Hoonyol Lee, Jae-Hee Lee Kangwon National University, Korea

1. IGARSS2008, Boston, MA, USA An Experiment of GB-SAR Interferometric Measurement of Target Displacement and Atmospheric Correction Hoonyol Lee, Jae-Hee Lee Kangwon National University, Korea Seong-Jun Cho, Nak-Hoon Sung, Jung-Ho Kim Korea Institute of Geoscience and Mineral Resources

2. Contents • GB-SAR System • Displacement Measurement • Atmospheric Correction • Conclusion

3. Introduction • GB-SAR: Ground-Based Synthetic Aperture Radar • “Synthetic Aperture Radar” • Imaging Radar • Azimuth aperture synthesis • “Ground-Based” • Fairly versatile system configuration • Multiple frequency (L, C, X, Ku, Ka, etc) • Full Polarization (VV, VH, HV, HH) • Ultimate SAR focusing • Zero Doppler centroid (stationary vehicle and target during Tx/Rx) • Accurate estimation of Doppler rate from geometry • Topography Mapping: Cross-Track InSAR • Surface Motion: Zero-baseline and short atmospheric path for high temporal coherency, DInSAR • Useful for new SAR concept design • GB-SAR Activities • EU and Japan for avalanche, landslide, glacier, building monitoring

4. GB-SAR System < Example > Center frequency : 5.3 GHz Range bandwidth: 600 MHz Range resolution: 25 cm Number of points : 1601 Maximum Range: 200 m Azimuth length : 5 m Azimuth Step : 5 cm Azimuth Resolution: 0.32 degree Azimuth width: 32 degree Power : 33 dBm Polarization: Full

6. System Configuration

7. SAR Focusing Algorithms

8. DF vs RD (Indoor) (a) DF algorithm (b) RD algorithm

9. DF vs RD (outdoor) (a) DF algorithm (2MB Memory) (b) DF algorithm (geocoded) (b) RD algorithm (128MB Memory)

10. GB-SAR Resolutions (b) Partial Focusing (Region I) (a) Full Focusing (Region IV)

11. Image Area (Bw = 200 MHz)

12. VV T1

13. VH T1

14. HH T1

15. DInSAR (T2-T1):Temporal baseline of 20 minutes VV

16. Cross-Track InSAR (T3-T2)Vertical baseline of 30cm VV

17. Delta-f InSAR (T4-T3)Carrier frequency shift of -10 MHz VV VV

18. Cross-Track and Delta-f InSAR (T4-T2)Vertical baseline of 30 cm, Carrier frequency shift of -10MHz VV VV VV

19. Wider View2cm Step, 2007. 3. 19 7:22pm- 4:20am, A1~A9 VV HH

20. System Phase Errors Ideal Case A6-A5, HH Azimuth scan shift of 2cm. A9-A1, HH Range System Shift of 2mm

21. Temporal Coherence Temporal Coherence of 9 acquisitions for 2 hours. Color scheme: black (0) to white (0.9), blue (0.9) to red (1)

22. Measurement of Target Displacement2007. 7. 18 3pm ~ 7pm

23. Image Area Image Area (KIGAM, Daejeon, Korea)

24. Precise Motion of the Trihedral Corner Reflector (160m away from the system) ↑ Radar Direction Displacements toward GB-SAR: 1, 6, 10, 30, and 40 mm A trihedral corner reflector on top of an acrylic plate with rulers on both sides

25. GB-SAR Images VV VH HH HV

26. Comparisons with Actual Displacement Co-polarization

27. Comparisons with Actual Displacements Cross-polarization

28. GB-SAR Interferometry in a Non-Dispersive Medium • GB-SAR phase in a medium: n = refractive index λ = wavelength R = range • Displacement sensitivity of phase: ex) -12.72 degree/mm for C-band when n = 1 (vacuum)

29. Refractive Index • n is a spatio-temporal function of temperature, pressure and humidity (Pipia et al., 2008). n = n (T, P, h) • Among them humidity has the strongest influence on n (Noferini et al., 2005). n = n (h)

30. Phase/Range vs. Humidity

31. Atmospheric Correction Algorithm • Strong linear trend between phase/range and humidity • Atmospheric correction algorithm:

32. Regression Coefficients

33. Comparisons – After Correction (total data) Co-polarization

34. Comparisons – After Correction (total data) Cross-polarization

35. Comparisons – After Correction (each pol.) Co-polarization

36. Comparisons – After Correction (each pol.) Cross-polarization

37. RMS Errors

38. Comparison with Pipia et al., 2008 • Pipia et al., 2008 • X-band (9.65GHz) GB-SAR system • HH polarization • Temp: 21°C • Humidity: 44 ~ 59% • Our algorithm in HH polarization at 52% humidity (average of Pipia et al.) is:

39. Wavelength Dependency of Phase Delay • n is constant over the wide range of electromagnetic spectrum (non-dispersive). • Phase delay is inversely proportional to wavelength. • Gradient ratio between X and C-band: 1.78 • Wavelength ratio between C and X-band: 1.82

40. So, what’s the point? Satellite SAR • Merely 11% of the humidity change (47%-58%) between two C-band SAR observations may cause: • a DInSAR-error of 3 mm at 200 m range, • a satellite DInSAR-error of 3 cm (one fringe)assuming 2 km range propagation through the tropospheric thick moist zone • 1.5mm DInSAR-error between near-range and far-range (100 m path difference for 2 km lower troposphere) for Envisat IS2 • Care should be taken of when we try to seek a geophysical meaning of one or two fringes. 2 km thick moist layer

41. Conclusion • We made a SAR system capable of highly accurate consecutive measurements. • GB-SAR displacement measurement have shown 2-3 mm error with moisture change of 11% (47-58%) at 160 m range. • Phase/Range vs humidity showed highly linear trend, resulting in a simple atmospheric correction algorithm in terms of humidity. • Comparison with an X-band experiment (Pipia et al., 2008) confirmed the non-dispersive nature of microwave. • Merely 11% moisture change both in time and space, for example, is enough to generate one or two fringes for satellite-based InSAR applications.

42. Thank You