IGARSS 2011, Vancouver, Canada

1. IGARSS 2011, Vancouver, Canada Session: 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

2. Contents • Ionospheric Effects in InSAR • Estimating TEC • Split Spectrum Method • Range Group – Phase Delay Method • Theoretical Performance • Experiments with ALOS-PALSAR acquisitions • Summary & Conclusion

3. Contents • Ionospheric Effects in InSAR • Estimating TEC • Split Spectrum Method • Range Group – Phase Delay Method • Theoretical Performance • Experiments with ALOS-PALSAR acquisitions • Summary & Conclusion

4. Ionospheric Effects in InSAR Yearly Average 2010 • Ionised gases at 50 – 1000 km • Spatial variations: typically over >100 km • effectively constant over SAR scene dimensions • Temporal variations: daily, seasonally, solar cycle • sun-synchronous orbits reduce temporal variation • Scintillation: rapid temporal & spatial changes 06:00 local, descending, 0 – 10 TECU 18:00 local, ascending, 0 – 30 TECU

5. Ionospheric Effects in InSAR • 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 Screen • TEC  ΔTEC (master – slave TEC) • constant ΔTEC  phase gradient in range • spatial variations in ΔTEC modulate ionospheric phase 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 time 68° W 25° S, scene 32 x 57 km

6. Ionospheric Effects in InSAR Existing and future SAR systems: interferometric phase sensitivity to VTEC at 35° incidence angle

7. Contents • Ionospheric Effects in InSAR • Estimating TEC • Split Spectrum Method • Range Group – Phase Delay Method • Theoretical Performance • Experiments with ALOS-PALSAR acquisitions • Summary & Conclusion

8. Estimating TEC / Ionospheric Phase Screen • Global Ionospheric Models (GIMs) from GPS • Low res ~100s of kms, little or no spatial resolution over SAR image, ~1 TECU RMSE • Single Image Techniques • Autofocus, Faraday Rotation (Polarimetric data, latitude dependent) • InSAR Techniques • Split 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 correlation • also exploits differing dispersive/nondispersive frequency behaviour, low resolution

9. Contents • Ionospheric Effects in InSAR • Estimating TEC • Split Spectrum Method • Range Group – Phase Delay Method • Theoretical Performance • Experiments with ALOS-PALSAR acquisitions • Summary & Conclusion

10. Split Spectrum Method interferometric phase at carrier frequency non-dispersive topography, atmosphere dispersive ionosphere Subband Range Spectra Lower Subband Upper Subband Optimal subband bandwidth? b = B / 3 (same as split spectrum/delta-k absolute phase estimator)

11. Contents • Ionospheric Effects in InSAR • Estimating TEC • Split Spectrum Method • Range Group – Phase Delay Method • Theoretical Performance • Experiments with ALOS-PALSAR acquisitions • Summary & Conclusion

12. Range Group – Phase Delay Method unwrapped interferometric phase at carrier frequency non-dispersive topography, atmosphere phase delay = group delay dispersive ionosphere phase delay = – group delay shift from crosscorrelation between master and slave take difference perform averaging

13. Contents • Ionospheric Effects in InSAR • Estimating TEC • Split Spectrum Method • Range Group – Phase Delay Method • Theoretical Performance • Experiments with ALOS-PALSAR acquisitions • Summary & Conclusion

14. Theoretical Performance • Split spectrum • Subband center frequency error  couples dispersive & non-dispersive components • Better performance at low carrier frequencies & high bandwidths • Range group-phase delay • Performance determined by crosscorrelation based group delay estimate • (In)Coherent CC  σ is (1.5)2 x less than split-spectrum range spectrum with nonuniform weighting

15. Theoretical Performance 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.

16. Comparison

17. Contents • Ionospheric Effects in InSAR • Estimating TEC • Split Spectrum Method • Range Group – Phase Delay Method • Theoretical Performance • Experiments with ALOS-PALSAR acquisitions • Summary & Conclusion

18. 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 coherence 150.21 W, 69.96 N 01-04-2007, 17-05-2007 2. Poor coherence 147.40 W, 66.62 N 01-04-2007, 17-05-2007

19. Experiments 250 m change in topography 0.5 cycles at 512 m hamb Fullband coherence average = 0.5 Topographic phase from external DEM

20. Experiments Fullband Phase 6 cycles in unwrapped phase – 0.5 cycles due to elevation = 5.5 cycles in differential phase = 5.5 cycles of ionosphere Fullband differential phase (DEM compensated) Fullband unwrapped phase

21. Experiments Split Spectrum Estimates Ionospheric phase ~5.5 cycles Ionospheric phase rewrapped Ionospheric phase

22. Experiments ΔSTEC Estimates average coherence 0.5 split spectrum σ(ΔSTEC) 0.04 TECU or 0.09 cycles Split spectrum res. 1 km x 1 km Range group–phase delay res. 2 km x 2 km

23. Experiments • Low coherence (<0.2) can cause difficulties for MCF-PU • Affects both split spectrum and range group – phase delay 0.25 <0.1 <0.1 Split-spectrum ionospheric phase Fullband avg. coherence 0.2 Fullband MCF-PU Split-spectrum ΔSTEC Range group–phase delay ΔSTEC

24. Contents • Ionospheric Effects in InSAR • Estimating TEC • Split Spectrum Method • Range Group – Phase Delay Method • Theoretical Performance • Experiments with ALOS-PALSAR acquisitions • Summary & Conclusion

25. Summary & Conclusion • 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“)