enables large baselines which allow for ultra high resolution DEMs withheight accuracies in the sub-meter range, but …
πϕ∆ 2=(height of ambiguity)
Compromise on Accuracy for Global DEM Local/Regional Ultra High Resolution DEMs
• use reduced baselines• additional acquisitions
for difficult terrain
acquisition scenariofor global DEM according to HRTI-3
• use multiple dataacquisitions with large and small baselines
regional DEMs withsub-meter resolution(e.g. HRTI-4)
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~ 70 %Global HRTI-3 DEM(incl. multiple acquisitions for difficult terrain)
TanDEM-X Mission Scenario for 3 Years
~ 30 %Additional Applications(local HRTI-4, ATI, new techniques, …)
Total Landmass:~ 150 Mio km2
TanDEM-X:~163 Mio km2/year
(stripmap, 140s/orbit)
1 - 4
4(+ asc. / desc.)
2(+ different hamb)
1(hamb ~ 35 m)
Number of Acquistions
~ 11 months< 20 %MountainousAreas
~ 26 months(incl. margin)
100 %Total
~ 8 months30 %Hilly Areas, Tall Forests
~ 7 months50 %ModeratelySloped Areas
Required Time(without RDP)
Percentage of Total LandmassTerrain Type
TanDEM-X Data Acquisition Strategy
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TanDEM-X Satellite Formation: HELIX
HELIX satellite formation enables safe operation• horizontal cross-track separation at equator by different ascending nodes• vertical (radial) separation at poles by orbits with different eccentricity vectors
(periodic motion of libration is compensated by regular manoeuvres)
horizontalbaseline
verticalbaseline
NH(desc.)
SH(asc.)
© Moreira,Krieger,Mittermayer, 2003
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Baseline Estimation and DEM Calibration
• Both satellites are exposed to almost identical orbit perturbations
- negligible azimuth modulation / twisting of DEM swath (∆B < 0.25 mm for 500 km swath and ‘unmodelled’ δa < 100 nm/s2)
- vertical bias and tilt of raw DEM swaths due to initial baseline estimation errors
• Precise baseline estimation by
- double-difference GPS carrier-phase measurements
- accurate orbit propagation model
- several studies predict a 3-D accuracy in the order of 1-2 mm
baselinevector
jAρ
jBρ
kAρ
kBρ
jk
GPS satellites
~ 1-2 mm
DEM
3mm/km1mm∆B
≈∂∂
=xh
1m/mm≈∂∂Bh
calibration with overlapping swaths, crossing orbits, external references, etc.
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Impact of Oscillator Noise
Major effects:• interferometric phase errors
(azimuth ‘modulation’ of DEM)• azimuth displacement • increased azimuth sidelobes• range drift (walk of receiving window)
m
m
X
Txτ
Rxτ
independentoscillators
Tx
Rx
≠
increasedsidelobes
mainlobedispersion
azimuthdisplacement
interferometric phase errors
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Phase Referencing in TanDEM-X
phase referencing can achieve a shortterm rmse below 1° in standard DEM acquisition mode by integrating multiple sync pulses (for B < 1 km)
Synchronisation LinkSynchronisation Link Analysis of Residual ErrorsAnalysis of Residual Errors
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S I R E V R a w D a ta
C h i r p S c a li n g
R a n g e F F T
R a n g e IF F T
R C M C o r r e c ti o n
R a n g eC o m p r e s s i o n
S e c o n d a r y R a n g eC o m p r e s s i o n
A z im u t h F F T
D e r a m p in g
A n t e n n a P a t te r nC o r r e c ti o n
P h a s e C o r r e c t io n
A z im u t h S i d e -l o b e S u p r e s s i o n
A z i m u t h IF F T
A zi m u t h F F T
S I R E V I m a g e
A z i m u t h S c a lin g
L O S - C o r r e c t i o n a n d F i r s t O r d e r M o t i o n C o m p e n sa t io n
A z i m u t h F F T
A zi m u t h IF F T
S e c o n d O r d e r M o ti o nC o m p e n s a t i o n
B0
CA
T s
τ
t
0f a
AC
Bτ
f a0
f r
C B A
0C B
A m p lit u d e
0C B
A m p lit u d e
B
0
AA m p l it u d e
0
BCA m p lit u d eI n t e g e r
R e s a m p li n g
Secondary Mission Goals & New Techniques
Pol-InSAR
Digital Beamforming
Bistatic Observations
Multi BaselineInSAR
Super Resolution
Along-Track Interferometry(fully polarimetric !)
(4 phase centres !)
(large bandwidth !) (bistatic angle !)
(HELIX formation !)
(flexible baselines !)
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Along-Track Interferometry
r+∆r r
t+∆t t
HELIX formation enables:• short along-track baselines(arbitray satellite shifts along the orbits)
• vanishing cross-track baselines(for specified latitude/incident angle combinations)
Example (bistatic mode):• Balong = 100 m• posting = 10 m• σ0 = -12 dBm2/m2
• θinc = 45°• vamb = 11 km/h
dv ~ 0.15 km/h (stdv.)
Balong
HELIX
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SAR Imaging with four Phase Centres
sensitive to fast movements
sensitive to slow movements
highly accuratevelocity estimatesfor slow and fast object movements
short baseline(∆t ≈ 0.2 ms)
long baseline(∆t ≈ 10-200 ms) split antenna
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SAR Imaging with four Phase Centres
without reconstruction with reconstruction
Ch. 2Ch. 1
SAR Proc.
P2(f) P3(f)P1(f) P4(f)
EnablesHigh
ResolutionWide
Swath Imaging
Ch. 3 Ch. 4
AmbiguitySuppression
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PolInSAR Example: Sunflower
polarisations
pdf(µmin)
pdf(µmax)
Parametersh = 1.2 mβ = 4.0 dB/mµmin = -7.0 dBµmax = 3.0 dBB⊥ = 4 kmθinc = 35°∆x = 30 m
Parametersh = 1.2 mβ = 4.0 dB/mµmin = -7.0 dBµmax = 3.0 dBB⊥ = 4 kmθinc = 35°∆x = 30 m
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“Double Differential SAR Interferometry”
h(t1)
∆h ~ ϕ2 - ϕ1
Grounding line detection, vegetation growth, snow/ice accumulation, … ?
e.g. difference between two single-pass cross-track interferograms
coherence between passes not mandatory
ϕ1pass 1
pass 2
∆h < 10 cm h(t2)
ϕ2
Bistatic Strip mapB = 3000 m ∆x = 12 m
Bistatic Strip mapB = 3000 m ∆x = 12 m
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TanDEM-X Summary
• TanDEM-X passed a phase A feasibility study with great success
• TanDEM-X has outstanding scientific and commercial potentials
• TanDEM-X will be implemented as a public private partnership
• TanDEM-X key technologies are:
– bistatic radar operation and phase synchronisation
– precise baseline determination
– close formation flying capability
– new algorithms for interferometric processing
• TanDEM-X plays a key role in the development of next generation SAR missions