IGARSS 2011, Vancouver, Canada

IGARSS 2011, Vancouver, CanadaSession: TU4.T02.2 - Ionospheric Effects on SAR, PolSAR and InSAR, Tuesday, July 26, 15:20 - 17:00

Ionospheric Effects in SAR Interferometry: An Analysis and Comparison of Methods for their Estimation

Ramon Brcic1, Alessandro Parizzi1, Michael Eineder1, Richard Bamler1 and Franz Meyer2

1 Remote Sensing Technology Institute (IMF), German Aerospace Center (DLR)2 Geophysical Institute, University of Alaska, Fairbanks

Institut für Methodik der Fernerkundung

Institut für Methodik der Fernerkundung bzw. Deutsches Fernerkundungsdatenzentrum

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IGARSS 2011, Vancouver, CanadaSession: TU4.T02.2 - Ionospheric Effects on SAR, PolSAR and InSAR, Tuesday, July 26


Contents

Ionospheric Effects in InSAR

Estimating TECSplit Spectrum Method

Range Group – Phase Delay Method

Theoretical Performance

Experiments with ALOS-PALSAR acquisitions

Summary & Conclusion



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Ionised gases at 50 – 1000 km

Spatial variations: typically over >100 kmeffectively constant over SAR scene dimensions

Temporal variations: daily, seasonally, solar cyclesun-synchronous orbits reduce temporal variation

Scintillation: rapid temporal & spatial changes

Yearly Average 2010

06:00 local, descending, 0 – 10 TECU

18:00 local, ascending, 0 – 30 TECU



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Dispersive medium for SAR signals

Group Delay, Phase Advance, Faraday Rotation

SAR images: location, phase shifts constant TEC STEC increases from near to far range group delay & phase advance increase

Repeat-pass interferogram: focus on Ionospheric Phase ScreenTEC ΔTEC (master – slave TEC)constant ΔTEC phase gradient in rangespatial variations in ΔTEC modulate ionospheric phase

22

cos

group phase

inc

STECK

f

TECSTEC

[m]

(Slant range TEC)

Ionosphere ~100 – 1000 km

equivalent SAR model thin layer, barycenter 400 km

TEC 13±0.2 STEC 16±0.5

07-07-2007, 18:30 local time68° W 25° S, scene 32 x 57 km



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Band P L C X

Carrier Frequency 435 MHz 1.27 GHz 5.405 GHz 9.65 GHz

Agency ESA NASA/DLR JAXA ESA DLR

Mission or Sensor BIOMASSDESDynI/

TerraSAR-LALOS-PALSAR Sentinel-1 TerraSAR-X, TanDEM-X

Range Bandwidth [MHz] 6 80 14, 28 100 100, 150, 300

Range Delay [m / TECU]

5.2 0.61 0.034 0.011

Interferometric Phase Advance[cycles / TECU]

7.5 2.6 0.61 0.34

Assumed Swath Width [km] 100 50 250 30

Interferometric range phase change over swath for constant ΔTEC

[cycles / TECU]0.89 0.15 0.18 0.012

Existing and future SAR systems: interferometric phase sensitivity to VTEC at 35° incidence angle



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Estimating TEC / Ionospheric Phase Screen

Global Ionospheric Models (GIMs) from GPSLow res ~100s of kms, little or no spatial resolution over SAR image, ~1 TECU RMSE

Single Image TechniquesAutofocus, Faraday Rotation (Polarimetric data, latitude dependent)

InSAR TechniquesSplit Spectrum or Delta-k [Rosen, Freeman, …]

exploit different behaviour of dispersive/nondispersive components in frequency

Range group – phase delay [Meyer, Bamler]nondispersive components have same sign, dispersive components have opposite sign

Subband correlationalso exploits differing dispersive/nondispersive frequency behaviour, low resolution



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interferometric phaseat carrier frequency

11

1

22

2

1 2 2 1 1 22

2 12

topo iono CC C

C

topo iono CC C

C

ionoC

C

ff

f f

ff

f f

f f f f

f f f

Subband Range Spectra

Lower Subband Upper Subband

2f

b

Cff

1f

B b

2 2 topo ionoC

C CC

f R K STEC

c cf

Split Spectrum Method

non-dispersive topography, atmosphere

dispersive ionosphere

C

Optimal subband bandwidth?

b = B / 3(same as split spectrum/delta-k absolute phase estimator)



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unwrapped interferometric phase at carrier frequency

shift from crosscorrelation between master and

slave

2iono C RC

2 2

topo ionoC C C

C

topo topo iono ionoR R C R C

R K STEC

cf

non-dispersivetopography, atmosphere

phase delay = group delay

dispersive ionosphere

phase delay = – group delay

take differenceperform averaging



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Split spectrumSubband center frequency error couples dispersive & non-dispersive components

Better performance at low carrier frequencies & high bandwidths

Range group-phase delayPerformance determined by crosscorrelation based group delay estimate

(In)Coherent CC σ is (1.5)2 x less than split-spectrum

22 13 3 1 [ ]

16Cfc TECU

K B N

erroriono iono topoC C Cf

range spectrum with nonuniform weighting

f



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Theoretical standard deviation of split-spectrum ΔSTEC and ionospheric phase for various SAR systems. Averaging over constant area of 1 km slant range x 1 km azimuth.

Mission, SensorAzimuth Resolution

[m]Range Bandwidth

[MHz]Resolution Cells

AveragedRange Phase Change

[cycles / TECU]

BIOMASS 12.5 6 3.2k 0.27 0.89

ALOS-PALSAR4.5 14 21k 0.13

0.154.5 28 42k 0.047

DESDynI / TerraSAR-L 10 80 53k 0.015

Sentinel-1 6 100 110k 0.034 0.18

TerraSAR-X3.3 100 200k 0.045

0.0123.3 300 610k 0.0087

( ) at 0.7ionoC



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Comparison

Split spectrum Range group-phase delay

Computational Performance

Filtering & InSAR processing of subbands

Minimal extra effort

group delay estimates from coregistration

phase delay estimates from fullband PU

Statistical Performance

Better statistical performance (theoretically)

Error Behaviour

Identical workflow for both subbands

common non-dispersive errors cancel

Different workflow for group & phase delay estimates

common non-dispersive erros may not cancel

2 x subband PU

more chance of error

1 x fullband PU

less chance of error

Sensitivity to range center frequency error



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Experiments

L-Band ALOS-PALSAR acquisitions over Alaska known to contain significant ionosphere (provided by Americas ALOS Data Node (AADN) and JAXA)

PALSAR PLR-mode, 14 MHz quad pole, HH channel

1. Good coherence150.21 W, 69.96 N01-04-2007, 17-05-2007

2. Poor coherence147.40 W, 66.62 N01-04-2007, 17-05-2007



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Experiments

Fullband coherenceaverage = 0.5

Topographic phase from external DEM

250 m change in topography0.5 cycles at 512 m hamb



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Experiments

Fullband differential phase (DEM compensated)

6 cycles in unwrapped phase–

0.5 cycles due to elevation=

5.5 cycles in differential phase=

5.5 cycles of ionosphere

Fullband unwrapped phase

Fullband Phase



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Experiments

Ionospheric phase rewrapped

Ionospheric phase

Split Spectrum Estimates

Ionospheric phase~5.5 cycles



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Experiments

Split spectrumres. 1 km x 1 km

Range group–phase delayres. 2 km x 2 km

ΔSTEC Estimates

average coherence 0.5

split spectrum σ(ΔSTEC)

0.04 TECU or 0.09 cycles



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Experiments

Low coherence (<0.2) can cause difficulties for MCF-PU

Affects both split spectrum and range group – phase delay

Fullbandavg. coherence 0.2

FullbandMCF-PU

Split-spectrumΔSTEC

Split-spectrumionospheric phase

Range group–phase delayΔSTEC

<0.1

0.25

<0.1



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Compensation of ionospheric phase for InSAR vital at P and L-Band

Estimation theoretically possible across P, L, C and even X-Band

Ionospheric phase screen estimation for repeat-pass InSAR confirmed with successful L-Band experiments

Given sufficient coherence for reliable phase unwrapping, split-spectrum and range group–phase delay approaches give similar results

Current / future work:Comparison with other methods

Fusion of all methods to obtain a better estimate (TH3.T02, Thursday, 13:20, Meyer et. al. “Potential contributions of the DESDynI mission to ionospheric research“)

Documents

IGARSS 2011, Vancouver, Canada