This document discusses methods for correcting distortions caused by the ionosphere in low frequency synthetic aperture radar (SAR) interferometric data. The ionosphere can cause displacement and defocusing in SAR images. The authors propose estimating the total electron content (TEC) and its spatial variation to model ionospheric distortions. They develop approaches using full-band and sub-band interferograms to estimate azimuth shifts and correct for ionospheric phase screens. Simulation results demonstrate the effectiveness of their correction methods, especially when using multiple sub-bands and increasing the number of looks, though this reduces spatial resolution. The authors conclude sub-band methods provide finer structure of the ionosphere compared to full-band approaches.

1. Correction of Ionospheric Distortions in Low Frequency Interferometric SAR Data Jun Su Kim, Pau Prats & Konstantinos P. Papathanassiou Microwave and Radar Institute German Aerospace Center

6. Interferometric phase: correction and results - = Original  FR Corrected phase

10. Full band vs. Sub-band az. shift estimation Low Middle High Full Low Middle High

11. Sub-band Correction scheme 6 sublooks 3 sublooks Full band

12. Differential az. sub-band interferogram GROUND ORBIT Ionospheric intensity S 1 S 2 S 3

14. Simulated 6MHz P-band data Without Ionospheric disturbances Coherence-range HH channel SLC Coherence with undisturbed master (N=81) Under Ionospheric disturbances (kp=3, Ckl=50)

15. Interferometric phase retrieval (kp=3, Ckl=50) Ionospheric phase screen Coherence as a function of range Nr of Looks =3 A = 3,000 m Nr of Looks =33 A = 300 m Nr of Looks =10 A = 1,000 m Nr of Looks =100 A = 100 m

16. Interferometric phase retrieval (Far Range) Ionospheric phase screen Nr of Looks =3 A = 3,000 m Nr of Looks =33 A = 300 m Nr of Looks =10 A = 1,000 m Nr of Looks =100 A = 100 m

17. Conclusion Air-borne motion compensation method can be applied. Combination of Az shift and  FR gives better  TEC estimation. Azimuth shift can be corrected for slow ionospheric variations. Azimuth full band approach Performance increases with increasing number of looks (i.e. degreasing azimuth apperture) on the cost of spatial resolution. Sub-band inter-interferogram is well coincidence with direct shift estimation Azimuth shift is compensated also in sub-bands. Azimuth sub-band approach Sub-band approach always give finer structure of the ionosphere

18. Correction of Ionospheric Distortions in Low Frequency Interferometric SAR Data Jun Su Kim, Pau Prats & Konstantinos P. Papathanassiou Microwave and Radar Institute German Aerospace Center