COMPACT POLARIMETRY MODE FOR A LOW FREQUENCY SAR IN SPACE Pascale Dubois-Fernandez, Sébastien Angelliaume, My-Linh Truong-Loi, Jean-Claude Souyris ONERA, Centre de Salon de Provence, France CNES, Centre de Toulouse, France
[email protected] 1. ABSTRACT In spaceborne SAR, a single-polarization on transmit has, over full polarization, the advantage of a double swath due to SAR system design issues. Without getting into the details deserving by themselves a full publication, we can just observe the swath characteristics associated with ALOS PALSAR dual-pol and full-polarization modes. This increased coverage has a direct impact on the revisit time, which is always a major drive for the Earth Observing community. The options chosen up to now for current dual-pol system designs (or single-polarization on transmit) rely on a linear polarization on transmit, with two orthogonal polarizations on receive. Souyris [1] in an earlier paper proposed a more pertinent alternative for the selection of the transmit polarization leading to a better characterization of the scattering mechanisms. In a previous paper, the analysis was pursued in more depth by including the effect of the ionosphere on the wave propagation and extending the applications to polarimetric interferometry (PolInSAR). A compact pol mode was proposed where the transmit polarization is circular while the only constraint on the two receiving polarizations is independence. Both the polarimetric SAR applications and the PolInSAR applications in the context of this compact polarimetry (CP) mode were explored. A pseudo covariance matrix can be reconstructed for distributed target and was shown to be very similar to the full polarimetric (FP) covariance matrix. A PolInSAR vegetation height inversion for P Band was presented and applied to the CP data with a level of performance similar to the one associate to FP (1.2m RMS Height on the RAMSES dataset over the Landes forest). A procedure was developed to correct for the ionospheric effects for PolInSAR acquisition in FP or CP mode and the simulation on airborne data proves the procedure to be efficient and robust. In this paper, we extend the analysis of the CP mode to other sites and other datasets. For PolInSAR inversion, CP mode could be very dependant of the dominant scattering mechanisms. The double bounce effect, which is an essential element of the scattering when topography is flat, has an interesting effect on the PolInSAR behavior of forest: When the DB is present, it is usually relatively strong and focused the bottom to the trunks, lowering the overall scattering phase center. In a CP polarimetry mode, the observed polarization states are only a subset of the FP set. In the unitary circle, the complex interferometric coherences associated with a given area describe a convex region when the polarization state varies. In the hypothesis of the Random Volume Over Ground model, this region is a line and the inversion relies on the accurate estimation of this line and its intersections with the unitary circle. In a CP mode, the length of the line will therefore be shorter, potentially creating a less accurate estimate of the needed line. The previous study explored the behavior of CP on the Nezer forest which has no topography. In this study, other sites will be included where topography is present and FP PolInSAR estimation will be compared to CP PolInSAR inversion.
2. REFERENCES [1] JC Souyris, N Stacy, T Ainsworth, JS Lee, P Dubois-Fernandez, “SAR Compact Polarimetry for Earth Observation and Planetology: Concepts and Challenges”, Proceedings of POLINSAR 20007, http:/earth.esa.int/workshops/polinsar2007. [2] P Dubois-Fernandez, JC Souyris, S Angelliaume, F Garestier, “Compact polarimetry at low frequency”, Submitted IEEE TGRS [3] S.R. Cloude and K.P. Papathanassiou, “A three stage inversion process for polarimetric SAR interferometry”, IEE proceedings, Radar, sonar and Navigation, Vol. 150, Issue 03, June 2003, pp 125-134
