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December 11, 2009
1
Feasibility of retrieving water vapor spatial variations at epochs of SAR acquistions from SAR Interferometry
A case study based feasibility assessment
Fringe2009, Frascati, Italy
Shizhuo Liu, Ramon Hanssen
Delft Institute of Earth Observation and Space Systems
December 11, 2009 2
Revisit frequency for S1A and S1B Days per revisit
Main difficulty:
Data exploration
Spatio-temporal variation between two acquisitions instead of spatial variation at each acquisition
Objective
Retrieving APS spatial variations at epochs of SAR acquisitions from InSAR
December 11, 2009 3
To investigate the feasibility of:
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Algorithm
Step 1. network forming
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In total 33 ASAR acquisitions in descending orbit Introducing baseline constrains
(Bp < 400 m, Bt < 6 months)
Spatio-temporal network 1. remove topographic phase2. unwrap phase3. assume negligible land deformation
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Step 2. parameter estimation
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eAPSAPS sp
mp
msp +−=ϕ eAxY +=matrix form
An example:p: image pixel
Constrained least-squares
December 11, 2009 8
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Constrained observation equation
( ) '111'' yQAAQA y
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Weighted least-squares estimation (WLSE)
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Stochastic model:Variance component estimation(VCE)
pseudo-observation
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Step 3. Testing
December 11, 2009 9
0213213 ≈=++ eppp ϕϕϕmeasurement with outliers
0*
213213 >>=++ eppp ϕϕϕ
( ) yQAAQAxy
Ty
T Δ=Δ −−−∧
111''bias:
measurement noise (with least-squares adjustment):
DIA:I.Detection: overall residueII.Identification: w-testIII.Adaptation: re-estimation of x after removing outliers
measurement outliers due to e.g. unwrapping error
p21ϕ
p32ϕ
p13ϕ1t
3t2t
Why?
0312312 =++= ϕϕϕeHow?Noise free:
Step 4. Spatial interpolation and filtering
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Raw result after testing Refined result after kriging
kriging
mm mm
Case study
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Case study
December 11, 2009 12
Result
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Zenith delay spatial variation (zero mean)
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pixel resolution: 1 kmmm
date:
ground size: 80 x 80 km
sorted in terms of delay RMS
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Mean RMS: 3.6 mm
Global climate map
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Result validation
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Mean RMS: 3.6 mm
Validation 1. Bias evaluation
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== ==Δ 1
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)(σσ
= 0.76 mm
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RMS of the bias:
Validation 2. Power spectral densities
December 11, 2009 18
Regime 2
Regime 1
-5/3, 2D turbulence
-8/3, 3D turbulence
1 km10 km100 km
-8/3
-5/3
Validation 3. Temporal correlation
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Correlation matrix
Main-diagonal: auto-correlation
Off-diagonal: cross-correlation
acquisition index
Mean correlation
0.1
Temporal correlated APS
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20060117 20071113
20080401
20070904
20060606 20070313
MERIS:20060117 MERIS:20071113 MERIS:20060606 MERIS:20070313
mm
North
Validation 4. Compare to MERIS
December 11, 2009 21
mmEstimates:
MERIS:
Summary
December 11, 2009 22
Internal validations
Cross validation with MERIS– good spatial correspondence
- estimation bias should be less than 1 mm (RMS 0.76 mm)
- PSD of estimated APS shows a power-lawbehavior which agrees with the turbulence theory
- low temporal correlation
Conclusion• It is feasible to retrieve APS spatial variation at the
epochs of SAR acquisitions, provided that:
1. Sufficient number of SAR acquisitions (reduce estimation bias)rule of thumb:
2. Short repeat orbit (prevent significant temporal decorrelation)every 6 days for Europe and Canada with Sentinel-1A/B
3. Larger critical baseline (a network with more acquisitions)>> 1100 m for Sentinel-1A/B (4 m range resolution)
4. Negligible or known deformation
December 11, 2009 23
Nbias 1
∝